The Battery Management System Design

I will try to glance upon the different important controllers of the electric powertrain system. Let's start with the battery controller a.k.a battery management system.

The brain of a pure electric vehicle is its battery management system. I will briefly go over the responsibilities of this controller in this article so as to give you an idea of what all this battery management system (or BMS) constitutes of. This could prove as a background in case you dive into this field in future.

As described in my previous article the battery constitutes of cells in parallel and series. There could be thousands of cells in a car battery which together produce hundreds of volts and a significant amount of current. Critical parameters of each cell are its voltage, current and temperature. Any one of these if goes out of control, can lead to a thermal runaway which can lead to an explosion. The entire battery is kept in control by its battery management system. The BMS is responsible for variety of function some of which are mentioned below.

State of Charge Estimation:

The BMS is responsible for estimating the charge remaining in the battery and hence the range. Hence the more effective the BMS estimates the range (taking into account factors like terrain, temperature, weather etc) lesser will be the range anxiety of the customer! There are various techniques used to calculate the state of charge in a battery (like coulomb counting etc)

Temperature/ Voltage/ Current Sensors:

It is very critical to measure voltages, currents and temperatures of all these cells. How these three quantities are measured depends on each manufacturer of the battery pack. Usually there is one current sensor measuring the current that the battery consumes or delivers. There are multiple voltage and temperature sensors at various locations to measure these cell variables at throughout the battery. All these are inputs to the battery management system (BMS) and the control logic in the BMS makes sure that these stay in bounds.

Battery Cooling Circuit:

It is also critical to sufficiently cool the battery. In an electric vehicle, batteries are generally cooled by separate pumps or via the radiator. The inlet and outlet temperatures are measured by two thermistors and are fed to the BMS which in turn controls the cooling fluid.

Contactors:

Contactors are nothing but switches that connect the battery to the motor/inverter or charger. Battery contains high voltage all the time. But when a car is not being driven, it’s not necessary to supply this voltage and hence current to the inverter. When a person puts the car into a state where he wants to drive it or charge it (where either the battery outputs or consumes current), these switches close and the loop (battery-charger or battery-motor) is complete. BMS decides when to close the contactors and let the battery drive or charge the car

Safety Monitoring:

BMS is also responsible for keeping the battery in safe condition. In any unsafe condition the BMS will not close the contactors and hence the high voltage will remain contained in the battery itself. Unsafe conditions can include any of the following

1. Over heating of the battery, over/under voltage and over current conditions: BMS open the contactors and isolates the battery from the car if any of the voltage/current/temperature sensors report the above mentioned conditions. 
2. Over charging: BMS stops charging if it detects an over charged cell. Overcharging can significantly reduce the battery health and lifespan. 
3. Unequal charging of the cells: BMS throws appropriate alerts if different cells are charging at different rates. It is also responsible for discharging/ bleeding the over charged cells (if any) and maintain the charge of all the cells to same level. 
4. Over discharging of cells: Over discharging cells also create problems for battery health. BMS prevents that. 
5. Grounding/ Isolation problems: BMS always keeps checking if the battery is properly isolated from the vehicle of not. In case of a short/ thermal runaway BMS makes sure that the damage is contained in the battery and does not propagate into the car. In case of a crash (head on collision) the BMS has provisions to cut the current supply from the inverter and isolate the battery.

For each bullet mentioned above, BMS has its own state machine and probably one person dedicated for its design in the industry! 

So what’s there for you guys here: Mechanical engineers play a crucial role in conceptualizing and modeling the BMS. EE, E/I engineers are generally responsible for designing a PCB that functions as a BMS controller while software engineers write the firmware that does the control. Hence it’s a super interdisciplinary board in an electric car.

I will finish with an interesting video clip explaining the BMS in short. Hope you like it :)

Interesting video clip on the BMS (Click Here)