One of the main upgrades to Trev is to increase the range of the car from 120 km per charge to over 250 km per charge. To do this, we have to fit a big battery into a small car.
Actually, the battery will not be that big. Currently, Trev has a 45 kg battery. We need to increase the battery mass to about 85 kg. The mass of the car with the larger battery will still be under 400 kg.
Choice of battery type is critical. We will use large lithium ion polymer cells, similar to those in our current battery, only larger. The advantages of lithium ion polymer are:
- they have high energy per kilogram (about 160 Wh/kg)
- they have high energy per volume
- the large, flat cells are easy to fit into tight spaces.
With most batteries there is a trade-off between energy capacity and power. However, high energy cells can deliver more than enough power for our low-mass car.
Last week we lifted the car up onto trestles to check that there was enough room beneath the car for a large battery. There was.
The diagram shows the structural tub chassis in green, and the proposed new battery box in white. The tub chassis is built from boards with an aluminium honeycomb core and fibreglass skins, with kevlar reinforcement on the floor. For the battery box, we are considering using polypropylene honeycomb with kevlar skins, which will be even lighter.
The entire high-voltage system—battery, fuses, battery management system, contactors and motor controller—will fit into the battery box. The motor controller is taller than the battery, but will stick up through the main floor under the rear seat. This layout will allow very simple wiring, from battery to motor controller to motor.
To accommodate the battery box, we will raise the car by about 60 mm. This will also allow us to increase the movement on our suspension so we can cope with rough roads. We will continue to use double wishbone suspension at the front, but will lengthen the suspension arms by putting the lower pivots on the underside of the main floor, where the battery box narrows near the front.
This design improves the overall simplicity of the car, by putting all of the high voltage components together in a box that can be easily separated from the rest of the car.
Now all we have to do is complete the detailed design and engineering analysis, then build it.