The role, composition, and working process of lithium battery protection board

Lithium element is in the third position in the periodic table, only one electron in the outer layer, is a very active metal, and lithium-ion batteries have a high discharge current, low internal resistance, long life, no memory effect, etc. are widely used, lithium-ion batteries are strictly prohibited in the use of overcharge, over-discharge, and short circuit problems; otherwise, the battery will fire or explosion and other issues. Therefore, a protection board circuit is usually added to the lithium battery circuit to protect the safety of lithium-ion batteries.

The role of the lithium battery protection board

A battery protection board usually has the following roles: overcharge, over-discharge, overcurrent, short circuit, and high-temperature protection. The above roles are also determined by the material of the lithium battery itself. A battery protection board usually has a protection circuit board and PTC devices. The protection circuit board will always monitor the voltage of the battery cell and the charging and discharging current and timely control the current circuit on and off.

Figure 1-1 Battery structure

Composition of the battery protection board

The protection board usually includes the control IC, MOS switch, resistor, capacitor, PTC, NTC, ID, and memory components, among which the control IC, under all normal conditions, controls the MOS switch on, making the cell and external circuit on. When the cell voltage or circuit current exceeds the specified value, it immediately controls the MOS switch off to protect the safety of the cell.

The protection mechanism of the battery protection board

Figure 1-2 Battery protection board circuit

Main functions of the individual PINs of the protection IC.

• VDD is the positive side of the power supply of the protection IC

• VSS is the negative side of the power supply of the protection IC

• V- is the over-current and short-circuit detection terminal of the protection IC

• Doubt is the discharge protection terminal

• Cout is the charge protection terminal

B+ and B- are connected to the positive and negative terminals of the cell, respectively, P+ and P- are the positive and negative terminals of the output of the protection board, respectively, T is the NTC port for temperature detection, and the NTC port needs to cooperate with the MCU for temperature protection of the battery. This port is also sometimes marked as ID for battery port identification. The reason for adding this port is that different manufacturers of different models of batteries often require different battery parameters (aging curve and other parameters). When it is ID, R3 is generally a fixed resistor resistance value, and the MCU's ADC will detect the voltage of ID so that different battery parameters can be used.

Figure 1-3 Protection board circuit

The working process of the protection board

1、The method of activating the protection board

When the protection board P+ and P- no output in the protection state, you can short circuit B+ and B- to activate the protection board, then Dout and Cout will be at a low level (protection IC these two ports are high-level protection, low-level normal state) state to open the two switches.

2、Charging

P+ and P- are connected to the positive and negative poles of the charger, charging current through the two MOS to charge the battery this time, the protection IC VDD and VSS is both the power supply terminal but also the cell voltage detection terminal (after R1), with the charging process, the voltage of the cell is increasing, when it rises to the protection voltage of the cell (overcharge protection voltage), this time COUT will output a high level to turn off the corresponding MOS switch. Corresponding MOS switch off, the charging circuit will also be shut down. After the overcharge protection, the core voltage will drop. When down to the IC voltage threshold (overcharge protection recovery voltage), Cout return to the low-level state to open the MOS tube.

3、Discharge

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

4、Over-current and short circuit

When the discharge process of the main circuit current is too large, due to MOS saturation conduction also exists internal resistance, so the current in the flow between B- and P- MOS tube ends will produce a voltage drop, the protection IC V- and VSS (after R2) will always detect the voltage at both ends, when the voltage rises to the detection threshold of the protection IC (generally 0.15V, discharge detection overcurrent voltage), Dout immediately output High level will correspond to the MOS tube shutdown, the discharge circuit is broken.

Another question, if the choice of low on-state resistance MOS or discharge overcurrent detection voltage of the protection IC, is it possible to obtain a large output current? The answer is yes, but do not forget to consider the choice of MOS power and the capacity of the core (usually cell phones and other mobile devices work at a high frequency, to reduce the ripple of the whole system, MOS tube on-state resistance The choice must be as small as possible).

5, the role of the NTC port

When the battery is working, there are no overshoot, overcurrent, and over-discharge problems, and the working time is too long, resulting in the cell temperature rise. NTC is close to the cell to monitor the battery temperature. As the temperature rises, the resistance value of NTC gradually decreases. When the resistance value drops to a set value, the CPU issues a shutdown command to stop charging the battery to protect the battery.

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