Involvement:Expert (understands the inner workings) - Ownership:more than 1 month
This protection circuit fits on the negative (-) side of standard 18650 lithium cells found in laptop batteries. It actually has 3 wires in the shape of thin stripes. The positive wire is the long one. It only taps the positive terminal of the battery. The shorter negative lead is connected to the battery's can and it is connected internally to MOSFETs that can break the circuit if certain conditions are met. Even though it shouldn't be used like that, it is entirely possible to use this circuit to stop the charging when the voltage reaches its maximum value.
It is incompatible with batteries that contain more cells in series. When first installed, the circuit is open by the protection and one has to connect the in- and out-negative leads briefly to kick-start the battery. Otherwise, one may connect the charger. When in the OFF state, the circuit will never come on by itself, an external voltage must be applied.
If you intend to use a cordless tool in the field, you might consider installing a button with a diode in series that will temporarily send voltage across the circuit into the load side to revive the tool after some overloading situation.
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