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TI Information – Selective Disclosure
Cell Balancing Methods
1
Battery Management Deep Dive
BMS Systems & Applications
Nov 7-9, 2011 Dallas, TX
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
Agenda
• The Problem-Cell Mismatches
• Cell Balancing & Implementation
• Cell Balancing Methods
• Passive Balancing
• Cell Balancing: What is Really Needed
• Active Cell Balancing – PowerPump™ – Isolated Bi-Directional DC-DC
2
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
Root Problem
The Problem – Cell Mismatches
3
Battery cell mismatch
State-of-Charge imbalances
Batteries age unevenly
Accurate SOC calculation and cell balancing is extremely important
SOC State of Charge
Cell Chemistry
Cell Age
Cell Temperature
Cell Voltage
Cell ESR Equivalent Series
Resistance Cell
History
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive 4
Cell Balancing What Causes Imbalance? • Capacity variation (1~2% cell-
to-cell for same model) • Charging state difference (i.e.
SOC difference) • Impedance variation (up to 15%)
causes voltage difference when charging/discharging
• Localized heat degrades cells faster than others, particular self-heating at high discharge rate
Why Balance? • Unbalanced cells lead to:
– Reduced run-time due to... • Premature charge termination • Early discharge termination
– Further cell abuse from cycling above or below optimal cell voltage limits
Time 2 4 6 8 10
4.4
4.2
4.0
3.8
3.6
3.4
3.2
3.0
Overcharge
Capacity Under-used
Undercharge
High Cell Count battery systems are more likely to see imbalance due to temperature gradients and cell self-
heating at high discharge rates.
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
Cell Balancing Implementation • Control Strategy – a Layered Approach
– Balance Cell Terminal Voltage • Easiest to understand – provides the basis for more complex control
– Balance Cell OCV estimates • Based on Pack current and Cell Impedance measurements • Compensates for impedance differences
– Balance for SOC at 100% • Based on how far each cell is from Full Charge Capacity • Compensates for capacity divergence
• Direct Measurements – Cell Terminal Voltage, Pack Current (using synchronous measurements)
• Derived Variables – Cell Impedance, OCV, SOC, Qmax
• Control Action – Move energy where and when its needed to minimize global imbalance
• Passive or Active Balancing
5
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
Battery Cell Balancing Methods
Dissipative (Passive) • ‘Balance’ current is dissipated as heat (wasted) • Best for very low charge/discharge currents • Cannot reclaim capacity lost from mismatch • Least Expensive
Charge Shuffling (PowerPump™) • Balance current is shuffled between adjacent cells • Balancing of currents of10mA to 1A and above • Balancing during Charge and Discharge • Inductive and Capacitive options
Isolated DC-DC Balancing • Current is moved between individual cells and module • Can charge and discharge any cell with high efficiency • Fully scalable to very high balance currents
6
TI Information – Selective Disclosure
Passive Balancing
7
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
Passive Balancing
8
3.3 3.4 3.5 3.6 3.7 3.8 3.9
4 4.1 4.2 4.3
Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 Cell 6 Cell 7 Cell 8 Cell 9 Cell 10
Volta
ge (V
)
Voltage Min Max
3.3 3.4 3.5 3.6 3.7 3.8 3.9
4 4.1 4.2 4.3
Cell 1 Cell 2 Cell 3 Cell 4 Cell 5 Cell 6 Cell 7 Cell 8 Cell 9 Cell 10
Volta
ge (V
)
Voltage Min Max
• Extra charge current is dissipated through a resistor
• Best for low charge/discharge currents • Low Cost Solution
Weakest Cell in the Pack
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
Cell Balancing: What’s Really Needed • Approach
– Higher balance current • balance within single cycle
– e.g. 20% imbalance in 1 cycle
– Balancing as needed during charge /discharge and /or idle
– Move energy when and where it is needed
– rather than bleed it off
• Benefits – Avoid cell abuse which leads to
further imbalance • Reduce time above 4.20 V
– Maximum runtime • Every cycle
– Enhanced safety • Stay within specified voltage
envelope
9
TI Information – Selective Disclosure
Active Cell Balancing
10
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
Active Balancing Extends Pack Life
Notes: 66 Ah 3.6V Cells, 1P96S 95 Good Cells: 3000 cycles to 20% capacity loss 1 Weak Cell: 3000 cycles to 25% capacity loss
Charge/Discharge Rate: 0.5-0.8C @ start of life Linear capacity degradation assumed
Nearly 20% more cycles!
ACB)
11
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
Active Cell Balancing Keeps Cells in Balance – For Safety & Performance
PowerLAN Multi-Cell Monitoring and Balancing Li-ion cells - 7S1P pack - Discharge/Charge Cycle
PowerPump Balancing Enabled after 1st Cycle - Runtime Extended to Maximum
2.75
3.00
3.25
3.50
3.75
4.00
4.25
0 50 100 150 200 250 300
Time [minutes]
Cel
l Vol
tage
[V]
Cell 1 VoltageCell 2 VoltageCell 3 VoltageCell 4 VoltageCell 5 VoltageCell 6 VoltageCell 7 VoltageOVL/UVL StatusOvervolt/Undervolt
Balancing Enabled
Cell balance restored within 40 minutes
Cells now reachundervolt limit together
All cells achieve same SOC levelwhen charging phase is terminated
Balancing EnabledAs supplied, cells showed variance in capacity and impedance - resultant multi-cell pack unusable without active balancing
(Balancing terminated by UVL )
12
TI Information – Selective Disclosure
PowerPump™
13
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
What is Active Cell Balancing? • Within a battery pack, ACB transfers charge from
one set of cells to another
• Allows for true balancing of cells with very little wasted energy
• TI's patented PowerPump™/ACB technology uses inductive strategy for high efficiency and wide balancing current range
• TI can provide solutions for balancing currents anywhere from 10mA to 1A and above, depending on application
• Possible applications: – Extended life packs – Extended runtime packs – Dissimilar Cell Capacity Packs
14
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
PowerPump™ Schematic for 3S pack
• Factorize the problem…
• Bidirectional PowerPump™ transfers energy efficiently between adjacent cells
• “Bucket brigade” allows redistribution anywhere in pack
• Move energy where and when its needed to minimize global imbalance
• PowerPump™ burst mode is time limited for each control loop iteration
• Operation of individual PowerPump™…
V3
V2
V1
15
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
PowerPump™ Schematic Operation
• Example : Pumping from Cell 3 Cell 2 – P3S frequency is 200 kHz, 33% positive Duty Cycle – P3S Turns PFET ON – DI/DT = V/L : Energy in Inductor builds
16
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
PowerPump™ Schematic Operation
• Example : Pumping from Cell 3 Cell 2 – P3S Turns FET Off – Current continues through NFET (body diode) – Energy transfers to Cell 2 – Time average Balancing current is 40 to 50 mA
17
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
Supported Balancing Modes
Rgs
Chf
Rgs
Chf
Chf
cell3
cell2
cell1
Q1
Q2
Q3
Q4
Q1
Q1
Q3
Rgs
Rgs
L
L
Rgs
Chf
Rgs
Chf
Chf
cell3
cell2
cell1
Q1
Q2
Q3
Q4
Q1
Q1
Q3
Rgs
Rgs
L
L
Rgs
Chf
Rgs
Chf
Chf
cell3
cell2
cell1
Q1
Q2
Q3
Q4
Q1
Q1
Q3
Rgs
Rgs
L
L
Rgs
Chf
Rgs
Chf
Chf
cell3
cell2
cell1
Q1
Q2
Q3
Q4
Q1
Q1
Q3
Rgs
Rgs
L
L
1 & 3 to 2 1 & 2 to 3 2 & 3 to 1 2 to 1 & 3
18
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
Design Considerations for PowerPump™ External Circuits -Design
• Size the MOSFET correctly (avoid basing the MOSFET current handling on the average balancing current consider the peak currents)
– Use very low resistance MOSFET (reduces heat dissipation)
• Inductor Saturation (Inductor saturation is based on peak currents)
• Traces width sizing – Peak currents cause large voltage drops and spikes that causes damage like high
voltage transients
• Size wires from the cells robustly
• Lower ESR in the output caps (High voltage ripple can be very spiky) – 2 - 10uf Caps in parallel are better than a single 20uF Cap
• Avoid long traces between MOSFETs and the Inductor
• Schottky forward voltage drop should be lower than the MOSFET body diode turn-on
– Voltage drop on the Schottky determines the max delta between cells
19
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
Design Considerations for PowerPump™ External Circuits -Layout • Peak inductor currents can be as high as 1 to 2 amperes or higher
depending on inductor value – Use Inductors with ratings 2x of peak to accommodate these high currents – Overhead for Temp as tem goes up, saturation currents goes down
• PowerPump™ components are located as close as possible to each other and connected with short, wide traces
• Minimize the impedance of the return path for the high-frequency signals.
20
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
Active Cell Balancing Pros/Cons PROS • Reusing available energy
• Extended runtime (on-the-go use)
• Extended lifetime (warranty)
• Significant degree of freedom in system design
• Solution size/cost can be quite small for the right app
CONS • Added cost and size of BOM
(inductors, schottkys, FETs)
• Exotic new gauging algorithm development for IT (BattSense)
• New voltage- or capacity-based balancing algorithms
• Risks inherent in brand new hardware
21
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive 22
Isolated Bi-Directional DC-DC Active Cell Balancing
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive 23
Isolated Bi-directional DC-DC Balancing
• Current is moved between individual cells and module
• Can charge and discharge any cell efficiently
• High performance regardless of chemistry
• Scalable to very high balance currents
- + - + - + - + - + - + - +
Gate Controller
Bi-directionalDC-DC
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive 24
Bi-Directional DC-DC Cell Balance Block Diagram • Architecture is based on
grouping of up to 7 cells
• Charge = Cell → Module
• Discharge = Module → Cell
• The chipset combination of EMB1428 and EMB1499 is controlled by a single command via SPI
-+
-+
-+
-+
-+
-+
-+
Charge Flow
Switch Matrix
Cell Balancing Engine
CellModuleModuleCell
Isolated Bi-Directional
DC-DC
...
EMB1428 Gate Controller
EMB1499 PWM Controller
Switch Matrix
Isolated Bi-Directional
DC-DC
EMB1428 Gate Controller
EMB1499 PWM Controller
1 of 7
1 of 7
SPI4
SPI4
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive 25
Switch Matrix
• EMB1428 Gate Controller
• 12 channel floating NFET driver that is designed specifically for BMS systems
• Automatically make multiple switch selections based on an input of a simple cell selection command (SPI)
• Goes to sleep mode when STOP command received (
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive 26
Bi-Directional DC-DC Controller
• The EMB1499 is specially designed to control the active clamp forward topology
• Has the ability to control the charge current in both directions (cell charge or discharge)
• Current can be adjusted via VSET voltage level
To Module(40 – 70 V)
To Cell via Switch Matrix
(2.x – 4.x V)
Controller (EMB1499)
1234
Q2Q1Q3Q4
Vsen
Isen1
Isen1Vsen
EMB1412
Driver
GNDFGND
Delay
Isen2
Isen2
12V12VF
Bias Supplies
ENDIR
VSET
From EMB1428
Gate Decouple
+Cell
-Cell
+Stack
-Stack
From microcontroller
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive 27
Efficiency
• More direct cell balancing… Much better than other architectures that require multiple transfers
• DC-DC Typical Efficiency: – Charge (Module → Cell) transfer efficiency ≈ 88% – Discharge (Cell → Module) transfer efficiency ≈ 86% – Between any 2 Cells in module, transfer efficiency ≈ 76%
• Example, cell 9 high, cell 1 low
-+
-+
-+
-+
-+
-+
-+
Switch Matrix
Isolated Bi-Directional
DC-DC
...
Switch Matrix
Isolated Bi-Directional
DC-DC
– Bidirectional DC-DC • 2 transfers,
∴eff≈ 76%
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive 28
Control Interface
• Simple Control – Single SPI start or stop command controls the cell selection and DC-DC
control – EMB1428 has built-in detection of illegal commands – EMB1499 has built-in fault detection mechanisms:
• Over-voltage, under-voltage, over-current and temperature threshold detection • Time-out detection • Controlled current ramp-down on fault • Fault codes reported back to microcontroller
– Very low microcontroller requirements
SPI
Gate Controller
ENDIR
DIR_RTDONE
FAIL
DC-DC Controller
3
4Microcontroller
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive 29
Pros and Cons
• Pros – Scalable from 6 – 14 cells – Adjustable current level – High current for large capacity cells – Fast time to balance – Near real-time compensation for cell mismatch – Higher worst-case efficiency between any cells in module
• Cons – Higher cost per channel – High component count – Large PCB area
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
Cell Balancing Methodology Comparison
30
Bleed Balancing PowerPump™ Isolated Bi-
directional DCDC
Cell Count >2 >3 >7
Cost of Implementation Low Mid High
Balancing Current Low Scalable Scalable
Time to Balance Slow Fast Fast
Efficiency Low Mid High
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive
• BMS Demo Box and GUI for FAE’s to demonstrate bi-directional dc-dc active cell balancing
ACB BMS (Battery Management System) Demo Box
31
TI Information – Selective Disclosure 2011 Dallas BMS Deep Dive 32
Thank you!
Cell Balancing MethodsAgendaThe Problem – Cell MismatchesCell BalancingCell Balancing ImplementationBattery Cell Balancing MethodsPassive BalancingPassive BalancingCell Balancing: What’s Really NeededActive Cell BalancingActive Balancing Extends Pack Life�Active Cell Balancing Keeps Cells in Balance – �For Safety & Performance�PowerPump™What is Active Cell Balancing?PowerPump™ Schematic for 3S packPowerPump™ Schematic OperationPowerPump™ Schematic OperationSupported Balancing Modes Design Considerations for PowerPump™ External Circuits -DesignDesign Considerations for PowerPump™ External Circuits -LayoutActive Cell Balancing Pros/ConsSlide Number 22Isolated Bi-directional DC-DC BalancingBi-Directional DC-DC Cell Balance �Block DiagramSwitch MatrixBi-Directional DC-DC ControllerEfficiencyControl InterfacePros and ConsCell Balancing Methodology ComparisonACB BMS (Battery Management System) Demo BoxThank you!