13: Protection circuit Part 1

Protection circuit is necessary when manufacturing lithium-ion batteries.
In rare cases, there is also a protection method to stop the discharge current by a fuse and so on instead of protection by an electronic circuit.

Normally, it is necessary to prevent accidents in various situations by protection circuit.
The contents of protection of the protection circuit that is normally done are the following three items.

  1. Stop charging during overcharging
  2. Stop discharging during over discharging
  3. Stop large current discharge such as short circuit

What is stopping of charging during overcharging?

In protection during overcharging, protection of each lithium-ion cell by charging voltage, rather than the total voltage in series, is performed.
For example, in the case of 2-direct, although the total voltage is 4.2V×2=8.4V, if the balance of the lithium-ion cell collapses and it becomes a combination such as 3V and 4V, there is a possibility of overcharging of the cell.

It is a function to monitor the voltage of each cell to prevent such case and to stop charging if there is at least one cell somewhere that is overcharged.
The overcharge protection voltage is normally set at 4.35V, but it is a setting considering the variation of voltage.

If the charging voltage of the cell is 4.2V, the charger will be impressed to 4.2V±0.05V and up to 4.25V.
If the overcharge protection voltage setting is set to 4.35V, it means that a margin of 0.05V is expected.

When the voltage of the lithium-ion battery reaches 4.35V, the protection circuit operates and stops charging.
When the voltage of the lithium-ion battery drops below the set voltage by discharging the battery (by leaving it in overcharge protection state and due to voltage drop through self-discharge), the overcharge protection state is released and charging becomes possible.

In addition, considering the case when the voltage of the detection part instantaneously exceeds 4.35V due to the noise, a delay is set.
There is also a setting method in which overcharge protection operates when the set overcharge protection setting voltage is exceeded.

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