MAXKGO Classroom (II) : What does BMS do

The basic function of BMS is to monitor and calculate battery parameters, including basic parameters and states such as voltage, current, and temperature, as well as the calculation of battery state data such as SOC and SOH. The field of power batteries also involves the calculation of SOP (state of power) and SOE (state of energy).

① Monitor

A BMS may monitor the state of the battery as represented by various items, such as:

· Voltage: total voltage, voltages of individual cells, or voltage of periodic taps

· Temperature: average temperature, coolant intake temperature, coolant output temperature, or temperatures of individual cells

· Coolant flow: for liquid cooled batteries

· Current: current in or out of the battery

· Health of individual cells

· State of balance of cells

②Computation

Additionally, a BMS may calculate values based on the below items, such as:

· Voltage: minimum and maximum cell voltage

· State of charge (SoC) or depth of discharge (DoD), to indicate the charge level of the battery

· State of health (SoH), a variously-defined measurement of the remaining capacity of the battery as % of the original capacity

· State of power (SoP), the amount of power available for a defined time interval given the current power usage, temperature and other conditions

· State of Safety (SOS)

· Maximum charge current as a charge current limit (CCL)

· Maximum discharge current as a discharge current limit (DCL)

· Energy [kWh] delivered since last charge or charge cycle

· Internal impedance of a cell (to determine open circuit voltage)

· Charge [Ah] delivered or stored (sometimes this feature is called Coulomb counter)

· Total energy delivered since first use

· Total operating time since first use

· Total number of cycles

· Temperature Monitoring

· Coolant flow for air or liquid cooled batteries

 

3.2 Management-balanced management

Each battery is different. Even batteries produced in the same batch by the same manufacturer have their own life cycles, and the capacity of each battery cannot be exactly the same. There are two reasons for this difference:
· Inconsistency in cell production
In the production process, the diaphragm is inconsistent, and the cathode and anode materials are inconsistent, resulting in inconsistent overall battery capacity.
· Inconsistency of electrochemical reactions
It means that in the process of charging and discharging the battery, even if the production and processing of the two cells are exactly the same, the thermal environment will never be the same during the electrochemical reaction process. The temperature of the battery is definitely lower than the middle, which causes the long-term inconsistency of the charging capacity and the discharging capacity, the inconsistent capacity of the battery cells, and the inconsistent aging speed.
This is why batteries need to be balanced.

In order to maximize the battery's capacity, and to prevent localized under-charging or over-charging, the BMS may actively ensure that all the cells that compose the battery are kept at the same voltage or State of Charge, through balancing. The BMS can balance the cells by:

• Passive balance：Wasting energy from the most charged cells by connecting them to a load (such as through passive regulators)
• Active balance：Shuffling energy from the most charged cells to the least charged cells (balancers)
• Reducing the charging current to a sufficiently low level that will not damage fully charged cells, while less charged cells may continue to charge (does not apply to Lithium chemistry cells)

3.3 Protection-Troubleshooting and Alarming

A BMS may protect its battery by preventing it from operating outside its safe operating area, such as：

· Over-current during charging

· Over-current during discharge

· Over-voltage during charging, especially important for lead–acid, Li-ion and LiFePO4 cells

· Under-voltage during discharging, especially important for Li-ion and LiFePO4 cells

· Over-temperature

· Charging while under temperature

· Over-pressure (NiMH batteries)

· Ground fault or leakage current detection (system monitoring that the high voltage battery is electrically disconnected from any conductive object touchable to use like vehicle body)

The BMS monitoring is matched with the hardware of the electrical system. According to the different performance conditions of the battery, it is divided into different fault levels (minor fault, serious fault, fatal fault), and different processing measures are taken under different fault levels: warning, power limit or Cut off the high voltage directly. Faults include data acquisition and rationality faults, electrical faults (sensors and actuators), communication faults and battery status faults.

A common example is that when the battery is overheated, the BMS judges that the battery is overheated according to the collected battery temperature, and then disconnects the circuit that controls the battery, performs overheating protection, and issues an alarm to management systems such as EMS.

3.4 Communication

The normal operation of the BMS is inseparable from the communication function of the BMS. Whether it is to control the battery during battery management, or to transmit the battery status and receive control commands, stable communication is required.

In the power battery system, one end of the BMS is connected to the battery, and the other end is connected to the control and electronic system of the vehicle. The CAN protocol is used in the environment, but the internal CAN is used between the internal components of the battery pack, and the battery pack is connected to the vehicle. Use the vehicle CAN to distinguish between them.

In contrast, the communication between the energy storage BMS and the internal communication basically adopts the CAN protocol, but the external communication (the external mainly refers to the energy storage power station dispatching system PCS) usually adopts the Internet protocol format TCP/IP protocol and modbus protocol.

With the rise of new energy sources, the prospect of battery applications will become more and more broad. As an indispensable part of battery development, BMS will continue to improve and better use the development of the industry.

The future BMS will be more integrated and simplified . This is also the direction that MAXKGO has been striving for. Compared with similar brand manufacturers, our BMS's size has been reduced to more than half of the competing products. Quality material, save more space and reduce weight for everyone's DIY creation.

Thanks for watching, see you on the next blog~