You have all heard that Lithium polymer batteries are dangerous if not handled properly. It’s so because of its high energy density and the chemical/mechanical structure of the battery. You have to ensure that the operating specs are maintained all the time. ie) Cell voltage shouldn’t exceed an upper or a lower limit, the current draw is maintained within a limit to adhere to the temperature specs. All these are made possible by a Battery Management System (BMS).

The most common chip you use for protection is a small 6-pin chip known as DW01(Available for less than $0.02). Check the schematic images. It’s usually paired with a Dual back-to-back MOSFET arrangement that gets controlled by DW01. You usually get this as a single IC package In normal operation both these MOSFETs are ON. For large charge/discharge currents, just connect multiple of these paired MOSFETs in parallel or use a pair of large power MOSFETs. You have 2 MOSFETs instead of one, because current can flow into a battery and out of it. When the MOSFET is OFF to disconnect, there can still be current flowing via the body diode which is prevented with with the Dual MOSFET setup.

Lets assume a charger that charges post the nominal cell voltage of a battery, the MOSFET M2 is turned OFF and the battery is cutoff from the charger. It’s turned ON when the battery either self-discharges or its voltage drops below a limit via a load. For the over-discharge case, when the battery falls below say 2.5V or so, M1 turns OFF and the load is disconnected from the battery. Now it turns back only when a charger pulls the voltage above 3V. It also has a short circuit current detector to cut off when a temp short happens.
So use the small protection circuit whenever you use a bare Lipo or Li-Ion cell. It’s very cheap to implement or buy off the shelf to add to your cell. Please don’t use a cell without it. It’s simply not worth the risk. I only talked about single-cell protection, for battery packs there are a lot of considerations, that I can go into in the future if there is interest.
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