I just found that AP9211 chip as cheap and alternative replacement to previous Microchip IC that price has skyrocketed and is unavailable until next year (maybe more).

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With previous chip there was a common GND I just got the safe voltage for the cell from 5V, cell had the circuit attached to it over diode, another diode was giving the 5V to the circuit, so when charger was connected cell was charged with a safe voltage but circuit took power from the charger, when there was no charger circuit took the power from battery, as simple as that. But there was a common ground to both cell and the circuit so two schottky diodes solved the power distribution.

To be honest I do not really understand this AP9211 approach. It seems to disconnect ground from the charger so the circuit is always powered from the cell? There can be no common GND with this chip? Does not this approach degrade cell in a situation when charger is connected all the time? I cannot just put common charger ground to the battery and the circuit because then AP9211 has no purpose.

How do I connect the battery with AP9211 to the rest of the circuit?

Is this chip charger only and prevent loading the cell?

Any hints welcome :-)


1 Answer 1


It looks like the AP9211 works by using dual N-Ch MOSFETs to switch the negative battery terminal. So in a fault condition, the negative terminal of the battery is disconnected from the rest of the circuit.

Remember that "ground" is arbitrary and can be any node in your circuit. In their diagram, they draw ground as connected to the negative terminal of the battery. However, it may be easier to think of your ground as the "P-" terminal in the diagram. Your "battery" then becomes a collection of both the battery cell and protection IC together.

Battery + Protection IC

The "common ground" in your circuit is then the 5V supply ground, charger ground, and P- terminal together. You can diode or the 5V supply and battery output together. In a fault case, the protection IC will work as expected. I did not have much time so here is a very basic diagram showing what this looks like.

Full System

Hopefully that answers your question...

  • \$\begingroup\$ Ah, so this is only the protection device, not the charger. Thanks! :-) \$\endgroup\$
    – CeDeROM
    Apr 9 at 22:32
  • 1
    \$\begingroup\$ @CeDeROM Correct, it appears this IC does not perform charging. It may do things such as overcurrent limit or overvoltage limit when charging but does not perform any regulation. \$\endgroup\$ Apr 11 at 0:30

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