As an electrical engineer, I thought that designing a battery pack for the motorcycle would be mainly about its energy capacity, power output, energy losses and electrical protection. Now that was the case for a while, when we were doing simulations to estimate the required capacity and power. Two full weeks were occupied by this simulation and the selection of the perfect LiPo (lithium-polymer) cell to use. The manufacturer of our cells can make them with all thinkable dimensions and ratings, each to serve a different purpose. In our case, the combination of the maximum voltage requirement of 700 V, energy capacity of 11.5 kWh and power output of at least 200 kW has led to some good candidates.
But then came the part I was not prepared for: fitting all the LiPo cells in the available space in between the frame of the bike. Luckily the exact dimensions of each cell are provided by the manufacturer, but it turned out that the cell types that were selected on their electrical properties could not fit in this space.
I learned a little 3D modelling in Solidworks in order to visualise the available space and designs.
We had to turn around the design process. Write a script that finds an optimal way of stacking each cell type in the battery pack. Each cell type that could fit enough cells in the available space was considered as a candidate. However, for safety reasons but also because of the required space of the connections within the battery pack each cell would require some extra space. Each subsequent detail that was taken into account reduced the list of possible cell types further to beyond the point where the initial candidates were ruled out. A frustrating process, I have to say.
Matlab works great to automatically generate simple visualisations of the cell stacking.
In the end, only one cell type stands out from the others, in terms of deliverable power, total voltage and losses. It also fits perfectly in the available space. This LiPo is the whole package.
When I say that it fits perfectly, I really mean perfectly. But that is perfection in theory, making many assumptions. The thermal design and mechanical safety design still have to be done, and brainstorming about that it turns out that we would probably need quite some more space in the battery pack for it all to fit. Back to the drawing table, I guess.
Another subject that was new to me was thermal design. Each LiPo cell has a certain internal resistance that generates heat when a current flows through the battery pack. This heat can reduce the performance of the cells or even damage them. Fortunately for us, a race only takes about 15 minutes, limiting the total energy that is lost into heat. If we would want to keep the temperature of the battery cells below 50 °C we would need more heat capacity than just what is provided by the LiPo cells themselves. Only the cells would result in a temperature of 60 °C, which enters the risky area for a LiPo cell. To solve this, we need to look into materials that are lightweight, but can still absorb much heat.
Now I am lucky to be in this multidisciplinary team, where I can enlist the help of a mechanical engineer and their knowledge of structures and thermodynamics. Realizing that you can not do everything alone may be annoying at first, but achieving a product that is worked out in every aspect of it gives a lot more motivation.