What is a protection circuit for?
A protection circuit is mandatory for batteries manufactured from Lithium ion cells. Basically, the protection circuit sits between the cells and the host device and shuts down the battery (via a pair of MOSFETS) if it monitors an abnormal condition such as over voltage (battery is being overcharged), under-voltage (battery is being over-discharged), over current (Battery is being charged or discharged at too high a rate) or short circuit (battery terminals have been short circuited). Protection circuits can be hard wired, software driven or both. Most multi-cell Lithium Ion batteries now have two levels of protection circuit with the first being software programmable and acting through a pair of MOSFETS and the second being a hard wired non resettable shutdown via a logic fuse.