Final Project report electric car change station

7/22/2019 Final Project report electric car change station

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Battery Changing

StationFinal Design Report

Group 28Dr J Bumby & Dr Z Racz

03/2014


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Executive SummaryThis report outlines the design for a fully automated battery change station for electric vehicles (EVs).

The station is to instantly recharge the EV by replacing the battery rather than charging it. This is to

overcome one of the main problems with EV ownership, which is short travel range on one full charge.Once the depleted batteries are removed from the EV they are transported via a conveyor system to

a storage area where the batteries are recharged. Due to the large number required the storage area

takes up a large volume. The roof of this storage area is covered with solar panels in order to

supplement the supply from the National Grid for the large power requirement. The batteries are

lifted from the subterranean conveyor and attached directly into the EV. In order to accommodate

the different types of batteries they will be mounted on a universal tray. The uniform size will allow

for simplification of the conveyor system, and each tray will include the electronics to regulate the

charging of the batteries.

These stations are to be built on major trunk roads in order to both have the greatest effect on

motorists, and easy access to high-power electricity supplies. The service will be paid for by eithercompanies or individual motorists by a recurring monthly contract, which will be priced competitively

with battery leasing schemes provided by EV manufacturers. Under contract, the batteries will be

owned by the station rather than the motorist.

The storage area consists of a series of angled shelving units with powered rails running down the

inside. The depleted batteries are loaded into the rear and are charged as they slide down the shelf.

The fully charged batteries are removed from the front when required. Each type of battery will be

stored on a different shelf to ensure that there is a charged battery ready for any type of EV wishing

to use the station.

The conveyor system has a modular design, with a series of powered rollers and low-friction tables tochange the direction of the battery. There will be a series of these tables mounted on pistons along

the open faces of the storage area to allow for safe transfer. There is also a specialised table around

the mounting point underneath the EV to align the battery to millimetre precision. There is a hydraulic

scissor lift system to raise and lower the batteries into and out of the EV. The battery will be held in

place by a manufacturer-designed quick release system, however there is an example system

described.

In order to ensure the vehicle is correctly positioned a conveyor system was designed. The EV can be

driven onto tracks which can manoeuvre it with millimetre precision. The conveyors can be realigned

to accommodate vehicles with different tyre widths and wheelbases, from information relayed

wirelessly from the EV. In order to ensure the driver is familiar with the system there will also be awalkthrough provided at first use, as well as clear signs and signals throughout the conveyor building.

The conveyor building has been designed to be aesthetically pleasing, since it is likely customers would

be drawn towards using a well-designed system.

Due to the top-down nature of this design, further refinement and analysis of the design would be

required before manufacture, however design for manufacture has been considered throughout.

Where possible, existing technology and construction methods have been the basis for the design,

and the majority of the system has a modular design to increase capacity where necessary. The design

described here is for a battery change station expecting a battery turnover of 450 per day, however

by adding more car bays and storage the system could be easily modified to accommodate a much

greater flow of customers.


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The design requires a reasonable amount of cooperation from EV manufacturers, since a quick release

mechanism would have to be built into the vehicles. However the system can cope with the difference

of batteries, which is has been the main problem with previous attempts at designing this sort of

system. It is likely that this system would attract government funding since it is promoting the use of

low-carbon transport, and also is providing green energy from the solar panels. All of these factors

increase the feasibility for the long-term success of this project.


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ContentsExecutive Summary .................................................................................................................................. i

Nomenclature ......................................................................................................................................... v

1 Introduction .................................................................................................................................... 1

2 Control Process ............................................................................................................................... 2

2.1 Design Concept and Detail Design .......................................................................................... 3

2.2 Commercial Considerations .................................................................................................... 3

3 Battery Storage ............................................................................................................................... 4

3.1 System Requirements ............................................................................................................. 4

3.2 Design Concepts ...................................................................................................................... 4

3.3 Design Development ............................................................................................................... 4

3.4 Detail Design ........................................................................................................................... 5

3.4.1 Storage Rack .................................................................................................................... 5

3.4.2 Rollers ............................................................................................................................. 8

4 Battery Charging and Electrical Considerations ............................................................................ 10

4.1 Design Concepts .................................................................................................................... 10

4.2 Design Development ............................................................................................................. 10

4.2.1 Solution 1Plug in ........................................................................................................ 10

4.2.2 Solution 2 - Wireless ..................................................................................................... 10

4.2.3 Solution 3Powered Rails ............................................................................................ 12

5 Battery Conveyor System .............................................................................................................. 15

5.1 Design Concepts .................................................................................................................... 15

5.2 Detail Design ......................................................................................................................... 15

5.2.1 Rollers ........................................................................................................................... 15

5.2.2 Change Direction Table ................................................................................................. 16

5.2.3 Battery Positioning Table .............................................................................................. 17

5.2.4 Lifting Mechanism ......................................................................................................... 175.2.5 Overall Layout ............................................................................................................... 18

5.3 Building and Construction Allowances ................................................................................. 19

6 Battery Handling ........................................................................................................................... 21

6.1 User Requirements ............................................................................................................... 21

6.2 Design Concepts .................................................................................................................... 21

6.2.1 Battery Attachment ...................................................................................................... 21

6.2.2 Battery Movement ........................................................................................................ 23

6.3 Design Development and Detail Design ................................................................................ 23


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6.3.1 Battery Tray ................................................................................................................... 24

6.3.2 Battery Attachment ...................................................................................................... 25

6.3.3 Lifter .............................................................................................................................. 27

6.3.4 Changes to Battery Packs .............................................................................................. 29

6.4 Design for Manufacture and Sustainability .......................................................................... 30

6.5 Discussion .............................................................................................................................. 30

7 Vehicle Conveyor System .............................................................................................................. 31

7.1 User Requirements ............................................................................................................... 31

7.2 Design Concepts .................................................................................................................... 31

7.3 Detail Design ......................................................................................................................... 32

7.3.1 Belt ................................................................................................................................ 32

7.3.2 Safety Rails .................................................................................................................... 337.3.3 Safety ............................................................................................................................ 34

8 Exterior Design .............................................................................................................................. 35

8.1 Design Development ............................................................................................................. 35

8.1.1 Concept 1 ...................................................................................................................... 35

8.1.2 Concept 2 ...................................................................................................................... 35

8.1.3 Concept 3 ...................................................................................................................... 36

8.2 Final design Choice ................................................................................................................ 36

9 Payment System............................................................................................................................ 37

10 Conclusion ................................................................................................................................. 37

11 Table of References .................................................................................................................. 39

12 Appendices ................................................................................................................................ 41

I. Design Brief ............................................................................................................................... 41

II. Process Flowchart ..................................................................................................................... 41

III. Source Code for Numerical Analysis ..................................................................................... 42

12.1.1 Solving the angle for the storage area .......................................................................... 42

12.1.2 Solving the roller dimensions ........................................................................................ 43

IV. EV Dimensions....................................................................................................................... 44

V. Engineering Drawings ........................................................................................................... 45

VI. Project Plan ........................................................................................................................... 53


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NomenclatureForce Frictional Coefficient Mass Acceleration

Gravitational Acceleration Time Distance Incline Continuous Beam Coefficient Length Youngs Modulus Second moment of area Thickness Width Mass Distribution

Stress Distance from Neutral Axis Bending Moment Radius Natural Frequency Inductance Capacitance Relative Magnetic Permeability of Conductor Permeability of Conductor Skin Depth Resistivity

Electric Frequency 1Power Current Resistance Cross Sectional Area Pressure Electric Vehicle EV

Radio Frequency Identification RFID

Active Reader Passive Tag ARPT

Active Reader Active Tag ARAT

American Wiring Gauge AWG


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1 IntroductionThis design report aims to solve problems created by the fact that current electric vehicles (EVs) have

a range of only around 60-1001miles. The solution presented here is to build battery change stations

along major trunk roads, which will replace depleted batteries for fresh ones with a speedy and fully

automated process. The system can be thought of as a petrol station for electric vehicles. Theconsumer will not have ownership of the physical batteries, and will instead lease the batteries from

the network of charging stations, for a cost which is competitive with current manufacturer battery

leasing schemes.

This solution comprises of a conveyor system which the motorist drives onto, which aligns the car

above a mechanical lifter which will remove the battery from underneath the EV. This lifter will then

lower, sending the battery to a charging and storage area via a conveyor system. A fully charged

battery will then be positioned over the lifter, and subsequently raised into the EV. After the battery

has been loaded the EV is free to drive away. The battery change will only be permitted if the EV has

an active subscription to the service, which will be checked by scanning the RFID tag added to the car

at time of commencement of subscription.

Figure 1.1 - Overall station design

1A report on this can be found at http://www.telegraph.co.uk/motoring/columnists/mike-

rutherford/9525189/Electric-cars-the-truth-about-the-cost-and-range.html


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2 Control ProcessThe control process for the whole station is summarised inFigure 2.1.The flowchart shows the path

an EV driver, a depleted battery and a charged battery take while within the station. These are

highlighted by the blue, black and grey paths respectively.

RFID communication is incorporated into the design of the station to ascertain certain dimensions of

the EV and monitor the batteries which are being held in the storage area. This allows the service to

be automated as nobody will need to be employed to monitor the batteries or to position the vehicle

conveyors.

Figure 2.1- Flowchart Detailing Station Operating Procedures


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2.1 Design Concept and Detail DesignThe RFID tags and readers can be either active or passive (Wikipedia - The free encyclopedia, 2014).

Passive tags require activation by a reader in order to start operation and be read. Passive tags are

usually read-only; these allow the reader access to a serial number which can be entered into a

database by the user, or WORM (write once, read many); these tags can only store one piece of

information which cannot be rewritten but can be read multiple times. Active tags require a small

battery as they periodically transmit their identification signal. These tags can be read-write storage

types; they allow information to be overwritten and stored to be transmitted (Bonsor, n.d.).

The RFID readers are also classed as active or passive, however this design only utilises active readers

which transmit interrogator signals when required.

There are two types of RFID system which are used in this design:

Active Reader Passive Tag (ARPT) system to read in information of the EV. Active Reader Active Tag (ARAT) system to monitor the depleted batteries while recharging.

The ARPT system will be used as an EV enters the station. A passive WORM tag installed in the vehicle

will store data relating to its dimensions. When this is scanned and read, the vehicle conveyors can

align to the correct wheelbase and the correct battery can be selected from the storage area.

The ARAT system will be used to monitor the charging of depleted batteries. The active read-write tag

will store data whilst periodically overwriting it to ensure factors such as charge level, battery wear,

current, voltage and temperature are accurately monitored. The tag and relevant monitoring

equipment will be stored in the battery tray, discussed in section 3. The information accessed by the

reader can alert an external user, i.e. a maintenance worker, to prevent any damage to the battery.

The standard range for this type of system can be up to 2m. This means that each tag doesnt require

its own reader and these are mounted directly onto the storage rack to allow close range to the batterytray.

2.2 Commercial ConsiderationsThe price for an ARPT system would be very cheap as each passive tag will be approximately 0.60

and the single reader will be in the range of 300 to 600 (RFID Journal, 2014). This means that these

tags can be quickly implemented and installed into the EV on commencement of the contract.

The cost for the ARAT system is significantly more as the active read-write tags cost in the range of

15 to 50. Since we expect battery number to be 420 assuming a factor of safety of 1.5 to assure that

the station is in surplus, this can lead to an overall average figure of 13,650. In addition to this,

42,000 would be required for the purchase of 140 readers in order to monitor this system.


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3 Battery Storage3.1 System RequirementsThe main requirement of the storage system is to compliment the rest of the system to provide a quick

and safe battery change. In order to ensure this, it must be easy to remove batteries from storage.

There must be a simple to implement charging system, and the entire system must be easy to maintainand easy to extend.

3.2 Design ConceptsThe first concept is very similar to the way shoes are stored in bowling alleys, with each battery being

stored in a slot in a wall. There would be a lifting machine, like a forklift, which would move the

individual batteries from their designated storage area to the machine to install it into the vehicle.

This would allow for each battery to have an individual charging unit.

The second concept is similar to a vending machine, where the batteries are stored and charged on a

shelf with similar batteries. When a battery of one type is required, it is removed from the front of the

shelf, while the battery it is replacing is returned to the back of the shelf, and all the other batteries

will slide forward to fill the space vacated. The batteries would be moved to the lifting system by a

conveyor system.

3.3 Design DevelopmentThe second concept was ultimately decided upon, since getting the batteries in and out of storage

would be a much simpler. For a battery change station which has two bays, it was calculated that the

storage area must have a capacity for at least 420 batteries. After considering different combinations

of dimensions, it was decided that each shelf would have space for 10 batteries, and the storage area

would be 5 shelves high. Therefore there would need to be at least 9 shelving units to accommodate

the batteries. Since the building is likely to be built on relatively cheap land, next to major trunk roads

2

,building out is likely to be far cheaper than building up. This has the added benefit of leaving a large

roof space, which will allow for a greater number of solar panels on the roof. Not only will this have

the effect of offsetting the cost of charging the batteries, it will also have a good public relations effect,

increasing the publics perception of EVs being environmentally friendly.Figure 3.1 shows an initial

concept for the shelving unit, which is 7 shelves high.

Figure 3.1 - Initial concept for the shelving unit

2

The charging stations are intended to be first built on trunk roads, however in the future there may be demandfor them in cities. Due to the increased price of land within major metropolitan areas, it may be cheaper to

design taller racks.


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3.4 Detail Design3.4.1 Storage RackThe angle required for the batteries to move under their own weight can be calculated by considering

the forces acting on the battery, as shown inFigure 3.2.

Figure 3.2 - Force diagram for battery on a slope

By resolving forces parallel to the slope, and perpendicularly, an equation relating mass, angle and

acceleration can be formed. By assuming constant acceleration and substituting =2/ thefollowing equation can be obtained. = (3.1)

Here the time taken for the battery to move along a distance is given as a function of the angle .Since the trays are 1500mm, and allowing for a 100mm gap between them, it was decided the time

taken for the battery to slide to the next index should be 2 seconds, the required angle needs to be

5o.

The rollers were to be mounted on a single beam, which would as a result, undergo a large bending

moment. In order to spread the weight, 5 upright supports would be on both sides of the rollers. Since

the angle at which the shelf is at is very small, its effect was ignored and the weight distribution and

peak bending moments in the beam were found using the continuous beam model (Steel Construction

Institute, 1994).Figure 3.3 shows the model with the required coefficients needed to find the reaction

forces and bending moments.

Figure 3.3 - Continuous beam model with coefficients

The vertical coefficients inFigure 3.3 are for the reaction force, which is simply given by

= , (3.2)where is the force acting over the span of length , and is the continuous beam coefficient. Thelength of each span is 3.2m, and the force can be approximated to

= 3 . 2

, (3.3)

0.

393

0.

393

1.

143

1.

143

0.

929

-0.107 -0.071 -0.107

0.077 0.036 0.036 0.077

R1 R2 R3 R4 R5


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where is the gravitational field strength and is the mass. The battery term can be substitutedstraight in as 187.5 kgm-1, assuming a 600Kg battery of length 1.6m. This is acceptable since the

batteries will all be the same way round on the racks. Similarly, the roller term is 9.4 kgm -1, since each

roller weighs 7.5 kg over a 400mm distance. Therefore the reaction force for each span is simply =

6182.

In order to calculate the required dimensions of the vertical columns, Eulers buckling formula,

equation (2.4), should be considered.

= (3.4)Here, is the youngs modulus, is the effective length of a column and is the second moment ofarea. Since the column here will be fully fixed at the base and pin-jointed at the top, = . Thesecond moment of area for a square column is given by equation (2.5).

=

(3.5)

Here is the thickness and is the width of the cross section, with being parallel to the directionof the moment. By combining the equations, the force on the column is provided by equation (3.6).

= (3.6)Since each column supports 5 shelves, there are five different forces applied to the column. At the

point where each shelf is attached to the column the force increases. At the top of the column only

one shelf is being supported, whereas the column below where the second shelf attaches has to

support the weight of two shelves. However at this point the length of the column decreases, since a

fraction of entire column is supporting the combined weight.Figure 3.4 shows this concept pictorially.

Figure 3.4 - Diagram showing forces in column supporting (a) one shelf and (b) two shelves

Since it is simpler to manufacture a prismatic column than one with a varying cross sectional area, the

column must be able withstand the maximum stress. By considering Eu lers buckling formula, it can

be shown that this occurs when the product of the length squared and the sum of forces is maximum,

i.e.

2

(3.7)

F FF

2F

LL2

(a) (b)


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This maximum is where the middle shelf attaches, where the column is supporting the weight of three

shelves. The exact length varies due to the angle of the shelves, however the stress always peaks at

the central shelf. Due to the several flaws with this analysis3and time pressure on this project, a factor

of safety of 10 was used when considering the force over the span, to ensure the columns are over-

engineered rather than likely to fail. The final equation generated from this analysis is therefore shown

below.

= 1 0 3 6 1 8 2 = (3.8)Here, is the distance from the foot of the column to the lowest point of the pin supporting thecentre shelf. It was decided that the columns should be designed from L Section so that they could be

connected together to provide extra stability. By considering standard dimensions for an L Section

(Beardmore, 2006) would help reduce the cost of the project, as there are many foundries which will

produce it, whereas ordering custom sections would be more costly. The outermost and innermost

columns should be constructed from 200x200x24mm L Section steel, whilst the other two columns

should be from 250x250x28mm L section, since they support a greater proportion of the load. Thefinal design is shown in figure 2.5.

Figure 3.5 - Final design for the shelving unit

The L section has a second set of holes drilled on the side not directly attached to the rollers, in order

to be bolted to the next rack along, providing rigidity in a different plane. These holes are drilled off

the centreline so that the bolts can be tightened without the two pieces of L section creating a box

preventing the construction workers from getting wrenches in. These bolts increase the risk of

electrocution, since if there was a contact between the power rail and the roller rack then the entire

group shelving units would be electrified. In order to prevent this the bolts will have rubber washers

as well as metal ones to insulate, in case there is a failure in the circuit breaker.

3This analysis ignores the weight of the beam itself, along with the fact that the theoretical Euler buckling load

is higher than the buckling load in practice. With more time the dimensions for the column cross section should

be calculated using a rearrangement of the Perry-Robertson equation, and equation (3.8) should be amendedto include the mass of the beam. The analysis also assumes that the reaction force at the connecting pin is spread

equally over the cross section of the column. Final analysis should involve finite element analysis.


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The racks are connected to the columns by means of a series of pins. These pins are made of two

threaded parts, which can be tightened with a wrench on each end. The neck at the edge has a

hexagonal bolt end cut into it, which will accommodate standard wrenches. In addition, there will be

a gap under the lowest shelf of at least 0.6m to allow access for construction and maintenance workers

to all of the columns. Although this is not enough headroom for standing, the increased cost of raising

the rack to head height would be too great, and it was therefore decided to be impractical.

The power rails, discussed in Section4,are insulated and connected to the shelf by sprung connectors.

The power rail is thin enough so there is some flexibility, and the springs are such that the gap between

the rails is slightly less than the width of the tray. When the tray is put on the shelf the rails are forced

apart, and the springs allow the rails to be fully in contact with the rolling contacts on the side of the

trays. This ensures an electrical connection is made and the batteries are able to charge.

3.4.2 RollersThe individual beam can be accurately described with a very simple model (Toll, 2013), shown inFigure

3.6.

Figure 3.6 - Bending moment diagram

The reaction forces can be found by symmetry, and the peak bending moment is found in the centre

of the beam, and has magnitude . The uniformly distributed load of magnitude W represents themass of the portion of the battery and tray which it is supporting. There are rollers of two different

widths, 2.1m and 1.6m. The centrelines of the rollers are separated by 200mm and the tray has

dimensions (2mx1.5m). Therefore, by dividing the length of the tray by the distance supported by each

roller, it is safe to assume that the rollers will take

and

5 of the weight of the trays each,respectively. Since the maximum battery weight is not likely to exceed 600kg, W for the rollers are

280.296Nm-1and 490.50Nm-1respectively.

As seen in the L1 Mechanics of Systems lecture course (Toll, 2013), for a beam under load, stress ()is given by

= (3.9)Where is the second moment of area and is the bending moment at position from the neutralaxis (Toll, 2013). The second moment of area for a hollow tube of radius r and thickness t is given by

= (3.10)By substituting equation (3.10) into equation (3.9) and rearranging, the radius required to prevent the

beam from yielding is given by

+ = (3.11)

2

W/Unit length

2


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This equation can be solved for or once the bending moment for the beam is calculated, since theyield stress for any given material can be found from data tables.

By expanding the bending moment found above into equation (3.11), a finalised equation can be

found, with the minimum radius and thickness to be found numerically, using the program in Appendix

III.This is shown in equation (3.12). 4 6 4 =

2 (3.12)

Since the rollers had been already created in the CAD model with a 100mm radius, it was decided to

leave this unchanged.

The tensile yield strength of A36 mild steel is 250MPa (Engineering Toolbox, n.d.), which is far greater

than the stress that would exist in a 100x1.6mm tube, a commonly manufactured tube size.

Overcompensating for the stress will also reduce chance of fatigue failure, although reducing the

thickness of the steel will help keep the cost of the project down, so a balance needed to be reached.


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4 Battery Charging and Electrical ConsiderationsIn order for this proposed station to continuously service electric vehicles, a method of recharging

depleted batteries is necessary. There are many ways to tackle this problem when only one battery is

to be charged, however on a larger scale these methods can become inefficient or unsafe. This is due

to the electric batteries stocked in the station being rated at different maximum voltages and currents,and allowing high voltages and currents in the charging system will lead to heating losses and safety

concerns due to the high temperatures. The required power would be sourced straight from the

National Grid with additional power being provided from an array of solar panels located on the roof

of the storage area. There is also the potential to incorporate this station into the upcoming Smart

Grids4, which would allow for the potential of significant government backing.

4.1 Design ConceptsAfter some initial research, it was decided that there were two possible solutions to the problem

outlined above:

1. High voltage plug in charging; a wired method of quickly recharging EV batteries similar tothose found in most charging stations.2. Inductive charging; a wireless method of charging EV batteries.

However, following further development of these solutions, it was found that both were insufficient

to meet the user requirements, and this will be discussed further in Section 4.3. A new solution was

then chosen, capable of meeting the user requirement:

3. Charging via a powered rail; a method of connecting depleted batteries to a single rail in orderto recharge, similar to overhead lines for trams or the Third Rail system for trains (Wikipedia,

2014).

4.2 Design Development4.2.1 Solution 1Plug inThis option was deemed inefficient and dangerous, and was therefore rejected. This is because each

of the batteries would have needed a set of cables to recharge, each carrying a voltage of 400-500V

and a current of 100-150A. This means that each cable would be carrying 75kW, and in order to allow

for this the cables were copper conductors specified to 1/0 AWG5would be required. These cables

would have a resistance of 0.3These cables incur large electrical losses (1.1kW), which cause a significant rise in temperature. They

could also create a trip hazard for anyone within the storage area itself.

4.2.2 Solution 2 - WirelessA neater solution to this problem was to inductively charge all the batteries while they are sat in their

connecting trays in storage. This was a more elegant proposal as it did not utilise any trailing wires,

but would also be more efficient as less energy would be dissipated as heat.

This method of wireless charging involves the creation of an inductive couple between the power

source and the electrical device. This induction charging system would work by excitation of an

inductive coil with an alternating current, generating an electromagnetic field. Once a secondary coil

is introduced into this field, an alternating current would be induced, inductively transferring power.

4A summary of Smart Grid Technology can be found at: http://en.wikipedia.org/wiki/Smart_grid5This specification describes a copper cable, 8.25mm in diameter, using American Wiring Gauge


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The power transferred this way is inversely proportional to the distance between both coils; as the

distance increases, less power is transferred and a lower magnitude current is generated. As a result,

large amounts of energy is dissipated to the surroundings, so the efficiency is lower compared to direct

contact. This would result in an increase in the time taken to charge a battery.

In order to increase the efficiency during the energy transfer, both the primary and secondary coilscan be tuned to resonate at the same frequency. Resonance can be achieved by selecting components

which satisfy Equation 4.1

= (4.1)Here is the natural frequency, is the inductance and is the capacitance. Since the primary coilwould resonate, it allows for the energy held in the magnetic field to dissipate less slowly, allowing

the secondary coil to collect most of it. This only occurs when the natural frequency,, is equal forthe primary and secondary circuits. In this instance, less energy would be dissipated as heat; common

resonant inductors have an efficiency of 80%.

An overall diagram of the layout of the inductive charging system can be seen in figure 3.1. The station

would receive power directly from the national grid in the order of 2MW at 50Hz. There is also power

which is delivered from the solar panels at 320W DC per module. The area of the roof of the storage

facility is adequate to hold 261 modules with dimensions 1.38mx0.9m, and each would be able to

change the angle it sits at, in order to achieve the maximum power output from solar energy.

Electricity generated via this method would be able to offset other energy costs, as it is generated

onsite. In order to feed the primary winding with an AC current, the current from the solar panels

would have needed to be converted so it oscillates at 50Hz. A solid state converter would be sufficient

to handle the input power and output at 50Hz (Witricity, n.d.).


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Figure 4.1 - Block diagram for the wireless charger

However, when the storage area design was finalised to steel as opposed to concrete, this concept

could no longer be implemented. This is due to the fact that the leakage flux, however minimised,

would magnetise the storage area. This provides both huge power losses, as well as posing a serious

safety hazard.

4.2.3 Solution 3Powered RailsThis system uses a set of two cylindrical conducting rails which are mounted onto the storage rack and

run alongside the battery whilst in its tray. One of these rails will be connected to an AC supply and

the other will be grounded, so the battery will begin to recharge as it comes into contact with both

rails. It will continuously charge as it moves down the row of the storage rack since it will not losecontact with the rails. There will be a step-down transformer before each set of rails to change the

high source voltage to 230V. This will minimise any damage caused to the battery due to overcharging.

Since the amount of current flowing through the rails is very high, they need to be constructed from

an appropriate material and be of the correct thickness. Aluminium was decided to be the best metal

to construct the rails as it is easily manufactured and meets the requirements listed above. It was

decided that 4/0 AWG6aluminium cables would be the best material. Since a high alternating current

will be flowing through the rails, the 4/0 AWG aluminium cables would be able to withstand the high

current and minimise the skin effect common in these types of conductors.

6This specification describes a stranded aluminium cable which has a diameter of 11.7mm


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The skin effect is characterised by current flowing on the outside of the conductor without actually

penetrating it too far beneath the surface. This leads to a high current density on the surface of the

conductor which radially decreases towards the centre of the conductor. The depth at which the

current density has decreased by 37% is known as the skin depth . Since the 4/0 AWG cable isstranded, it allows the current to fully flow through the conductor, therefore minimising this effect.

For a cable such as this, the skin depth is calculated using Equation (4.2).

= (4.2)Here is the resistivity,is the frequency of the current, is the relative magnetic permeability ofthe conductor and is the permeability of free space. The resistivity of aluminium at 20C is 2.65x10-8m(Engineering Toolbox, n.d.), therefore the skin depth for aluminium is 10.3mm. The resistance,

R, of the cable can be approximately calculated using Equation (4.3).

(4.3)Here D is the diameter of the conductor. For the stranded cable the resistance is calculated to be 4.61

m. Due to the high current, there will be a lot of heat generated from power losses which could pose

a serious threat. The amount of power lost is given by Equation (4.4).

= (4.4)The amount of current (I) flowing through the rail will be a maximum 1500A as there are 10 batteries

per row. Substituting the current and resistance into equation (4.4), there is a heating loss of roughly

10kW per rail. This can pose serious electrical or heat risks to anybody working in close proximity to

the storage area. Therefore, a thermal and electrical insulator was added around the rails as this would

absorb any heat losses to the surroundings. However a small area was left uninsulated to allow the

battery tray to come into contact. If anyone were to accidently touch a rail, the risk posed to them

would be reduced by this, though additional protection, through implementation of circuit breakers

would increase this protection further.

Power is transferred from the rails to the battery via a connecting tray. The connecting tray contains

all the relevant circuitry required to fully charge the battery:

A rectifier to transform the alternating current into direct current RFID and battery monitoring equipment mentioned in section 1 An intelligent charger similar to that found in an EV to control the charge levels of the battery

and to cease charging once it is at 80%. A solid state relay could be used to achieve this.

The initial design for the tray is shown below inFigure 4.2.It shows copper pads mounted on spring

loaded pistons; these would allow for permanent contact between the rail and the pad. A connection

would be made from the pads to the internal circuitry within the tray. However there were problems

with this design; the constant movement of the batteries would lead to the copper pads wearing out

very quickly and they would therefore need frequent replacement. As well as this, there was also a

dangerous risk of sparking when the copper conductor comes close to the charged rail. Since the tray

would have to slowly squeeze its way into the rack, the close proximity and extended time between

both conductors could cause a spark to occur.


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Figure 4.2 - Initial tray design

The design for the tray was subsequently finalised and is shown inFigure 4.3.This design features

rolling aluminium contacts along the edge of the tray. These allow for movement with minimalized

friction and also reduce the effect of sparking as the time taken to squeeze in between the rails willhave decreased. Brush contacts will be included within the tray to transfer the current from the rolling

contacts on the tray.

Figure 4.3 - Final design for the battery tray


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5 Battery Conveyor SystemFrom the early stages of our design development, it was noted that a key factor to be considered was

the movement of the batteries, which were both discharged and charged, between the storage area

and the location where they would be installed into the vehicle. Certain elements of the design, such

as the minimum time for changes to be able to occur and the overall dimensions the station, woulddetermine the manner in which the system operated. Additionally, due to the nature of the design

there were extra features that needed to be accounted for and it led to the following specification list

for the battery handling system:

Efficient movement of batteries between the storage area and installation locations, whilststill being safe.

Easily extendable for stations of varying sizes or number of lanes. Strong enough to be able to withstand the movement of batteries weighing up to 600kg each. Be able to be constructed in a compact area to reduce amounts of groundwork required for

construction.

5.1 Design ConceptsHaving decided on these specifications, two systems were seen as plausible. The first would be a single

conveyor between the storage area and installation location, which could travel in both directions

acting in a discrete manner. The second being a continuous loop system, which would work by

transporting batteries from the storage area, to the installation area, then back into the other side of

the storage area. These systems are illustrated inFigure 5.1.

Figure 5.1 - Concepts for the conveyor system

5.2 Detail Design5.2.1 RollersHaving decided that the battery transportation system would act in a continuous manner, it was

necessary to consider the possible methods of moving the battery around the circuit. It was decided

that for any movement over distance in excess of a few metres, a system of rollers would be used,

similar to those that can be seen on luggage carousels or in warehouses. These rollers are made from

varying lengths of 100mm diameter 1.6mm thickness steel, which was deemed to be strong enough

by considering beam theory across a hollow cylinder, see Section 3.4.2. Having identified that a steel

tube of thickness 1mm would be sufficient, the nearest standard size of tube had 1.6mm thickness so

this was chosen. Although this alone would create a beam that was both strong enough and

lightweight, the low coefficient of friction between the battery trays and the steel would mean the

accuracy of delivery would be low. In order to combat this effect, it was decided that the rollers shouldbe coated in a layer of vulcanized rubber, which would greatly increase the amount of friction between

Storage Area

Installation

Area

Storage Area

Installation

Area


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the rollers and trays. At each end of the roller a solid steel section would be installed in order to

improve rigidity, and a ball bearing race would also be installed to allow for low friction movement.

The final roller design can be seen here inFigure 5.2.In order to ensure that the batteries continue to

move through the system, small electric motors would be positioned every 5 rollers, powering the

rollers and helping transport the trays. These electric motors would be connected to the rollers via a

small drive belt, in order to ensure good power transfer. This system is particularly important in order

to move the batteries up the return path to the higher end of the storage rack.

Figure 5.2 - A pair of rollers

5.2.2 Change Direction TableA secondary issue that was encountered was deciding upon a method to change the direction of the

battery as it moved through the system, to allow it to navigate corners. Two obvious solutions were

noted for this initially; firstly that the batteries would travel around the corners, much like the way a

car does at a junction, secondly that the batteries orientation relative to the storage area would not

change as it travelled around the system. Although the first of these two solutions seemed more

intuitive it was found that during initial design numerous challenges and flaws arose when considering

implementation of this system, as it provided low accuracy in positioning of the tray, along with being

relatively bulky as it required a large amount of space to turn the battery. Conversely, the second

option could be compact as it only required a space the size of the tray to change its direction whilst

also granting the opportunity for much more precise control of the battery tray through the system.

Having decided that this was the most appropriate method for changing the direction of the battery,

various options were considered for the way in which this would be implemented andFigure 5.3 shows

the final chosen design.

Figure 5.3 - Change direction table


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The two perpendicular sets of powered wheels are each able to be raised and lowered independantly

of one another. This allows a battery to move in from one direction, with that set of wheels risen,

before these lower and the other set rise, allowing for the battery to be moved onwards through the

system. This did however lead to there being a requirement for rollers of different length; finalised at

1600mm and 2100mm, to allow for the two different orientations of the battery. Protective blanking

plates would additionally be fitted on the sides of the table where there was not a set of rollers to

ensure the batteries were unable to fall off of the conveyor. Otherwise this could pose a serious risk

to both the equipment and any employees.

5.2.3 Battery Positioning TableOnce the batteries had been transported from the storage area to the installation area, it was

necessary to design a method of accurately positioning the batteries prior to their installation.Figure

5.4 shows the final design, where the four wheels found at the corners of the table are each

independently mobile on their axles. This means that the battery is able to freely move both laterally

and rotationally, allowing for accurate positioning prior to the lifter (which rises through the gap found

in the centre of the device) raising, installing the battery into the electric vehicle above. The small

cylinders that can be seen are captive ball bearings, which allow low friction movement both laterally

and rotationally.

Figure 5.4 - Battery positioning table

5.2.4

Lifting MechanismIn order to retrieve the batteries from the storage area, the same change direction tables that were

described earlier are positioned at each end of the storage area and mounted on four hydraulic

pistons, each positioned at one of the four corners of the table. This system allows for rapid and

precise raising and lowering in order to collect batteries from the correct height of the storage area.

Three of the different potential heights of this system can be seen below inFigure 5.5.


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Figure 5.5 - The lifting mechanism in three states: (a) Compressed (b) Half raised (c) Fully raised

5.2.5 Overall LayoutThe final layout of the conveyor system is shown inFigure 5.6.This system, in summary can be seen

as the simplest, yet most efficient method of transmission of batteries around the system. Due to the

standardised nature of the trays within which the batteries are located it allows for a single conveyor

system and battery storage system, meaning changes don't need to be made for alternate battery

sizes. Additionally, this method is incredibly flexible, due to its modular construction, and therefore

allows for simple enlargement.

Figure 5.6 - The final battery transportation system placed within the stations groundwork. The two lines of

tables for raising can be seen as well as the rollers to transport the batteries

(a)

(b) (c)


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5.3 Building and Construction AllowancesThroughout the entirety of our design process, certain factors were considered in order to ensure that

eventual construction would be as cost efficient as possible. For example, whilst components were

being chosen for parts, current standard dimensions were used to ensure easy sourcing of materials.

In particular, for the roller design, it was found that for a 100mm diameter tube, 1mm thickness would

have been more difficult to source than 1.6mm7. Additionally, the entire design had aimed to be

modular in nature. This would allow for easy variation for future station layouts, and enable a variety

of overall capacity of this system, as extra bays would then be able to be added, or a larger storage

area to be constructed with little additional design.

The first major design allowance to consider was whether it would be better to have the electric

vehicles rise up on a ramp prior to the battery change, as this would mean that little excavation would

be required, with the majority of the works occurring at or above ground level, or conversely having

the driver remain at ground level throughout the process. From an aesthetic perspective the second

was decided upon, as it would reduce the overall profile of the design. Driver acceptance of the

scheme was also considered, and it was thought that by allowing the driver to remain at ground levelfor the entirety of the exchange procedure would reduce reluctance towards the idea whilst also

increasing the safety for passengers, as in the event of an emergency, or malfunction with the

machinery the occupants of the vehicles would simply be able to step out of their vehicles and walk

out of the exchange station. Another option that was considered was to drive the car onto a lifter at

ground level, prior to raising it above the exchange position, though the increased time that this would

take to position the car appropriately was deemed too great, so this concept was quickly discounted.

Additionally, as the main storage area for the battery racks is sloped, opting to keep the vehicles at

ground level is beneficial as it reduces the total amount the far end of the storage rack needs to be

risen by from 1.25m to only 0.4m on the far end of the storage area, as the ideal amount to sink the

lower area by is 0.85m. Therefore the total ground works required for the installation of the stationare those that can be seen in figure 4.7. An additional slope can also be found on the nearside of this

design, and allows for the return of batteries up to the higher end of the storage rack. It is intended

that the majority of the rest of the station would be manufactured off site and installed once these

initial foundations are completed. As the entire system is modular by design, a variety of sizes of

station can be created, dependant on the requirements of the operator at the location of the station

implementation. For sites with limited dimensions, the storage area can be located away from the

installation area, providing there is the ability to construct the conveyor system for moving the

batteries between storage and installation.

7This can be found from many building suppliers, for example: http://www.themetalstore.co.uk/


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Figure 5.7 - The complete groundwork required for the station

Besides the works required constructing the main body of the station, the main challenge outside of

this is ensuring there is adequate electrical power supply to the station, as it will potentially require a

very large supply8at peak times. It is possible that this project would be able to be built with support

from the national grid however, as it is a prime candidate for inclusion into the UK governments aim

to move towards smart grids for the distribution of electricity on a national scale. These stations

would be good to implement into this system, as they have the opportunity to control charging rates

so that greater power is required overnight, whilst the wholesale price of electricity is lower. This has

the benefit of both reducing the price for running the station whilst also gaining additional support

from national governments. Additionally, by fitting solar panels on the roof of the main storage area,

providing additional power during the day, when wholesale electricity prices are more expensive,

along with improving the green image of the station. These issues of electricity supply should

however be minimised as it is likely that these stations would be situated along main roads through

the country, which are often also the location of large power distribution networks, so it should be

possible to ensure adequate power supplies to each of these stations, as it the power requirements

predicted, in the region of 2MW, is not uncommon for medium scale industrial applications.

8As discussed in Section 3, if all batteries are required to be charged at the same point, this can be in excess of

2MW.


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6 Battery Handling6.1 User RequirementsFollowing the movement of the batteries from the storage area to the installation area, it was found

that a system was required which was able to remove the discharged batteries from the vehicles,

deliver them to the transportation system and reinsert a fully charged battery in the waiting vehicle.It was decided early on the main requirement for the system was to change batteries as quickly and

safely as possible. Ideally this should be within one and five minutes, a similar time for a conventionally

fuelled car to refill its tank. The battery exchange process had to be fully automated, with no need for

the driver or an attendant to have an input into the process during its normal operation.

One of the main problems with battery exchange stations is the perceived need for battery

standardisation. Previous systems have only been compatible with one set of battery dimensions and

thus a limited range of car models. For example, the Better Place battery exchange systems were

only compatible with specially designed Renault Fluence Z.E. electric vehicles (Wikipedia, 2014). This

would require manufacturers to agree on a single standard battery for all vehicles, which they are

reluctant to do so due to the differing power requirements and the effect the batteries have on car

weight and handling. Therefore, one of the main advantages this system has over previous systems is

its compatibility with batteries of different dimensions. This eliminates the need for a single

standardised battery across all vehicles and allows a range of different vehicles to be used. Whilst

some design adjustments are required for compatibility and the cooperation of manufacturers is

essential, the system has been designed to be compatible with different battery dimensions and allow

flexibility for manufacturers with the possibility of adapting current models through aftermarket

solutions.

As this system has been designed with the future in mind throughout, an additional consideration was

the potential future uptake of electric vehicles, and therefore it was imperative to ensure that thissystem was expandable in order to provide solutions for the future. One of the main disadvantages of

electric vehicles is their current restricted range. This system alleviates the effects of this, which may

potentially make the decision to use a full electric vehicle more desirable and thus help improve

uptake, increasing future demand on stations. This station has been designed to be expandable as

required by either the purchaser or operator. It can be built with a minimum of one operational lane

(working with a single vehicle at a time) and expanded with further lanes without fundamentally

redesigning elements or building a new station; conveyers can be extended and new lanes can be

added, with the size of the battery storage racks tailored to the needs of the station operator.

For the handling and installation of batteries, it was decided that the following components would

need to be designed and implemented:

A battery attachment/detachment mechanism allowing the battery to be removed andreinserted from the bottom of the car

A transportation system to move the battery from the car to the conveyer system A method of connecting the battery to be charged whilst stored in the station

6.2 Design Concepts6.2.1 Battery AttachmentBefore considering methods for how to raise or lower the battery into the car it was deemed that

considerations needed to be made about how the batteries are physically connected to the car. For

most commercially available electric vehicles currently, the main battery modules are located in the

undercarriage, and connected to the car by high strength mechanical brackets. In order to weather-


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proof these modules, an under floor protective casing is then usually installed. Due to the fact that

most batteries are located in this position, it was decided that a vertical raising mechanism would be

the most effective way of removing the batteries, whilst also requiring the least change in

manufacturers design processes. An additional issue that arose was that current connection points

are often located at the sides of the batteries, and these would need to be raised to the top in order

to allow for easy installation. The concept of changing battery modules is not totally revolutionary. It

is currently possible, however only if carried out by service engineers with specialist tools.

The first solutions investigated worked around the current attachment mechanisms, allowing vehicles

on the road to be immediately compatible. The initial idea was to use automated robotic arms

approaching the side of the battery to unscrew bolts attached to the brackets and use magnets to

move and hold the brackets during the battery swap, as shown inFigure 6.1.It was found this system

would be complicated due to the restricted room available underneath the car and the small

tolerances the brackets had to be placed at in order to properly position them. This was also less than

ideal, as it would be difficult to implement a system to allow the arms to move freely to accommodate

different battery dimensions without interfering with the conveyer system.

Figure 6.1 - (a) Layout of initial attachment design (b) Single automated robotic controller

It was therefore decided that the implementation of a quick release detachment system integrated

into the car, rather than an external removal system was the preferred solution. This reduced the

complexity of the system and allowed the handling systems to easily accommodate different battery

dimensions. This also allows a speeding up of the removal process, allowing more time for positional

adjustments to be made to compensate for the cars position. This requires car manufacturers co-

operation to design and implement it into their vehicles, either in their initial design or through

aftermarket solutions. It was decided to design an example system for car manufacturers to prove the

feasibility of the idea and that it can be flexible to their needs. After consideration it was decided to

focus on the following parameters for the attachment mechanism:

Securityholding the batteries in place during normal driving conditions Footprintreducing space underneath the car, as little interference with the suspension as

possible

Keep weight to a minimum

Allow airflow for cooling of batteries

(a) (b)


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6.2.2 Battery MovementAnother important aspect was how to move the batteries between the conveyer system and vehicles.

This has to deliver the batteries quickly to the main system and be able to handle the large weights of

the batteriesthese can range from 100kg for the Renault Twizy (Renault S.A., 2013) to around 500kg

for the Tesla Model S (Tesla Motors, Inc, 2014).

To move the battery from the conveyer system to the car, it was decided to use a hydraulic scissor lift.

Generally scissor lifts are able to operate quickly can be designed to different heights and sizes and

compress to a smaller sizethis would be essential where there was limited room. To accommodate

the different battery sizes it was desirable to lift the batteries from the centre. This also allowed more

flexibility with placing the batteries on the conveyer systemdue to the lifts ability to compress and

fold to a small height, they could easily place the battery on the conveyer without reducing the

available conveyer area available and thus the driving force moving the batteries from their position.

Another consideration made was whether the batteries should be moved on the conveyers on their

own or whether to place them in supporting trays. Using a universally sized tray would make it easier

for the conveyer system to accommodate different battery dimensions and keep them in a fixedposition for refitting. It also could be used to incorporate further electronic features including charging

circuitry and RFID chips for identification. It was therefore decided that despite the costs necessary

for producing custom made trays, their use would help simplify the conveyer and charging systems.

6.3 Design Development and Detail DesignWhilst performing the analysis two example batteries were modelleda large battery for cars, vans

and a smaller battery for smaller vehicles. Table 5.1 shows the approximated dimensions for these

models the larger battery is based on the dimensions of the Nissan leafs battery (Nissan North

Amerca, Inc, 2013) and slightly enlarged and the smaller based approximately on the Renault Twizy.

The weights and dimensions of the test batteries are slightly larger than for the normal models in

order to allow the system to compensate with a large range of new potential battery dimensions,

allowing it to work with new vehicles without fundamental redesign.

Table 6.1 Dimensions of batteries

Battery Large Small

Dimensions (w x l x h) (m) 1.2 x 1.6 x 0.275 0.9 x 1.3 x 0.275

Maximum mass (kg) 600 400

Minimum mass (kg) 250 200


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6.3.1 Battery TrayThe final design of the trays, as shown inFigure 6.2,allows the tray to support the batteries, maintain

their position and holds the required charging connections and circuitry. It is able to hold batteries up

to the size of the large battery model (1.2x1.6x0.275m) and maximum weight of 600kg. The batteries

are connected to charging circuitry and supporting using support shafts in the centre, and rolling

electrical contacts are constructed on the trays edges.

Figure 6.2 - Final tray design

The dimensions of the main tray body were determined by analysing its bending stresses when

supporting the model batteries. The analysis was derived from the fundamental equation below (Toll,

2013).

= (6.1)

Where is the bending moment in the tray, the largest distance from the centroid and the secondmoment of area (Toll, 2013). From basic theory it was realised the maximum stress would be in the

centre of the tray so this was the main focus of the analysis. For the analysis, two scenarios were

considered: the first looking at the bending along the batterys length, looking at a cross section across

it with where it This was analysed along the batterys width, where the tray is supported by the

conveyers, and looking at the cross section along the batterys length, looking at the central section

where it is left unsupported immediately after lifting.

The moments, looking at the bending moments of the length of the tray, are shown in equation (5.2).

= ( ) () (6.2)

Figure 6.3 - Diagrams of the bending scenarios examined


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Here, , are widths of the battery, conveyer in contact with tray and tray respectively, thebattery mass. This was also derived looking along the batterys length at the central section where it

is unsupported by the conveyer system. The bending moments here were found using equation (5.3).

=

(6.3)

Here is the length of the battery. As the tray contains the charging circuitry and contains aninternalised gap to do so, its cross section changes along its length and width. In order to

accommodate for this, the analysis was performed on two separate casesonce using a thin cross

section, looking at a cross section where the gap for charging circuitry is present and again using a

thick cross section where the gap is not present. The results of the analysis are shown in table 6.2.

Table 6.2 Results of the tray analysis

Battery Model Large Small

Mass (kg) 600 250 450 200

Dimensions 1.2 x 1.6 x 0.275 0.9 x 1.3 x 0.275Minimum stress(Mpa) -1.97 -0.82 -1.92 -0.85(Mpa) -1.04 -0.43 -1.01 -0.45Maximum stress(Mpa) 4.23 1.76 2.58 1.15(Mpa) 3.57 1.49 2.18 0.97

From this it was decided to construct the main bodies of the trays using a high strength plastic like

nylon-6. It is able to withstand the compressive stresses found as well as withstand the high

temperatures of the battery charging circuitry and battery itself during charging (MatWeb, LLC, n.d.).The relatively low density of the plastic also helped reduce the overall weight of the tray compared to

original concepts with full steel or steel reinforced bodies.

The tray is also able to hold the batteries in a fixed position as they are moved through the battery

system. Using a standard size tray meant that to accommodate larger batteries, the movement of

smaller batteries within this system had to be reduced. This is achieved using support shafts of 75mm

diameter and 100mm height coming from the base of the tray, including the central shaft for the main

electric connection between the battery and the tray.

The tray also contains the main electrical systems required for charging the battery. The tray contains

charging circuitry and equipment as designed in the car, so only needs to be supplied with an inputvoltage and current. The rollers on the side are used to provide a rolling electric contact between the

rails and the charging circuitry.

6.3.2 Battery AttachmentAn example design has been produced to show that a quick release mechanism built into the car is a

feasible solution and can meet the needs of manufactures. Car manufacturers are free to change and

develop to suit their own needs.Figure 6.4 shows the overall layout of this system.


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Figure 6.4 - Battery attachment system

The system works by using hooks attached to a main support frame on the battery pack. The hooks

are driven by electric linear actuators, each driven with their own motors to reduce the complexity of

the system, allowing it to connect to the existing power sources on board the car. Using six hooks

helped to optimise the system by reducing the loading on each actuator during running and meant

they could quickly move. The whole system was designed to be as lightweight as possible and reduce

added weight on the car as well as not interfere with wheels or suspension. Using an aluminium frame

this system would add a maximum weight of 50kg, with further reductions possible through

optimisation of the support frame.

The battery attachment system has to be able support the battery throughout normal use within the

car and allow it to quickly be attached and detached from the car. One of the concerns with keeping

the control system in the car is the worry of the battery falling out of the bottom of the car. This also

has to be ensured within the control system as well. The hooks must not be removed from the battery

except during an exchange when the tray is in place or during servicing if battery removal is necessary.

The most important factor was the loading of each of the hooks. In this example system the traction

battery was 600kg the maximum weight of the models examined. The hooks were designed looking

at the worst case scenario, being able to hold half the battery weight before yielding. If one of the

hooks failed it was desired for the other hooks to remain intact to prevent the pack from falling due

to the risk of exposure to hazardous materials from the main battery modules.

The hooks are driven to the attachment hooks by electric linear actuators. Under normal loading

conditions (where all hooks are able to hold the weight) they are able to move retract in a short space

of time and withstand the full weight of the lifter and load. It is expected that they would draw power

from the main traction battery, with the pressure of the lifter on the battery maintaining the electricalconnection as the attachment mechanism is being driven. It is feasible since the power of each

actuator motor is insignificant compared to the main traction motor getting the car to the station. The

chance of a fully discharged car entering the system was minimal, and if it did happen a service worker

could manually connect the car to a charging cable to top it up to the required level.


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6.3.3 LifterThe overall system used for lifting the batteries in and out of the car is a scissor lift powered by a

hydraulic piston as an actuator as shown inFigure 6.5.The lifter has a small overall area in order to

allow the battery tray to be placed on the conveyer system.

Figure 6.5 - (a) Hydraulic scissor lift with battery (b) with battery

One of the most important factors of this system is its structural integrity. All of the structural

members within the lifter must be able to easily withstand the loading of the battery, tray and main

platforms. Since the system would require multiple changes of batteries per day, all of the systems

had to be able to handle repeated loading.

This lifter system had particular design challenges to face compared to typical uses. The platform had

to be relatively small compared to the height of the lifter in order to allow the tray to be placed onto

the conveyer and be driven off once the lifter was detracted. In order to hold the battery tray in

position, support shafts were added to the top of the platform to insert into bores underneath the

tray.

Through the design process it was decided to use a two tier scissor lift. Although a single tiered scissor

lift was desired due to reduced complexity it was found this was not feasible due to the required length

of each lifter arm in comparison to the platform for the desired height. It was also found using multiple

tiers helped reduce the compression height and clearance between platform and conveyers.

The main analysis performed was the ability for the lifter arms to cope under pressure. They had to

hold the weight of the whole system, including themselves for the lower tier arms. The lifter was

constructed out of steel frames as shownFigure 6.6.Each had a tapered end to allow them to rotate

easily, especially when connected to the platform and bottom frame supports.

(a) (b)


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Figure 6.6 - Diagram of lifter support frame

The frame columns were examined by looking at their buckling stresses under loading. As detailed in

(Toll, 2013), the critical stress for buckling is found using Equation 6.4.

= (6.4)Where is critical stress,is the elastic modulus, second moment of area,cross sectionalarea and length. Due to the changing cross sections at their ends, they were modelled as having aconstant rectangular cross section of 100 x 200mm. It was found having each frame of length 1.25m

allowed the lifter to meet its height requirements (reaching a height of 1.8m and lowering to a height

of 0.8m to reach the vehicle and lie below the conveyer set at 1m) and was able to withstand the

loading weight to a factor of safety of 1.5.

Using hydraulics proved to be faster than using electric motors for the lifting, as well as being a more

cost efficient solution. The piston, shown in Figure 6.7, was designed to withstand the required

pressures, support the weight of the entire system and provide the driving force to enable the lifting

motion.

Figure 6.7 - Hydraulic piston


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The main lifting forces were calculated by analysing the pressures on the different parts of the inner

shaft using the correct formulae (Engineering Toolbox, n.d.).

= () (6.5)

= () (6.6) and are the pistons pushing and pulling forces respectively, and the minimum andmaximum cross sectional diameters of the inner piston respectively and P the internal pressure of the

outer piston. For delivering the required actuator forces it was found 77bar and 43bar pressures would

be needed for pushing (lifting) and pulling (retracting) respectively. Looking at the required pressures,

the yielding stresses of the piston were calculated using the Tresca criterion, described as

= (6.7)Where is the yield stress of the cylinder, the maximum hoop stress and the maximum radialstress (Coates, 2014). It was found constructing the outer piston from steel with a thickness of 75mm

would allow it to withstand the necessary operating pressures to a factor of safety of 1.5.

6.3.4 Changes to Battery PacksThroughout the design process the aim was to reduce the changes that would need to be made to

the battery packs themselves. However it was found that some design changes have to be made in

order to make the quick battery exchange feasible.Figure 6.8 shows an example of a suitable battery

pack.

Figure 6.8 - Battery pack with suggested changes

The connections to the battery have to be made from the top of the battery to allow the electrical

connections to be made during the process of lifting the battery up. This would also require a circuit

breaker embedded at this connection to allow disconnection in an emergency situation. Further

electric contacts need to be embedded at the base in order to make the connection with the tray and

the charging circuitry.


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For compatibility with the hook attachment mechanism, a supporting frame has to the exterior of the

battery pack. This is to allow the hooks to hold the battery pack without having to enter the inner

volume of the pack and thus reduce the capacity of the pack for battery modules and thus itscharging

capacity.

6.4 Design for Manufacture and SustainabilityThe main feature to consider for manufacture and sustainability are the supporting trays. Whilst mostof the components of the battery handling system can easily be sourced from suppliers and the battery

attachment mechanism is the responsibility of the car manufacturers, the battery support trays will

have to be custom manufactured.

Due to temperature and stress requirements it is difficult to recycle the plastic used or construct them

from recycled plastic. In order to reduce waste from the station and reduce manufacturing costs it

was decided to construct the trays from larger panels, allowing the circuitry to be placed and accessed

for repair. This reduces the plastic waste from the station and extends the working life of the trays.

6.5 DiscussionA lot of the systems used in the battery attachment mechanisms have been over engineered to acertain degree. The main aim of the design process was to ensure that the structures were sound and

would not fail critically or wear easily during their use. However there are some parts of the design

that could be further optimised to reduce unnecessary weight including the battery trays and the lifter

arms themselves. More careful and accurate analysis of these systems would further optimise the

design and reduce the manufacturing costs for the station.


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7 Vehicle Conveyor SystemThe aim of the system is to accurately position the EVs above the battery exchange equipment to allow

the exchange to take place. This will be done using a conveyor system which will stop when the EV is

over the equipment and then move the car forward once the exchange has taken place.

7.1 User RequirementsAs this is the main part of the system that the driver is involved with, safety is paramount. The simplest

solution is to automate as much as possible, and ensure the driver remains in the vehicle. It was

therefore decided that the system must:

Hold the vehicles in position over the battery change equipment. Be easy for the driver to use. Not damage the vehicle. Be able to accurately position all electric cars. Release the vehicle after the battery is changed. Not be susceptible to human error. Be able to release the vehicle and user in an emergency.

Since the vehicle must be positioned correctly, to within 10 mm above the lifter, it was necessary to

design a precision positioning system. It would be unreasonable to assume that all drivers can park

this accurately. Current positioning systems include vehicle assembly lines and simple car wash

systems. The decision was that a car wash system would be a better solution as road users are already

used to the process and it is possible to drive straight on to these systems.

7.2 Design ConceptsWhen looking at how to move the EVs along the bay, the first solution considered was push conveyor.

This is the most common type of conveyor system found in car washes and can either be chain and

roller or belt systems, shown inFigure 7.1 (Tommy Car Wash Systems, 2014)

Figure 7.1 - (a) Chain roller conveyor (b) Push belt conveyor

Although these are easy to source and familiar to road users, a push conveyor would not be accurate

enough to align the vehicle. Therefore the decision was to add another roller in front of the tyre to

hold it position as shown inFigure 7.2.Although this is a far more accurate, it would be difficult for

the vehicle to drive out of the system in an emergency.

(a) (b)


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Figure 7.2 - Two roller system

7.3 Detail Design7.3.1 BeltIn order to solve this, a solid plate belt was designed. The final design was to use a solid plate belt

design with gaps of length 250mm for the vehicle tyres to sit in. This would allow the accuracy required

as the tyre will sit in the centre of the gap, each of which is about 5m apart to ensure that only the

front wheels of the EV are captured. Once the vehicle is in the gap, the driver should apply the

handbrake. This will hold the vehicle in place until it has been moved to the end of the conveyor, at

which point the driver will be signalled to remove the handbrake and drive away. Due to the low

profile of the gap it will be easy to drive out of, whilst still providing definite feedback to the driver

when they enter it. The system is shown inFigure 7.3

Figure 7.3 - Belt conveyor

Figure 7.4 shows how far the wheels drop below the belt, but it has been assumed that the tyres will

not deform, so the actual values will be greater than this and depend also on tyre pressure.

Additionally, it can be seen that a compromise needs to be made on the distance of the gap, so that

large wheels are firmly held, whilst small wheeled cars are able to exit the gap at the end of the

conveyor. This therefore requires the battery exchange equipment to handle small differences in

heights of the vehicles. The width of the conveyor plates is 400mm as this allows space for even the

largest production tyres. This will ensure that no tyres will be damaged by the conveyor system.


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Figure 7.4 - Wheels resting in gap

In order to accommodate vehicles with different wheelbases, the conveyors can be moved laterally.

The movement will be symmetric to ensure the battery will always be located above the lifting

mechanism. The movement will be controlled by a worm screw powered by an electric motor, as seen

inFigure 7.5.The wheelbase and tyre width information will be retrieved from the vehicles RFID tag

as it enters the station.

Figure 7.5 - Lateral movement

7.3.2 Safety RailsTo stop cars driving off the conveyor, a safety rail was designed. This was initially simply a solid barrier

simply bolted to the sides of the conveyor. However to accommodate different tyre widths 9the rails

need to move laterally. Powered screws at each attachment point control the movement, which is

symmetric due to opposing threads, which keeps the wheel in the centre of the conveyor. The angled

rails at the end help guide the vehicle onto the conveyor. Beneath the angled r

Documents

Final Project report electric car change station