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
- isolating contactors
- battery management system
- a 12 V battery for instrumentation and emergency lighting
- a DC-DC converter to charge the 12 V battery from the traction battery
- ventilation fans
- battery balancing circuitry
- a battery charger.
The battery must fit into the under-floor tray, which is 510 mm wide, up to 2300 mm long, and 50-80 mm high. The contactors, motor controller and BMS high voltage components must also fit in the battery tray.
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.
When Team Trev drove around the world with a 13 kWh battery, Trev had a reliable range of about 200 km and a maximum range of about 250 km, with one person in the car. This is not as good as expected. Testing is required to find the source of the unexpected energy losses. We suspect:
- the rear motorcycle tyre had unknown, but probably high, rolling resistance
- brake drag
- aerodynamic drag from three-spoke wheels
- unoptimised motor control (six-step commutation rather than more efficient control methods).
Lithium ion polymer
Lithium ion polymer cells have a specific energy of up to 200 Wh/kg, but are expensive.
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.
Trev's original battery of thirty-six Kokam SLPB 90216216 40 Ah cells had:
- energy capacity: 5300 Wh
- mass: 33 kg
- 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.
Team Trev's pack of thirty-five Kokam SLPB 70460330 100 Ah cells had:
- 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.
Team Trev used this last pack for their around-the-world trip. The maximum distance driven on one charge was about 250 km, with one occupant.
Lithium ion phosphate
Lithium ion phosphate cells have a specific energy of up to 100 Wh/kg, and are commonly used for EV conversions.
EV Power in Western Australia sell Sky Energy cells. The 70 Ah cells are 61×113×206 mm with a mass of 2.5 kg, giving a specific energy of 92 Wh/kg.
A pack of 34 cells would give an energy capacity of 7.8 kWh, and a range exceeding 100 km.
Nickel metal hydride
Nickel metal hydride cells have specific energy up to 70 Wh/kg.
The Toyota Prius uses NiMH modules, each containing six cells. Hybrid Interfaces has some good information.
The 7.2 V, 6.5 Ah modules are 280×130×20 mm. Specific energy is 46 Wh/kg.
A pack of 54 cells would give an energy capacity of 2.5 kWh, which would give a range of only 50 km.
The battery tray under the floor should contain:
- traction cells
- battery contactor (switching the 130 V line)
- battery management system (BMS)
- cooling fans.
The BMS monitors the voltage of each cell and several battery temperatures, and outputs these values on the CAN bus.
The high voltage system is isolated from the (0, 12 V) lines and from the car.
Team Trev did not have fans in the battery box, and did not have any (high) temperature problems.
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.
Team Trev used a separate 12 V battery to provide 12 V to the car, charged from the (0, 130 V).
The driver's key switch has three positions:
- Accessories: provides power to the main (0, 12 V) lines, but does not enable the high voltage battery
- ON: provides power the the main (0, 12 V) lines, and enables the high voltage battery.
The UniSA prototype used a 450 W DC-DC converter from Vicor Power.
The Vicor converter is set to 13 V, to maintain charge on the 12 V battery.
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.
Team Trev used an Elithion battery management system with a cell board monitoring each cell.