15: Protection circuit Part 3

Stopping large current discharging such as a short circuit

Over current protection prevents large current from continuing to flow due to accident such as external short circuit.
Normally it stops when the voltage at the ON resistance of the FET for discharge switch exceeds the set value for current detection.

For example, for a laptop computer pack, if the maximum discharge current is about 3A, the over current protection current is generally about 2 to 3 times of it.

It is common to use the standard protection IC released by several manufacturers in order to carry out these three kinds of protection.

Using standard IC by combining it
If it is 7-direct, there is also a method of protection by separating ones for single cells into seven.
FET used for charging and discharging switches must withstand the full voltage.

Because the protection circuit cannot reduce the breakdown and malfunction to zero, double protections are required except for battery packs for mobile phones.
The first protection circuit becomes main, and the second protection function protects when the main protection function does not operate.

Please note that after the second protection circuit operates,the function of lithium-ion batteries will not recover.
When using a fuse (current, temperature and so on) as a protection function, if the fuse breaks, the battery pack will not function.

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