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Abstract - In electric vehicles, battery management is of utmost importance. Since the batteries are the most expensive and limiting factor that prevents the massive deployment of EV’s, special care must be taken to ensure that the batteries pack supply the maximum possible energy, guaranteeing also reliability and longevity of this critical element. In this article it is presented a modular battery bank intended to be used in electric vehicles, that can be easily connected to the data and control network of the vehicle in order to share all battery relevant information, namely state of charge and critical values, like low or excessive temperature or voltage. Equalization of individual cells is mandatory with Lithium chemistry and in our module this is done by dissipative method, which is of simple implementation, robust and with acceptable energy loss. With this module energy banks of the desired nominal voltage and capacity (connecting modules in series, parallel or series/parallel) can be built. Applications could range from electric bicycles to full-sized electric cars. The batteries in the bank are connected through a CAN bus network, where the communications and equalization algorithms are implemented in a simple 8 bit microcontroller.
I. INTRODUCTION
Electric vehicles (EV), hybrid or battery only, are
considered key elements to sustainable mobility [1]. In fact,
today’s mobility is based on vehicles with Internal Combustion
Engine (ICE), that consume mainly a scarce source – petrol -
with rising costs, supplied from politically and social instable
world countries. ICE vehicles contribute heavily to city
pollution (CO2, particles,…) and the emission of pollutants is a
major contributor to the global warming effect and climate
changes occurring nowadays.
Battery Electric Vehicles (BEV) have, locally, zero
emission of pollutants, and if electric energy is supplied by
renewable sources – wind, photovoltaic or hydroelectric – they
can be truly zero emission globally. Other advantages of
electric vehicles are, among others, simplicity of motor
construction and drive train, higher reliability and no noise
emission. The main limitation of BEV is range. This restriction
is directly related to the low value of specific energy of the
batteries. On the other hand the ICE engine has a much lower
efficiency (somewhere between 20% to 30%) when compared
to an electric motor (bigger than 90%) [2].
Since the batteries are the most expensive and limiting
factor that prevents the massive deployment of BEV, special
care must be taken to ensure that the batteries pack supply the
maximum possible energy guaranteeing reliability and
longevity of this critical element.
The battery bank is constituted by series of cells (grouped
in modules) that must be continuously monitored – voltage,
temperature and current – when supplying current to the
electric motor, recovering energy from regenerative braking,
or in the charging process, to ensure that they work within
bounds in order to assure their longevity. The electronic
system responsible for this task, built around a low cost 8 bit
microcontroller, is presented is this article. It monitors a 4
cells module (equivalent to a 12V battery) and shares all the
relevant information with a Master node, using CAN network
[3].
The microcontroller Single Board Computer, network and
protocol used in the battery bank is also described, including
the global Batteries Management System (BMS). First
prototypes implementation and tests are presented and the
article finishes with a conclusion part.
II. BATTERY BANK AND 12V MODULES
A. Cells and Batteries
The cells and batteries used in electric vehicles can be of
diverse chemistry, being the most common the ones showed in
Fig. 1 and Table 1.
Fig. 1. Typical energy densities of different battery types [4]
Pb (Lead Acid/Gel) batteries suffer from low energy
density, high recharge time (greater than 8 to 10 hours for full
charge), but they are tolerant to overcharge voltage. The
Peukert effect is noticeable in these kind of batteries, lowering
substantially the available energy when high currents are
needed (which is true for electric vehicles). NiMH despite the
good cycle life stated by manufactures (Table I), that value is
usually for low discharge rates. The self discharge of 20-30%
per month is also an issue [5].
Lithium batteries have high energy density, long cycle life,
can supply large currents without loosing capacity (virtually no
Peukert effect). As a matter of fact, the current trend in the
automobile industry clearly points to the utilization of Lithium
batteries. On the other hand these are less tolerant to extreme
Lithium Modular Battery Bank for Electric Vehicles
Luís Marques1, Verónica Vasconcelos
1, Paulo G. Pereirinha
1,2,3, João P. Trovão
1,2
1IPC/ISEC, Polytechnic Institute of Coimbra, R. Pedro Nunes, P-3030-199 Coimbra, Portugal
2Institute for Systems and Computers Engineering at Coimbra (INESC-Coimbra), Portugal
3APVE, Portuguese Electric Vehicle Association.
lmarques, veronica, ppereiri, [email protected]
conditions, namely over or undervoltage limits. To cope with
this, electronic system must be used to exercise tight control
on voltage and cell temperature.
TABLE I
Battery Types Available for Electric Vehicles [6]
Property (Unit) Lead Acid NiMH Lithium
Cell Voltage (V) 2 1,2 3,2-3,6
Energy Density (Wh/Kg) 30-40 50-80 100-200
Power Density (W/Kg) 100-200 100-500 500-8000
Maximum Discharge Rate 6-10C 15C 100C
Useful Capacity (DOD %) 50 50-80 >80
Charging Efficiency (%) 60-80 70-90 ~100
Self Discharge (%/Month) 3-4 30 2-3
Cycle Life (Number of
Cycles)
600-900 >1000 >2000
Robust (Over/Under Voltage) Yes Yes Needs BMS
Technology Maturity Old Mature New
Price Low Medium Medium to
High
From Fig. 1, Table 1 and subsequent comments it should
be evident that Lithium battery is nowadays the available
technology that best fits the needs of electric vehicles. Among
them, one of the variants of Lithium cells, the LiFePO4
(Lithium Iron Phosphate), has a good compromise between
electrical characteristics and price [7]. Example of LiFePO4
cylindrical cells (from Headway) and discharge graph are
showed in Fig. 2 and Fig. 3.
Fig. 2. Cylindrical LiFePo4 Cells from Headway
Fig. 3. Discharge Profile of LiFePO4 cell
These Lithium cells have nominal voltage of 3.2V,
maximum charge voltage of 3.65V and cut off discharge
voltage of 2.0V. They can be charged to 5C and the maximum
discharge current is 10C. They present very good cycle life of
2000 cycles with discharge rate of 1C and 80% depth of
discharge (DOD). The energy density is 105Wh/kg and power
density 850 W/kg.
B. 12V Module and Battery bank
The battery bank can be composed of any number of 12V
modules - 4 Lithium cells in series. The 12V Module is
showed in Fig. 4.
Fig. 4. LiFePO4 12V Module Battery
The 12V module is composed of four LiFePO4 cells
connected in series (with 12,8 V nominal voltage), a thermistor
attached to each cell to sense individual temperature and a
board with an 8-bit microcontroller - the Single Board
Computer, SBC – internally named SBC2680, view figure 4 –
plus an equalization board. This SBC was designed and built
by the author’s and is used in other projects related to electric
mobility, also running in our institution [8]. For the first
version of equalization system a dissipative type was used.
The global battery bank schematic is depicted in figure 5.
Fig. 5. Modular Battery Bank (with N 12V modules)
The battery bank network architecture follows the
distributed paradigm, with all 12V modules connected using a
digital shared serial network, namely the Controller Area
Network (CAN), which is the most popular networking
technology used in the automotive domain, today. This
popularity is due to several features, namely:
• high efficiency with short data transfers;
. . .
CAN
SBC2680 Equal.
Board
CAN Bus (Vehicle data and control network)
CAN Bus (battery bank)
CAN
SBC2680 Equal.
Board
12V Module - 1 12V Module - N
CAN
SBC2680
CAN
Master
node
CS
Modular Battery Bank
+
CAN
SBC2680 Equal.
Board
-
• medium to high information data rate (up to 1Mbit/s);
• robust error detection and automatic retransmission of
corrupted messages;
• distinction between transient and permanent errors;
• asynchronous medium access with priority arbitration;
• low cost.
The CAN protocol is, however, rather simple, which is an
advantage but also a limitation. Therefore, there are several
higher layer protocols developed to work on top of CAN and
provide extra features as needed. One such protocol, which
was selected for use in our bank, is FTT-CAN (which stands
for Flexible Time Triggered communication over CAN [9].
III. SINGLE BOARD COMPUTER AND BATTERIES
MANAGEMENT SYSTEM
The Single Board Computer - SBC2680 - is based in a
Microchip PIC18F2680 microcontroller [10]. This
microcontroller has very appealing characteristics to be used in
the 12V Battery Module, namely:
• clock speed of 40 MHz;
• internal memory of 64 kbytes;
• 24 digital I/O, that can be configured as input or
outputs;
• 10 bit ADC, with 10 multiplexed inputs;
• integrated CAN controller;
• serial UART;
• SPI communication;
• low cost.
The SBC2680 construction is modular, allowing easy
replication and adaptation to all subsystems, resulting in a
homogeneous network with simplified deployment and
management.
Fig. 6. SBC2680 module, based on Microchip 18F2680 microcontroller
In Lithium batteries the voltage of individual cells must be
monitored closely in order to not allow a voltage bigger than
maximum or lower then a minimum. If this is not assured,
irreversible damage will occur in the cell(s), rendering the
bank unusable, leading to full stop of vehicle and to big
maintenance costs.
Battery cells are never identical. Even batteries produced
in the same batch present always small differences in self-
discharge rate, capacity and impedance. Therefore,
individual cells in a battery pack will show different
voltage levels after a full charge.
The management system must guarantee that individual
cells never pass a maximum value and do not go lower than
minimum voltage. In the charging process, if any cell reaches
the maximum value, the process must be stopped and one or
more cells in the pack are not fully charged. The obvious
consequence is that the battery pack full capacity is not
available for discharge (see Fig. 7). In the discharging process,
the available capacity cannot be fully used because the output
of battery must be cut if any of the cells reaches minimum
capacity (first cell in Fig. 7.b). Even worse, with charge-
discharge cycles cell capacities start to drift, leading to a
decrease in the battery available capacity [11].
a) b)
Fig. 7. Example of charge level imbalance in the four cells of the battery
pack: a) during charge; b) during discharge
To cope with the above stated problem the battery pack
needs to be balanced. So, to avoid damaging the cells due
to imbalance in the battery pack, cell equalization is used
to reduce the difference in voltage between the cells. This
gives cells longer lifetime and more available capacity [11]
[12].
The algorithm used in the charge process and to do cell
equalization was the following:
Charge and Equalization algorithm for 12V Module
Do 20 times per second, for each cell (i=0 to 3)
1. Measure cell voltage
2. if voltage(Cell[i]) > THRESH_HIGH then bypass
Cell[i] (turn ON parallel MOSFET[i])
3. if voltage (Cell[i]) > THRESH_OVER then send
ALARM message, in order to cutt-off charger.
Do each second
1. Measure all individual cell temperature
2. if temperature(Cell[i]) > THRESHOLD_TEMP
then send ALARM message (in order to cutt-off
charger).
The action 3 in the algorithm is performed by the Master
battery node and when done it sends a message to all 12V
modules in order to disconnect all the MOSFETS and bypass
resistors.
In the discharge process the SBC2680 is responsible for
monitoring individual cells voltage and to cut-off output of
module (by sending Alarm message to the Master) if any cell
voltage goes lower than a preset minimum value.
The module will also send a CAN alarm message to the
MASTER to inform if over temperature situation occurs.
IV. IMPLEMENTATION AND TESTS
In order to test the battery module, the algorithms to
determine the SOC and to equalize individual cells, batteries
from A123 were used, depicted in Fig. 3. These cells are a
scale down of the intended cells allowing carrying out the first
tests with the proposed system.
Fig. 8. Lithium cells used in testing of concept
To get the charge profile of cells several charge/discharge
cycles were done, being the values monitored with a PC
connected to the microcontroller trough the serial port. The
charge algorithm used constant current/constant voltage, as
recommended in the cells manual. Discharge was done using
resistive loads. In Fig. 4 the profile of charge and discharge of
cell number 1 of the 12V LiFePO4 module is shown.
Fig. 9. Charge profile of tested cell nº1 (1C)
Fig. 10. Discharge profile of tested cell nº1 (1C rate)
From the experimental results it can be seen that the
capacity of cell is 1000 mAh, as stated in the cell manual.
The system used to test the charge and discharge and
equalization algorithms is shown in Fig. 11.
The next step to test the module is to implement in the
microcontroller the algorithm to control the module charge and
the discharge limits. The implementation was done using C
language.
The Master SBC is also responsible for the State of Charge
(SOC) estimation. In the first prototype of the battery bank, the
SOC estimation was done using one of the various techniques
used in the literature, as for instance in [12]. We used
Coulomb counting plus heuristics with satisfactory results.
Fig. 11. Experimental setup
V. CONCLUSIONS AND FUTURE WORK
In this article it was presented a modular battery bank
intended to be used in electric vehicles, that connects the
various 12V modules through a CAN bus. The 12V modules
are continuously monitored by SBCs, being these also
responsible for the equalization process of individual cells.
Using the Master Node and the CAN network, the battery bank
shares the relevant information with the vehicle data and
control network. Several charge/discharge cycles were done in
order to test the proposed system. Future work involves the
design and implementation of more accurate SOC estimation
algorithms. On the other hand, techniques to minimize losses
in equalization process [13][14][15] will be evaluated in terms
of price, effectiveness, energy conservation and complexity.
VI. REFERENCES
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Fuel Cell Electric vehicles key to sustainable mobility”, [Online].
Available: www.avere.org
[2] M. Ehsani, Y. Gao, and E. Amadi, Modern Electric, Hybrid Electric,
and Fuel Cell Vehicles: Fundamentals, Theory, and Design”, 2nd
Edition, Boca Raton, CRC Press, August 2009.
[3] Controller Area Network (CAN) specification – version 2.0., Bosch
GmbH, 1991.
[4] Maxim Integrated Products, Inc, (2011, April 20). Electronic
Publication: Application Note 3958. [Online]. Available:
www.maxim-ic.com
[5] J. Aditya and M. Ferdowsi, “Comparison of NiMh and Li-ion Batteries
in Automotive Applications”, IEEE Conference on Vehicle Power and
Propulsion Conference, 2008.
2
2,5
3
3,5
4
0 600 1200 1800 2400 3000 3600 4200
Cell Voltage
1C
Time (s)
2
2,5
3
3,5
4
0 600 1200 1800 2400 3000 3600 4200
Cell Voltage (V)
1C
Time (s)
[6] Axeon. Inc, (2011, April 22). Electronic Publication: “Our Guide
Batteries”. [Online]. Available: www.axeon.com
[7] W. Jiayuan, S.Zechang and W. Xuezhe, “Performance and characteristic
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Vehicle Power and Propulsion Conference, September 2009.
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M. Silva and P. Tavares, “The Electric Vehicle VEIL Project: A Modular
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[9] L. Almeida, P. Pedreiras, J.A. Fonseca, “The FTT-CAN protocol: Why
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[10] Microchip Technology Inc, Electronic Publication:
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[11] J. Xu et al, “Estimation of SOC for Lithium-ion Battery Pack in Electric
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