Lithium-ion batteries are widely used in different applications. The material chemistry of lithium-ion batteries cannot withstand overcharge, over-discharge, over-current, short circuit and ultra-high temperature. So lithium-ion batteries need BMS to ensure the reliability and safety of the battery. The battery management system is the brain of the lithium battery, reporting the status and health of the battery. Let's get a better understanding from this article.

What is a BMS system?

The BMS on the battery is a circuit protection element. The battery management system is used for lithium-ion batteries, lifepo4 battery packs and lithium polymer batteries. BMS monitors and controls the voltage and current of the battery.

The main components of the battery BMS:

1.Printed Circuit Board: Common PCB board types include single board, double-sided board and four-layer board.

2.IC: The best BMS IC for lithium battery must use brand-name IC, which determines the price and quality.

3.Mosfet: Acts as a switch in the circuit. However, the on-resistance of MOSFET affects the battery performance. High-quality Mosfet has a smaller on-resistance, which makes the lithium battery have a smaller on-resistance and a stronger load. In addition, high-quality MOSFET has extremely low power consumption.

4.NTC: Measures the side temperature of lithium-ion batteries.

How does BMS work?

1. Activate the battery management system
When BMS P+ and P- are in the protection state, there is no output. Short B+ and B- to activate BMS, and Dout and Cout will be at a low level (the two ports of protection are high-level protection). The state supports the switch to be turned on.

2. Charging
P+ and P- are connected to the positive and negative poles of the charger. The charging current charges the battery through the MOS. The VDD and VSS of the protection IC are the power supply end and the battery cell voltage detection end. The voltage of the battery cell continues to rise. When it rises to the protection voltage of the battery cell (overcharge protection voltage), COUT will output a high level at this time, turn off the corresponding MOS switch, and the charging circuit will also be turned off. After overcharge protection, the battery cell voltage will drop. When it drops to the IC voltage threshold (overcharge protection recovery voltage), Cout returns to a low level state and turns on the MOS tube.

3. Discharge
When the battery is discharged, the VDD and VSS of the IC will also detect the battery voltage. When the cell voltage drops to the IC threshold (over-discharge protection voltage), Dout randomly outputs a high level and turns off the corresponding MOS tube. The discharge circuit is disconnected. After over-discharge protection, the cell voltage will rise. When it rises to the threshold voltage (over-discharge protection recovery voltage), Dout returns to a low level and turns on the MOS switch.

4. Overcurrent, short circuit
When the main circuit current is too large during the discharge process, due to the internal resistance of the MOS saturation conduction, when the current flows between B- and P-, a voltage drop will occur at both ends of the MOS tube to protect V - and the VSS of the IC (through R2) will detect the voltage at both ends at any time. When the voltage rises to the detection threshold of the protection IC (generally 0.15V, discharge detection overcurrent voltage), Dout immediately outputs a high level to turn off the corresponding MOS tube, and the discharge circuit is disconnected.

5. NTC working process
There will be no overcharge, overcurrent, and over-discharge when the battery is running. However, due to long-term operation, the battery temperature will rise, so the NTC is placed close to the battery cell to monitor the battery temperature. As the temperature rises, the resistance of the NTC increases. When the resistance drops to the set value, the CPU will issue a shutdown command to stop charging the battery, thereby protecting the battery.

BMS has protection functions such as overcharge, discharge, short circuit, and temperature protection.

Main functions of the battery management system

1. Overcharge protection
Overcharge protection refers to the fact that when the lithium battery is charging, as the voltage rises beyond the reasonable range, it will bring uncertain dangers. The overcharge protection function of the protection board is to monitor the voltage of the battery pack in real time. When it is charged to the top of the safe voltage range, it will cut off the power supply to prevent the voltage from continuing to rise, thereby playing a protective role.

During charging, the protection board will monitor the voltage of each string of the battery pack in real time. As long as one of the strings reaches the overcharge protection value (default charging voltage 3.75V±0.05V), the protection board will cut off the power supply and the entire group of lithium batteries will stop charging.

2. Over-discharge protection
Over-discharge protection refers to the fact that when the lithium battery is discharged, as the voltage drops, if all the power is discharged, the chemical substances inside the lithium battery will lose their activity, resulting in failure to charge or a decrease in capacity. The over-discharge protection function of the protection board is to monitor the voltage of the battery pack in real time. When the battery voltage is discharged to the lowest point, it will cut off the power supply to prevent the voltage from continuing to drop, thereby playing a protective role.

During discharge, the protection board will monitor the voltage of each string of the battery pack in real time. As long as one of the strings reaches the over-discharge protection value (the default over-discharge voltage of ternary batteries is 2.7V±0.1V, and the default over-discharge voltage of iron-lithium batteries is 2.2VV±0.1V), the protection board will cut off the power supply and the entire lithium battery pack will stop discharging.

3. Overcurrent protection
Overcurrent protection means that when the lithium battery supplies power to the load, the current will change with the change of voltage and power. When the current is large, it is easy to burn the protection board, battery or equipment. The overcurrent protection function of the protection board is to monitor the current of the battery pack in real time during the charging and discharging process. The overcurrent protection circuit cuts off the current when the current exceeds the safe range to protect the battery or equipment from damage.

During charging and discharging, the protection board monitors the current of the battery pack in real time. Once the set overcurrent protection value is reached, the protection board will cut off the power supply and the entire lithium battery will stop charging and discharging.

4. Short circuit protection
When the positive and negative poles of the battery are directly connected without any load, a short circuit is formed. Short circuits may cause damage to the battery and equipment.

When the lithium battery accidentally causes a short circuit (such as wrong wiring, wrong wiring, water ingress, etc.), the protection board will cut off the flow of current in a very short time (0.00025 seconds), thereby protecting the battery. Protective effect.

5. Temperature protection
Temperature control protection: The temperature control probe of the hardware protection board is welded on the main board inside the protection board and cannot be plugged in or out. The temperature control probe can monitor the temperature changes of the battery pack or the working environment in real time. The temperature control protection system of the battery pack will disconnect the charge and discharge when the temperature exceeds the set value (default: charging -20~55℃, discharging -40~75℃). When the temperature returns to a reasonable range, the system will reconnect the charge and discharge.

6. Balance protection
Passive balancing means that when there is a voltage inconsistency between battery strings, the protection board will adjust the voltage of each string to be consistent during charging.

When the protection board detects a voltage difference between lithium battery packs. When charging, when the number of high-voltage strings reaches the balance value (LiNiCoMnO30: 35V, LiFePO2: 4.13V), the protection board discharges (consumes) about 4-3.525mA from the high-voltage string through the balancing resistor, and the other low-voltage strings continue to charge until they are fully charged.

Battery BMS is the heart of the battery pack. The battery management system (BMS) reports the battery status and performance of the lithium-ion battery pack.