What to take into account, when designing a battery pack for a solar car?
The battery pack is an important part of the electrical system of a solar car. It is used to store excess energy, generated by the solar cells or by recuperation, that is not immediately needed to power the motor. If the solar cell power is not sufficient to drive at desired speed, due to cloudy weather or a steep ascent of the road, this stored energy is used additionally.
Main Part of the Battery Pack: Battery Cells
The main parts of the pack are the battery cells. The rules of the World Solar Challenge define how much these are allowed to weigh when using a certain battery chemistry. We decided to use lithium-ion cells, mainly because they are the most established technology in rechargeable battery cells. The rules allow 20kg of this battery cell type, so we tested various 18650 cells to find the one with the highest energy density while being as light-weight as possible. We also tested other factors, for example how the capacity changes under high temperatures, before committing to a particular battery cell to use.
After choosing a battery cell it is simple to estimate the allowed number of cells to use in the pack. Based on this number a circuit diagram can be provided. It is beneficial if the pack provides a high voltage connected with a low current at the same demand, because this allows us to design the cables with lower cross section, which leads to reduced weight. It is also important to take the efficiency of the motor controller and the MPPTs into account, which depends on the voltage and thus on the serial connected cells.
The next task is the arrangement of the cells. We wanted them to use as little space as possible while at the same time being easily exchangeable in case of damage, so we arranged them in 18 identical modules with high packing density. The cells of each module are hold in place by a 3D-printed fixture. We experimented with different 3D-printing techniques and materials to produce components which have as low tolerances as possible and are very stable especially at high temperatures.
Although we designed the pack for high temperatures there are still dangerous operating ranges, that have to be avoided. Thus, the temperature of the cells must be regulated. We decided to use fans to provide an air cooling of the pack. With the help of the BMS it is possible to control the fans to run only when required, so that only a minimal part of the valueable solar cell energy is used.
Battery management system
BMS stands for Battery Management System, it has many different jobs. It can identify if parts of the pack are at dangerous temperatures, using multiple temperature sensors. In this case, it is able to automatically control the fans to cool the battery pack down. With numerous voltage sensors, the BMS is monitoring the voltage of the battery cell modules in order to protect them from depth discharge or overvoltage. It also switches the main contactors, balances the battery cells and communicates with the main components of the solar car. In addition, the BMS is able to estimate the state of charge of the battery which is very important especially for the driving strategy during the challenge.
Finally, everything has to be placed into a box to be protected from environmental influences. This box has to be light-weight, it has to fit into the carrying structure and there must be room for all the different parts of the battery pack, that have been mentioned above: The battery cell modules with the circuitry, the fans and the Battery Management System.