100 Chapter 8 The other important aspect of the BMS is its ability to communicate with the external system. Most advanced BMS systems will have the capability to both send and receive messages from the vehicle and or motor controller. Typically, the BMS will send requests to reduce or stop the battery current (discharge) entirely and will send data on the status of the pack itself such as remaining capacity and energy, which can be converted into range and life for the end user. Finally, the BMS will decide when to open and close the contactors in the system thereby allowing current to flow from the pack to the electric motors or to flow from the charging system into the battery.
Software and Controls The “unsung hero” of the BMS system is the software that controls it all. Most manufacturers will guard the core software very closely as this is the “core IP” of the entire BMS design. Most of the hardware is based on “off the shelf” components, but the software is custom designed and may consist of tens of thousands of lines of code. The code may also be referred to as a series of algorithms. Essentially it is a series of mathematical formulas and calculations to understand all of the SOx states of the battery, how much energy and power is available for instantaneous use, what the current SOC is, how much SOC is left, and how much life is left.
The algorithms are a very complex set of models that are usually based on a specific cell. Most often the BMS designers will operate the chosen cell in a controlled laboratory environment in order to understand how it operates under different conditions and then overlay this onto the code. Through a series of repetitive steps, it is possible for the software designers to end up with a set of algorithms that can accurately predict the performance of a cell under most conditions. This is also the reason that it is generally not possible to take a BMS designed for a chemistry and integrate it into a system designed with a different chemistry.
For instance, NMC cells operate at a nominal voltage of 3.7V, while LFP type cells operate at a nominal voltage of 3.3V and LTO cells operate at about 2.2V. So the algorithm must be designed such that it understands the maximum and minimum ranges that it can operate within. Now there are several BMS manufacturers out there who have developed multiple sets of software for their single set of hardware thereby making them usable with multiple chemistries.