Written 11/08/22 by Tom Johnson

UH26 (last years car) had a 7.812 kWh, 588V accumulator. This was done as we were attempting to maximise the overall potential difference of the accumulator, which in turn would reduce the peak current (P=IV) required to reach the 80kW maximum power, and therefore we could reduce the thickness and mass of our HV cabling, as well as making the system safer overall. In hindsight, it wasn't the best idea. In the end, our potential difference had increased by less than 10 Volts, meaning we still had pretty much the same current draw and therefore didn't actually change the wiring.

In order to achieve that 588V, we ended up increasing the number of modules used. This is bad for two reasons. Firstly, its extra mass, which as all automotive engineers should know, is bad for the handling, acceleration and deceleration of your vehicle. Secondly, it added extra capacity to the accumulator, which already was over the top for the endurance event, with other teams running between 6.8 kWh and 7.5 kWh as standard. Instead, we should have focussed on making the pack smaller and bringing regenerative braking into the scope of the project, which is definitely something I want to do this year.

Another talking point is the cooling. I worked with an excellent engineer named Jack Farley, who took charge of the overall TSAC design and cooling for the system. He chose to implement a 'transverse' cooling approach, where the air effectively goes across the accumulator, rather than through it, as had been used on previous UH cars. This was chosen as it increased the number of cooling paths (the accumulator was longer than it was wide). It was one of the guiding principles that led the layout of the HV System, so once we had started, it was very difficult to change. I'm planning to revert to the original 'front to back' approach, as we can more effectively redirect the laminar flow of air from the sidepod, rather than having to force the air sideways, which would introduce turbulent flow. Additionally, this will have the added benefit of slimming down the accumulator, meaning we have the opportunity to slim down the chassis as well.

Last year we also chose to 'install' the accumulator through the firewall (behind the drivers seat), rather than the rear bulkhead, meaning the bulkhead could be made smaller, decreasing aerodynamic losses to drag. Geometries are hard to visualise at the moment, as I have started this project before the chassis has been designed, but it is certainly something to consider again for UH27.