Have you ever wondered where the State of Charge SOC and the remaining electric range on the dashboard in your EV actually come from? Surprisingly this number cannot be directly measured but needs to be estimated or calculated. This is only one task of a battery management system BMS in an electric car. The BMS monitors the battery to keep it healthy and safe over lifetime, but how?
A high voltage battery pack in an electric vehicle is typically located underneath the floor. It consists of several individual battery modules which themselves consist of individual battery cells. Each of those battery cells has a certain operation window in which they operate safe and reliable over lifetime.
There is a certain minimum and maximum voltage that should not be exceeded otherwise you overcharge or overdischarge the battery which can lead to severe safety issues. The same is true for minimum and maximum charge and discharge currents as well as a certain temperature range that this battery is safe and comfortable to operate at. One major role of the BMS is maintaining these healthy and safe limits over lifetime.
The voltage, current and temperature limits for a safe operation are different for different battery chemistries – depending on the anode and cathode material chosen. While the most popular anode material today is Graphite, for the cathode side the two most popular chemistries are LFP, lithium iron phosphate and NMC Lithium Nickel manganese Cobalt Oxide.
Different material pairings of anode and cathode provide different voltage levels in battery cells. Watch this video for more on battery chemistry.
Importantly, pretty much every car manufacturer has their own battery design, using various battery cells, formats, chemistries, cooling systems, module and pack designs – so there is NO “one BMS fits all” approach – a BMS architecture needs to be designed, it needs to be developed, it needs to be calibrated for every new battery design.
A BMS architecture has a modular structure and typically consists of three subsystems: the Battery Management Unit BMU, the Cell Supervisor Unit CSU and the Intelligent Battery Junction Box (BJB).
The cell supervisor unit CSU monitors cell voltages and temperatures of each individual lithium-ion battery cell and performs cell balancing to compensate for inconsistencies between battery cells. The battery management unit BMU is the brain of the battery management system. It controls every other subsystem inside the BMS and it hosts the main safety controller. The intelligent battery junction box is responsible for controlling contactors, the high voltage relays and pyro elements. It also senses battery pack currents and voltages.
In case of a wireless BMS the communication between BMU and the individual cell supervisor units happens without a wired connection which can significantly ease the assembly and production of battery packs.
Based on the individual battery cell voltages and temperatures plus the pack current, the battery state of charge SOC can be estimated and information can be transmitted to the vehicle control unit and on-board charger through e.g. CAN communication.
If we look at a battery management system in a very simplistic way, we can say that the physically measured quantities such as voltage, current and temperature are input variables which the BMS uses in different algorithms to output quantities such as the State of Charge, the State of Health and several Status and Fault messages.
Let’s have a closer look at the State of Charge. The SOC is an expression of the present battery capacity as a percentage of the maximum capacity and it is the quantity that finally shows up on your dashboard in your EV together with the electric range. It obviously depends on the battery voltage, current and temperature but cannot be measured directly. Common options to estimate the SOC in a battery are either direct voltage measurement, coulomb counting – so basically measuring how much charge is going in and out of the battery – or a combination of both.
The challenge with determining the SOC solely from voltage measurements becomes clear when looking at the OCV (open circuit voltage) curve of a battery – the open circuit voltage vs. the State of Charge. You can see that at very low and high State of Charge there is a clear relation between battery voltage and SOC. But as soon as you get into this flat part around 50% State of Charge already very small variations in voltage lead to significant changes in the SOC. This gets even more severe when you change over from an NMC to a LFP chemistry that has an even flatter OCV curve.
So, overall – very often direct voltage measurements are combined with current measurements and coulomb counting to get the most accurate SOC. The accuracy of voltage, current and temperature measurements has a direct impact on the predicted electric range:
Please feel free to watch the entire video with Mark Ng, General Manager EV powertrain at Texas Instruments to get even more detailed information.
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