I'm trying to design a simple protection/charging circuit for a single LiIon 18650 Cell and think I made a mess with the Diodes Incorporated AP9211 IC [1]. It's my own fault as I skimmed through that data sheet but keyed off the typical application circuit and bread boarded that circuit. So the Over discharge circuit, which is the 'typical application circuit' given in the datasheet [1] worked.

My problem came when I decided I'd try to charge the 18650 again. I connected my charging voltage 5V to the P+ and P- connections of the 'typical circuit'. That resulted in no charging happening, so I went back to the datasheet and reading that discovered that the negative charging voltage should be connected to the VM Pin, Oops.

So I corrected my mistake, but again no charging happening, which is likely down to me having applied 5V across P+/P- so I might have blown the IC.

I only ordered up one to try this out but given I have to re-order to try again I have a few questions which someone probably has a much better idea of.

I'd like to have some indication that the cell is charged, either an LED turning on or off, and not sure how to add that to the AP9211 IC.

Secondly that IC is a bit fiddly to solder in that package and wonder has anybody got any advice/experience with a simple solution to over charge/over discharge protect a single 18650. I thought that IC looked the part but not sure now.

Thanks for any advice.

[1] https://www.diodes.com/assets/Datasheets/AP9211.pdf


1 Answer 1


If you want to add a LED indicator, you can do it with a zener diode, or an opamp. For rechargeable Li-on battery (4.2 V max) you will need a low dropout voltage LED (red - 2V) and a zener diode with a very small zener voltage (Vz - 1.2V) and that's very tough to find.

So I will suggest an opamp variation. If that's your complete board, you'll have to add a voltage reference of aobut 4V for the comparator. The LED will lights when the battery is charged.

Anyway, you can try to find a charger IC with a LED output. If you want an easier handling, I would suggest not to use a QFN package. You can take a look at bq24091 - TSSOP package with LED charged output.

  • 1
    \$\begingroup\$ Thank you for that advice. Been looking over the datasheet for the bq249x and looks perfect. In a shopping basket so I'll try it out. Thanks again for your help. \$\endgroup\$
    – jwhitmore
    Apr 18, 2021 at 20:58

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