Production journal – battery
Probably the most important component in an electric car is the battery. It represents the most important lever for reducing the purchase price because it reflects the highest production costs. In addition, the range is increased mainly via the battery. In our Huawei Sonnenwagen, the battery is not the most important part, but nevertheless very important to us. Today we will explain what exactly makes the battery so important for us! So stay tuned! 🙂
What has to be taken into account, when designing a battery pack for a solar car?
The battery pack is a combination of a certain amount of battery cells and an important part of our Huawai Sonnenwagen.. 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
How do you start when you want to build a pack? Of course with the battery cells. The rules of the Bridgestone World Solar Challenge specify the weight of the different cell types. We have opted for lithium-ion cells, among others, because this is currently the most established technology on the market. According to the rules, 20 kilos of lithium-ion cells are allowed. With this specification, we selected the cells with the highest energy density per kg. For this purpose, we subjected the cells of the various suppliers to a hard test. A few other factors also played a role in the selection, for example how their capacity behaves at high temperatures. From the various investigations, we have selected a winner for our field of application.
After choosing a battery cell it is fairly 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 (What were MPPTs again? More on that in our solar cell blog) into account, which depends on the voltage and thus on the serial connected cells.
Since we now know the interconnection, the next task is to arrange everything as space-saving as possible. In the pack for the Huawei Sonnenwagen, it is also important for us to replace broken parts quickly and easily, which is why we split the cells into 18 equal modules. Each of these modules is held together by additive cell brackets. For this purpose, we have experimented with various additive manufacturing processes and materials to produce components with high tolerances, which remain stable even 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 Battery Management System (BMS) it is possible to control the fans to run only when required, so that only a minimal part of the valuable solar cell energy is used.
The Battery management system (BMS)
BMS stands for Battery Management System and it has many different jobs. It can identify if parts of the pack are at dangerous temperatures, using multiple 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.
By the way: All components of our Battery Management System are provided by Sensortechnik Wiedemann. At this point again a huge thanks for it! 🙂
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. And then our battery is already complete. It was tested, calculated and built. Our first pack is ready and our second pack will soon be built. By the way: Due to different directives in the aviation sector, our battery has to travel to Australia by sea much earlier than the rest of the car. That is why the deadlines for our battery crew are much earlier. Congratulations to the first pack! 🙂