Abstract:
BMS board protection is conducted by a custom designed circuit board, it keeps the battery in a safe status during charging/discharging.
Lithium ion battery is a portable energy unit, it’s made of chemical materials which designed and put in a limited space shell.
The specified chemical features and physical structure bring a risk of exploration and burning, so it need custom protections.
Lithium battery failure risks from chemical features
After learned the lithium ion battery cell structure , you will know that the lithium ion battery cell is mainly made of anode chemical material pole, cathode chemical pole, seperator, electrolyte, shell.
The electrolyte contains organic solvent and it’s heat stability is not very good, so the if the lithium ion battery themperature rised too much when overcharged, over discharged, short circuit, or in a high temperature working environment, it may cause thermal runway and swelling then cause a exploration and burning.
So it need BMS board protection to prevent possiable chemical risks.
Lithium battery failure risks from physical structure
When the lithium ion battery is overcharged, overdischarged, crushed, the seperator may broke which will cause a internal short circuit then swelling and then cause a exploration and burning.
In this case, the BMS board protection can prevent the lithium battery from overcharged and overdischarged, but it not work for preventing physical crush.
How BMS board conduct protection to the lithium battery pack
Below diagram is a simple illustration of BMS board connection with the lithium ion battery pack, each cell will be monitored by the IC.
The IC will monitor the voltage of each cell to prevent from cell overcharging and overdischarging.
A NTC or thermal controller will monitor the temperature of battery cell/pack and BMS board to prevent from high/low temperature.
Mosfets are used to control block/conduct the output/input current.
Lithium battery BMS protection types
BMS has hardareware protection and software protectioin.
Hardware protection like PTC fuse or thermal controller, they monitor battery temperature and trigger cut off the circuit directly by themselves.
Software protection mainly refer to the protection monitored and conduct by the IC, thery are programed code/software along witht the IC and some smart BMS usully support I2C/UART/CANbus communication or bluetooth communication.
Lithium battery BMS board types
There are common port BMS and seperate port BMS based on the charge/discharge output circuit layout as below.
Seperate port lithium battery BMS
Seperate port BMS as below, charge circuit port and discharge circuit port are seperated. In this way, usually only 1 mosfet to control the charge ,then usually the max charge current is ≤5A; Almost all ebike battery use separte port BMS.
Common port lithium battery BMS
Common port BMS as below, charge circuit port and discharge circuit port are shared. The charge circuit & mosfet are same, so the max charge current & discharge current are same,usually it requires to use common port BMS if customers need very fast charging.
Software smart lithium battery BMS
Software smart BMS usually has a UART/RS485/CANbus communication port as below picture.
Charge Port Short Circuit Protection Lithium battery BMS
Currently there are 2 general type charges.
Type1: Old version with a continuous ouput which mean the charges will always has a voltage/current output.
Type2: New version with battery connection detective fuction by use pulse, the pulse charger will have output voltage/current only when it detects the battery is connected.
Some pulse charger manufactures set a short time pulse which can not activate the mosfet on the BMS, experienced pulse manufactures would set a longer time pulse or ask a BMS sample to test and adjust the pulse time to fit the BMS mosfets.
But most time, it can not know the charger type from the charge specification, the charger manufacturs would ask us if need type1 or type 2 when we purchase chargers from them.
If the BMS charge port has output voltage, then both type 1 and type 2 chargers can work.
If the BMS charge port has voltage insulation design, then type 1 and type 2 with longer time pulse can work.
How to choose a suitable BMS for my lithium battery pack design
It mainly depands on the specific product design requirements as below.
What’s the specific application when use the lithium battery?
What’s the local market regulations reuirements for your product?
How long do you expect to full charge the lithium battery pack?
What‘s the working environments of the lithium battery?
What’s the requirements of lithium battery pack dicharge current?
