Function design method of lithium battery protection board
The protection function of lithium battery pack is usually completed by a protection circuit board and current devices such as PTC. The protection board is composed of electronic circuits, which can accurately monitor the voltage and charge of the battery cell at all times under the environment of -40℃ to +85℃. Discharge the current of the loop, and control the on and off of the current loop in time; PTC prevents the battery from being badly damaged in a high temperature environment.
Ordinary lithium battery protection boards usually include control ICs, MOS switches, resistors, capacitors and auxiliary devices FUSE, PTC, NTC, ID, memory, etc. Among them, the control IC controls the MOS switch to turn on under all normal conditions to make the cell and the external circuit conduct, and when the cell voltage or loop current exceeds the specified value, it immediately controls the MOS switch to turn off to protect the cell’s Safety.
When the protection board is normal, Vdd is high, Vss and VM are low, DO and CO are high. When any parameter of Vdd, Vss, VM is changed, The level of the DO or CO terminal will change.
1. Normal state
In the normal state, the 'CO' and 'DO' pins of N1 both output high voltage, and both MOSFETs are in the on state, and the battery can Charge and discharge freely. Because the on-resistance of the MOSFET is very small, usually less than 30 milliohms, its on-resistance has little effect on the performance of the circuit. In this state, the current consumption of the protection circuit is μA, usually less than 7 μA.
2. Overcharge protection
The charging method required for lithium-ion batteries is constant current/constant voltage. In the initial stage of charging, it is constant current charging. As the charging process, the voltage will rise To 4.2V (depending on the positive electrode material, some batteries require a constant voltage value of 4.1V), switch to constant voltage charging until the current becomes smaller and smaller. When the battery is being charged, if the charger circuit loses control, the battery voltage will continue to be charged with constant current after the battery voltage exceeds 4.2V. At this time, the battery voltage will continue to rise. When the battery voltage is charged to more than 4.3V, the battery’s chemistry Side reactions will intensify, causing battery damage or safety issues. 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, different ICs have different values), its 'CO' pin will change from high voltage to zero voltage. Turn V2 from on to off, thereby cutting off the charging circuit, so that the charger can no longer charge the battery, playing a role of overcharge protection. At this time, due to the existence of the body diode VD2 of V2, 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 length of the delay time is determined by C3 and is usually set to about 1 second to avoid errors caused by interference. judgment.
3. Over-discharge protection
In the process of discharging the battery to an external load, its voltage will gradually decrease with the discharge process. When the battery voltage drops to 2.5V, its capacity has been When the battery is completely discharged, if the battery continues to discharge to the load at this time, it will cause permanent damage to the battery. During the battery discharge process, when the control IC detects that the battery voltage is lower than 2.3V (this value is determined by the control IC, different ICs have different values), its 'DO' pin will change from high voltage to zero voltage, making V1 Turning from on to off, which cuts off the discharge circuit, so that the battery can no longer discharge the load, playing a role of over-discharge protection. At this time, due to the existence of the body diode VD1 of V1, the charger can charge the battery through this diode. Because the battery voltage can't be lowered in the over-discharge protection state, the current consumption of the protection circuit is required to be extremely small. At this time, the control IC will enter a low power consumption state, and the power consumption of the entire protection circuit will be less than 0.1μA. There is also a delay time between when the control IC detects that the battery voltage is lower than 2.3V and when the V1 signal is turned off. The length of the delay time is determined by C3 and is usually set to about 100 milliseconds to avoid errors caused by interference. Judgment.
4. Short circuit protection
When the battery is discharging the load, if the loop current is so large that U>0.9V (this value is determined by the control IC, different ICs have When the value is different), the control IC judges that the load is short-circuited, and its 'DO' pin will quickly change from high voltage to zero voltage, so that V1 will be turned from on to off, thereby cutting off the discharge circuit and playing a short-circuit protection role. The delay time of short circuit protection is extremely short, usually less than 7 microseconds. Its working principle is similar to that of over-current protection, but the judgment method is different, and the protection delay time is also different. In addition to the control IC, there is also an important component in the circuit, which is the MOSFET, which acts as a switch in the circuit. Because it is directly connected in series between the battery and the external load, its on-resistance affects the performance of the battery. Influence, when the selected MOSFET is better, its on-resistance is small, the internal resistance of the battery pack is small, the load capacity is also strong, and it consumes less electric energy when discharging.
5. Overcurrent protection
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