What does the battery control module do?

This blog will explain the function of the battery control module and answer the following questions: what are the different types of battery management systems? Why does the battery discharge despite a battery control module?

What does the battery control module do?

The battery control module monitors the state of charge of the SOC of the battery in the vehicle. The state of charge is the amount of energy available in the battery, based on the SOC the battery control module can decide the amount of charging current required to charge the battery. The Battery control module maintains the battery at 80 percent SOC and reduces the charging current if the battery is overheating.

The location of the battery control module depends on the type of battery present in the vehicle. If the vehicle is a plug-in hybrid or electric vehicle then there can be more than one battery control module.

Similarly, if the vehicle has an additional battery many manufacturers use a dedicated battery control module to stabilize the electrical system.

It is also possible to program a battery monitoring function into a control unit. This reduces the number of control units in a vehicle and also simplifies the wiring harness. The control unit with the battery monitoring function can easily send the data to the engine control unit and set the correct charging current for the battery.

Apart from the battery monitoring function, the battery control module can also record data of 100 driving cycles and 100 stationary cycles of the vehicle. In the driving cycle, the rate of charging should be positive as it indicates the battery is getting charged from the generator or the alternator. 

The data in the stationary cycle should not show a negative charge balance as it means the battery is losing energy when the ignition of the vehicle is OFF. There can be a parasitic drain in the vehicle or the battery can simply be unable to retain the charge.

 What are the different types of battery management systems?

The different types of battery management systems are given below:

12 V battery management system

The 12V battery is very commonly used in passenger cars, the battery management system of the 12 V battery can be a separate control unit or it can be a programmed function. As the amount of the voltage and current in the system is not high, it is possible to control the charging voltage and current by a LIN signal.

The battery management system of a 12 V electrical system uses a battery sensor that is attached to the negative terminal. The battery sensor measures the temperature and the voltage of the battery and sends this data to the BMS over a single wire LIN.

After the data from the battery sensor is received, the BMS can determine the state of charge of the battery and send the signal to the ECU to control the excitation current of the alternator. If any component is directly attached to the battery, the BMS cannot compensate for the additional current drains as the data of all the control units present in the vehicle is already coded in the BMS.

Some of the passenger cars use two 12 Volts batteries, one is the main battery that supplies the power to the onboard electrical network and the other is an additional battery that can be used to support additional functions like automatic engine start-stop or engaging the parking gear.

The BMS is responsible for monitoring the state of charge of both batteries. An additional battery relay is controlled by the battery management system and the control unit actuates the relay when the state of charge of the additional battery is too low.

48 V battery management system

This battery management system is found in mild hybrid vehicles, the BMS is integrated into the 48 V high voltage battery. The battery management system in the 48 V system controls the charging voltage and current from the alternator.

Also, the battery management system acts as a DC/DC converter. There is a 12 V battery in the vehicles with a 48 V battery management system, the DC/DC converter is used to charge the 12 V onboard electrical battery.

Data from the battery sensor present on the 12 V battery is taken into account before the charging voltage is converted from the DC/DC converter. The BMS is also responsible for the thermal management of the 48 V battery.

There is a coolant circuit to cool the BMS and the 48 V battery system as heat is generated when the 48 V battery is being charged. If the cells of the battery overheat, oxygen is formed in the battery which reduces its capacity.

The mild hybrid systems use the 48 V system to power the components like the air conditioning compressor and PTC heater booster. It can also support the engine and develop additional power but the vehicle cannot be driven only on electrical power.

High voltage battery management system

The battery management system for high-voltage batteries is very complex. There can be more than three battery management systems, the number mostly depends on the size of the battery pack. In high voltage batteries, the BMS is responsible for charging and also monitoring the health of the individual lithium-ion cells.

If the cells get too hot, the percentage of oxygen increases and the cell voltage can drop below the specified value. The Battery management system also operates the master circuit breaker in the high voltage electrical system.

If a crash signal is detected, the battery management system will open the circuit breaker and prevent an electrical fire. It also checks the electrical voltage in the interlock circuit, if there is an open circuit in the interlock the high voltage electrical system will not work.

The temperature of the lithium-ion battery is constantly monitored by the BMS. in cold countries, the outside temperature can go below 0 degrees Celsius. The high voltage battery cannot be charged at cold temperature. Hence, the BMS used PTC heater boosters to keep the cell temperature above 7 degrees.

Why does the battery discharge despite a battery control module?

The battery discharges despite a battery control module if the charge balance becomes negative when the battery is not getting charged and there is still a current drain from the battery. The cause of the current drain can be a malfunctioning control unit that remains active even after the vehicle is turned off or there is cell damage in the battery.


This blog explained the function and use of battery management systems in different types of onboard electrical systems. The battery management systems can even operate the battery disconnect switch if the main battery voltage is going below 11 V to ensure the next engine start is possible.