Detailed analysis of the principle of lithium battery protection board

The reason why lithium batteries (chargeable) need protection is determined by its own characteristics. Because the material of the lithium battery itself cannot be overcharged, over-discharged, over-current, short-circuited and ultra-high temperature charge and discharge, the lithium battery lithium battery assembly will always follow an exquisite protection board and a current fuse.

Lithium battery protection function is usually performed by protection circuit board and PTC current device coordination, protective plate is composed of electronic circuits, and 40 ℃ to + 85 ℃ under the environment of time accurate monitoring batteries voltage and circuit current and the control current loop in time on and off; PTC prevents severe battery damage at high temperatures.

Ordinary lithium battery protection boards usually include control ICs, MOS switches, resistors, capacitors, and auxiliary devices FUSE, PTC, NTC, ID, memory, and so on. The control IC controls the MOS switch to be turned on under all normal conditions, so that the cell and the external circuit are turned on, and when the cell voltage or the loop current exceeds a prescribed value, it immediately controls the MOS switch to turn off, and protects the cell safety.

Detailed analysis of the principle of lithium battery protection board

When the protection board is normal, Vdd is high level, Vss, VM is low level, DO and CO is high level. When any parameter of Vdd, Vss, VM is changed, the level of DO or CO will be A change has occurred.

1. Overcharge detection voltage: In the normal state, Vdd gradually rises to the voltage between VDD and VSS when the CO terminal changes from a high level to a low level.

2. Overcharge release voltage: In the state of charge, Vdd gradually decreases to the voltage between VDD and VSS when the CO terminal changes from low level to high level.

3. Over discharge detection voltage: In the normal state, Vdd gradually decreases to the voltage between VDD and VSS when DO terminal changes from high level to low level.

4. Over discharge release voltage: In the over discharge state, Vdd gradually rises to the voltage between VDD and VSS when DO terminal changes from low level to high level.

5. Over current 1 detection voltage: In the normal state, the VM gradually rises to the voltage between VM and VSS when DO changes from high level to low level.

6. Over current 2 detection voltage: In the normal state, the VM rises from OV at a speed of 1 ms or more and 4 ms or less to the voltage between VM and VSS when DO terminal changes from a high level to a low level.

7. Load short-circuit detection voltage: In the normal state, the VM rises at a speed of 1 μS or more and 50 μS or less from OV to the voltage between VM and VSS when DO terminal changes from a high level to a low level.

8. Charger detection voltage: In the over-discharge state, the VM gradually drops to OV and the VM-VSS voltage changes from low level to high level.

9. Current consumption during normal operation: In the normal state, the current flowing through the VDD terminal (IDD) is the current consumption during normal operation.

10. Over-discharge current consumption: In the discharge state, the current flowing through the VDD terminal (IDD) is the over-current discharge current consumption.

1, Normal state

In the normal state, the "CO" and "DO" pins of N1 output high voltage in the circuit, both MOSFETs are in conduction state, and the battery can be freely charged and discharged. Since the on-resistance of the MOSFET is small, it is usually smaller than 30 milliohms, so its on-resistance has little effect on the performance of the circuit. The current consumption of the protection circuit in this state is μA level, usually less than 7μA.

2, Overcharge protection

Lithium-ion batteries require a constant current/constant voltage. In the initial stage of charging, they are charged at a constant current. As the charging process, the voltage rises to 4.2V (depending on the cathode material, some batteries require a constant voltage of 4.1V), turn to constant voltage charging until the current is getting smaller and smaller. When the battery is being charged, if the charger circuit loses control, the battery voltage will exceed 4.2V and continue constant current charging. At this time, the battery voltage will continue to rise. When the battery voltage is charged to over 4.3V, the battery chemical side reactions will increase, causing battery damage or safety problems. In a battery with a protection circuit, when the control IC detects that the battery voltage reaches 4.28V (this value is determined by the control IC and different ICs have different values), its "CO" pin will change from high voltage to zero voltage. In order to turn V2 from on to off, the charging circuit is cut off, so that the charger can no longer charge the battery, which acts as an overcharge protection. At this time, due to the presence of the V2 diode VD2, the battery can discharge the external load through the diode. There is a delay time between when the control IC detects that the battery voltage exceeds 4.28V and when the V2 signal is turned off. The duration of this delay is determined by C3 and is usually set at about 1 second to avoid misjudgment due to interference.

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