Delivering high performance at
reduced cost by applying a
tailored approach to BMS
strategy
(Battery pack optimization)
January 10, 2013
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© MIRA Ltd 2012
© MIRA Ltd 2012
AGENDA
Introduction
BMS functions
Approach
Cell characterisation tests
Using the Cell characterisation data
Simulation results
Conclusion
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Introduction
Battery management system (BMS) can be:
- A simple device for monitoring the critical system parameters, and
controlling the battery pack contactors. OR
- More sophisticated providing real-time information on the current
status of the battery pack.
Maximum battery pack performance, can only be achieved if you know
the safe operating limits of the cells.
Automotive applications demand dynamic feedback on battery status
in real time.
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Typical BMS functions
The basic functionalities of a BMS are:
- Monitor cell and or pack voltage
- Monitor cell / battery pack temperature
- Cell balancing ( keeps cell voltages equal during charging)
- Thermal management - Maintain battery pack temperature within the safe
operating limits
- Maintain battery pack voltage within safe operating limits
- Protect against over discharge / charge
- To estimate the state of charge (SOC) and state of health (SOH) of the
battery
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Approach
Establish safe operating limits for the cell to prevent unintentional abuse and
damage too the battery pack, due to:
- Overcharging
- Deep discharging
The BMS calculates the instantaneous available power in the battery and
communicates this to the vehicle controller thereby reducing the opportunity
for unintentional abuse
Factors that have an influence on the available power from the pack are
-
Cell/pack voltage
Cell/pack temperature
SOC
SOH
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Cell characterisation tests
Safe operating limits for the cell are established by performing cell
characterisation under laboratory conditions.
Cell performance changes with temperature, therefore cell characterisation is
conducted at different temperatures.
To establish the performance of a cell and it safe operating range the following
tests must be performed:
- Discharge capacity test
- Charge capacity test
- Open circuit voltage test (Voc)
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Cell characterisation tests - Cont
Discharge capacity test
- Constant current discharging (varying C-Rate) at a constant cell
temperature
o This test provides the cell discharge voltage curve vs. capacity (Ah)
- See 3 Examples charts below
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Cell characterisation tests - 0.5C Discharge
0.5C Discharge at Different Temperatures
4.2
10⁰C = 38.88Ah
23⁰C = 42.39Ah
30⁰C = 43.35Ah
40⁰C = 43.69Ah
Supplier 23⁰C = 39.95Ah
4.1
4
3.9
Voltage (V)
3.8
3.7
10degC
23degC
3.6
30degC
3.5
40degC
3.4
Supplier Data at 23degC
3.3
3.2
3.1
3
0
5
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15
20
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Capacity (Ah)
30
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Cell characterisation tests - 1C Discharge
1C Discharge at Different Temperatures
4.2
0⁰C = 30.17Ah
10⁰C = 34.43Ah
23⁰C = 40.39Ah
30⁰C = 42.09Ah
Supplier 23⁰C = 38.40Ah
4.1
4
3.9
Voltage (V)
3.8
3.7
10degC
23degC
3.6
30degC
3.5
0degC
3.4
Supplier Data at 23degC
3.3
3.2
3.1
3
0
5
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Capacity (Ah)
30
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Cell characterisation tests - 3C Discharge
3C Discharge at Different Temperatures
4.2
10⁰C = 27.06Ah
23⁰C = 36.53Ah
30⁰C = 39.11Ah
Supplier 23⁰C = 38.25Ah
4.1
4
3.9
Voltage (V)
3.8
3.7
10degC
3.6
23degC
3.5
30degC
Supplier Data at 23degC
3.4
3.3
3.2
3.1
3
0
5
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Capacity (Ah)
30
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Cell characterisation tests - Cont
Charge capacity test
- Constant current charging (varying C-Rate), followed by constant voltage
charging at a constant cell temperature,
o This test provides the cell charge voltage curve vs. capacity (Ah)
- See 3 Examples charts below
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Cell characterisation tests - 0.5C Charge
0.5C Charge at Different Temperatures
4.3
4.2
4.1
4
Cell Votlage (V)
3.9
3.8
10degC
3.7
23degC
3.6
30degC
3.5
40degC
10⁰C = 39.56Ah
23⁰C = 42.69Ah
30⁰C = 43.24Ah
40⁰C = 43.92Ah
Supplier 23⁰C = 41.48Ah
3.4
3.3
3.2
3.1
Supplier Data at 23degC
3
0
5
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Capacity (Ah)
30
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Cell characterisation tests - 1C Charge
1C Charge at Different Temperatures
4.3
4.2
4.1
4
3.9
Voltage (V)
3.8
3.7
23degC
3.6
30degC
Supplier Data at 23degC
3.5
3.4
3.3
23⁰C = 38.81Ah
30⁰C = 42.14Ah
Supplier 23⁰C = 41.46Ah
3.2
3.1
3
0
5
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Capacity (Ah)
30
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Cell characterisation tests - 3C Charge
3C Charge at Different Temperatures
4.3
4.2
4.1
4
3.9
Voltage (V)
3.8
3.7
10degC
3.6
23degC
30degC
3.5
Supplier Data at 23degC
3.4
10⁰C = 34.41Ah
23⁰C = 36.54Ah
30⁰C = 38.96Ah
Supplier 23⁰C = 41.13Ah
3.3
3.2
3.1
3
0
5
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Cell characterisation tests - Open cct voltage test
Open circuit voltage test (Voc)
- Applying short pulse discharge and charge currents, followed by rest
intervals to allow the cell voltage to stabilize to determine the open circuit
voltage, throughout the operating voltage range of the cell
- See 3 Examples charts below
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Cell characterisation tests – Voc Test: 0.5C
Discharge
0.5C Discharge Voc
4.4
25
4.3
4.2
4.1
20
4
3.9
3.8
15
3.6
3.5
10
3.4
Current (A)
Voltage (V)
3.7
3.3
3.2
5
3.1
3
2.9
0
2.8
2.7
2.6
-5
0
2000
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4000
6000
8000
Time (s)
Voltage (V)
Current (A)
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10000
12000
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14000
16
Cell characterisation tests - Voc Test: 0.5C
Charge
0.5C Charge Voc
4.4
25
4.3
4.2
20
4.1
4
Voltage (V)
3.8
3.7
10
3.6
3.5
Current (A)
15
3.9
5
3.4
3.3
0
3.2
3.1
3
-5
0
2000
4000
6000
8000
Time (s)
Voltage (V)
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12000
14000
Current (A)
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Cell characterisation tests - Voc Hysteresis
Cell Voc vs Capacity
4.30
4.20
4.10
Cell Open Circuit Voltage (V)
4.00
3.90
3.80
3.70
3.60
3.50
3.40
3.30
3.20
3.10
3.00
0.00
5.00
10.00
15.00
20.00
25.00
Capacity (Ah)
0.5C Discharge
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30.00
35.00
40.00
45.00
0.5C Charge
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Using the Cell characterisation data
Using cell characterization data in combination with actual cell voltage,
current and temperature measurements, improves the estimation
accuracy of the following battery pack parameters:
- State of Charge (SOC)
- Cell internal resistance
- State of Health (SOH)
- Capacity left in the pack (Ah)
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Using the Cell characterization data - Cont
SOC – Indicates when the battery needs recharging and used by vehicle
manufactures to calculate vehicle range.
- Two commonly used methods for SOC prediction are:
o Knowledge of the relation between cell open circuit voltage (Voc) and its depth
of discharge (DOD)
o Current integration (coulomb counting)
DOD can be derived from cell voltage and temperature measurements and
cell data.
- It can be refined by using discharge data and voltage drop measurements
during pre-charge to determine the initial DOD.
- Coulomb counting method can be used once the initial DOD is known.
Combining both methods further improves the estimates of DOD and hence
SOC.
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Using the Cell characterization data - Cont
Instantaneous power predictions
- Cell Internal resistance can be calculated from Voc, cell voltage
and current measurements
- Internal resistance together with the knowledge of the Ah capacity
left in the pack can be used to calculate instantaneous available
pack power
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Simulation results (Measured vs Supplier data)
SOC prediction vs time for a typical vehicle drive cycle
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Conclusion
Cell performance varies greatly with temperature (not always clear from cell
supplier data sheets)
Poor knowledge of cell characteristics can lead to over stressing of the cells in
operation.
A stressed battery pack will have a shorten battery life and is prone to
premature shut downs.
Improvements in SOC estimation accuracy gives designers the confidence to
optimise the size of the battery pack.
Providing opportunity to reduce
battery pack weight, size and cost.
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