What should I do if the voltage of the lithium battery exceeds its withstand voltage during the charging or discharging process?
Obviously, for a single-cell lithium battery, its voltage is limited, and it has a maximum operating voltage value. If the voltage exceeds the maximum value, the lithium battery will crack and explode like a capacitor. Therefore, it is necessary for engineers to add a circuit for the overvoltage protection function of lithium batteries.
How to design this circuit?
In this BQ294682 chip application circuit, it is one of the solutions for overvoltage protection of lithium batteries.
The V1 pin of the chip is connected to the positive electrode of the lithium battery through the resistor R1, and the negative electrode of the lithium battery is directly connected to the VSS pin of the chip.
Relying on the function of this V1 pin, the BQ294682 chip can monitor the voltage of the lithium battery from time to time; once the voltage of the lithium battery exceeds the threshold set by the chip, the chip will start the program to protect the lithium battery.
The threshold value here is a key indicator, which can directly determine the magnitude of the overvoltage voltage that protects the lithium battery. In the BQ294682 chip, its threshold is 4.225V.
That is to say, as long as the voltage of the lithium battery exceeds 4.225V during the charging or discharging process, the chip will protect the lithium battery.
The question is, how is its protection achieved?
The 6-pin of the chip is an OUT output pin, which directly controls the gate of a MOS tube. And this MOS tube is directly connected to the positive pole of the lithium battery pack.
The chip can realize the protection function of lithium battery overvoltage by controlling whether the MOS tube is turned on or not. When the lithium battery voltage exceeds the threshold voltage of 4.225V and lasts for more than 4 seconds, the OUT pin of the chip will output a high level, and the MOS tube will be turned on.
At this time, the positive electrode of the lithium battery pack will be pulled down due to the conduction of the MOS tube, which is equivalent to a direct short circuit to the ground wire GND.
What happens to a short circuit?
The self-recovery fuse in the circuit will be blown, and the lithium battery will be disconnected from the external circuit, thus playing a protective function.
If the voltage of the lithium battery is lower than 3.925V, the chip will resume its normal working state, and the lithium battery will also enter the normal working mode.
Resistor R1 and capacitor C2 are an RC filter, which can filter out clutter and make the voltage entering the V1 pin of the chip more stable.
The VDD power pin of the chip is connected to the positive pole of the lithium battery through the resistor R2, where the resistance value of the resistor R2 is 200Ω for current limiting. Capacitor C1 is for filtering.
The rated current of the resettable fuse can be selected according to the actual needs of the project. Here, the chip brother chose 2A.
This is the lithium battery overvoltage protection circuit scheme of the BQ294682 chip.