Battery
From Trevipedia
You should choose a motor before you start designing a battery, as the motor will determine the operating voltage of the traction system.
The battery subsystem comprises:
- a high-voltage traction battery
- isolators
- a 12 V battery for instrumentation and emergency lighting
- a DC-DC converter to charge the 12 V battery from the traction battery
- sensors for measuring cell voltages, battery current, charge and temperature
- cooling fans
- battery balancing circuitry
- a battery charger.
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Energy requirements
The prototype car had a 5300 Wh battery, and was able to travel up to 120 km at 90 km/h with one person in the car. Based on these figures, the energy use was about 45 Wh/km.
The extra energy required per kilometre to carry each additional kilogram is about 1000 g crr/η, where crr is the rolling resistance coefficient of the tyres and η is the efficiency of the drive system. For Trev, the extra energy required for each additional kilogram is roughly 1000 x 9.8 x 0.01 / 0.75 = 130 J/km = 0.036 Wh/km.
The specific energy of the high-power Kokam batteries used in Trev was 117 Wh/kg.
A 10 kWh battery will increase the mass of the battery by 40 kg and increase the energy requirement to 46 Wh/km, giving a range of 220 km.
A 12 kWh battery will increase the mass of the battery by 57 kg and increase the energy requirement to 47 Wh/km, giving a range of 255 km.
Adding a 70 kg passenger will increase the energy requirement to 50 Wh/km, giving a range of 240 km.
Traction battery
The UniSA prototype car used thirty-six 40 Ah lithium ion polymer cells from Kokam. The current equivalent is the Kokam SLPB 100216216H high power 40 Ah cell. These high-power cells are able to deliver up to 400 A, and so the battery is able to deliver up to 26 kW continuous and 53 kW peak.
A pack of thirty-six Kokam SLPB 90216216 40 Ah cells would have:
- energy capacity: 5300 Wh
- mass: 33 kg
- size:
- specific energy: 160 Wh/kg
- power: 5300 kW continuous, 26 kW peak
- estimated range: 120 km with one occupant, 110 km with two occupants.
A pack of thirty-five Kokam SLPB 53460330 70 Ah cells would have:
- energy capacity: 9065 Wh
- mass: 59.5 kg
- size: 1645 x 455 x 42 mm (five modules x seven cells, under the floor)
- specific energy: 152 Wh/kg
- power: 9.1 kW continuous, 39 kW peak
- estimated range: 199 km with one occupant, 188 km with two occupants.
A pack of thirty-five Kokam SLPB 70460330 100 Ah cells would have:
- energy capacity: 12950 Wh
- mass: 81.2 kg
- size: 1645 x 455 x 56 mm (five modules x seven cells, under the floor)
- specific energy: 159 Wh/kg
- power: 13 kW continuous, 40 kW peak
- estimated range: 280 km with one occupant, 265 km with two occupants.
Who else makes large cells with high specific energy?
The battery box under the floor contains:
- battery cells
- battery fuse
- battery contactor (switching the 130 V line)
- battery management system (BMS)
- cooling fans.
The connections to the battery box are:
- (0, 12 V, battery enable) in: 12 V powers the BMS; the 12 V battery enable line operates the battery contactor
- (0, 130 V) out
- (CAN low, CAN high).
The BMS operates when 12 V is applied. The battery contactor closes when 12 V is applied to the battery enable line, and opens when 12 V is removed from the battery enable line.
The main contactor switches the 130 V line.
The BMS monitors the voltage of each cell and several battery temperatures, and outputs these values on the CAN bus.
The (0, 130 V) lines are isolated from the (0, 12 V) lines and from the car.
The cooling fans are operated by the BMS as required. The air intake for the battery box is inside the front compartment of the car.
Do we want a "service disconnect" plug, for isolating the traction battery while working on the car?
12 V battery
The UniSA prototype did not have a 12 V battery. Instead, 12 V was supplied directly from the DC-DC converter.
Some jurisdictions require a separate 12 V battery so that hazard lights can operate if the traction battery fails.
Trev will use a separate 12 V battery to provide 12 V to the car. The 12 V battery will be charged from the (0, 130 V).
Key switch
The driver's key switch has three positions:
- OFF
- Accessories: provides power to the main (0, 12 V) lines, but not to the battery enable line
- ON: provides power the the main (0, 12 V) lines, and to the 12 V battery enable line.
DC-DC converter
The UniSA prototype used a 450 W DC-DC converter from Vicor Power.
Can the Vicor converter be configured to charge the 12 V battery?
Cell balancing
Small differences between cells can cause charge level to vary amongst cells after many discharge and recharge cycles. This reduces the effective capacity of the battery. Charge balancing addresses this problem by one of the following methods:
- putting resistors across full cells during charging
- transferring charge between imbalanced cells.
The first method is simple, and the energy lost is small.
