Battery Management System Testing & Its Challenges
A battery of an electric vehicle usually consists of about 96 to 200 cells per pack, which are monitored and controlled by a battery management system (BMS). If cells get overcharged, deep discharged, exposed to very high or low temperatures or have chemical issues, the whole battery would be no longer useable. Thus, the battery is only as good as the weakest link in the chain, which means the weakest battery cell.
One of the main challenges are the cell voltages, which drift apart because of manufacturing tolerances and aging. Therefore, to extend the battery lifetime, a BMS needs to do a good balancing between the cells, to recover bad ones and save good ones. To test these balancing algorithms and security features, the battery cell simulator (BCS) is a key tool. The emulated cells are configurable in all their characteristics and therefore can simulate all the needed states and failures. Additionally, to test both charging and discharging, for each cell a source and a load is needed at the BMS.
The use of real batteries on the other hand would be very expensive, because it is not only slow and badly reproducible, but also dangerous and sometimes even not feasible in extensive system testing. So, the cell simulation does not only save time because you don’t have to charge and discharge real chemical cells but also runs tests safely which is very important in development of a BMS.
One of the main tasks of a BMS, as mentioned above, is the balancing of the voltages between the cells. Due to unavoidable production tolerances the aging of the cells is different, which leads to different cell voltages over the time. If one cell reaches a critical voltage level, the discharging would have to be stopped before the cell gets deep discharged and therefore unusable. But all other cells still could have enough capacity to be discharged. This also applies for charging. A fully charged cell should not be charged any longer when the other cells are not fully charged. In this case there are two known ways of balancing, the common passive balancing and the modern active balancing.
The passive balancing is focusing on the lowest cell and discharge the other cells by means of resistors until all reached the same voltage level. While the cells are balanced, the energy of the fuller cells just gets transformed into heat. This is not very efficient but comes at low expense and low complexity.
Further, to test a BMS automatically and reproducible, a BCS needs to have several characteristics. At first precise control and measurement of cell voltages are mandatory. The typical state of charge (SOC) curve of a lithium-ion cell is very flat because the voltage rises only from approx. 3.4V (0% SOC) to 4.2V (100% SOC). The BMS should identify at minimum differences of 1% SOC.
Figure 3 State of charge of a lithium-ion cell
Therefore, to simulate every step of the SOC, the BCS needs to have a pretty high accuracy at every cell, which should be at least in the range of +/- 1mV. This applies to both the DC part of the voltage generator and the AC part (ripple), where it’s more difficult to achieve. Additionally, because of the stack voltage up to 1500V in comparison with the single cell voltage of 0.01 – 8V, traditional measures like shielding of cables, diodes or coils for attenuation, etc. can only be used to a limited extent. Thus, the residual ripple of the cell emulator output must be kept to a minimum. At the same time the voltage setting needs to be controlled without overshoot within a few milliseconds.
In summary a BCS needs to handle simultaneously the following points: low single cell voltage, high stack voltage, low residual ripple, currents of several amps, electrical faults simulation, controlled voltage balancing of the supply line deviation, fast settling time in the range of only a few milliseconds and emulation of a battery stack with a lot of cells.
The BCS itself is only the core of a BMS test system. To complete such a HiL (hardware-in-the-loop) system, even more test and simulation hardware is required. The figure below gives an overview of what the other main components of such a test bench are and how they are linked to the BMS.
Figure 4 Design of a BMS test bench
For a long time, there wasn’t such a solution, which would have covered the expectations of OEMs and Tier1’s, Comemso was founded in 2009 by CEO Dr.-Ing. Kiriakos Athanasas and his wife Dipl.-Ing. (FH) Anita Athanasas, having searched for advanced testing solutions in the automotive area without success. As a result, they founded Comemso (complex embedded solutions) to care for complex applications for the e-mobility market. Comemso developed its first own battery cell simulator and launched in 2011
Most of our innovations are driven by challenges of the customers, which develop products for society and a sustainable future. Therefore, comemso supports them to maintain the quality of the e-mobility market and to push the boundaries of efficiency even further. Just as the CEO Dr. Kiriakos Athanasas summed it up “We want to make our contribution against the climate change and for a sustainable future by staying innovative and always giving the best”.
Dr.-Ing. Kiriakos Athanasas, CEO
Dipl.-Ing. (FH) Anita Athanasas, Head of Sales and Product Management