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I came across this answer How do I test rechargeable batteries? to the question of how to test rechargeable AA batteries and plan to try it. What I am wondering is how to apply this to other types of batteries. In particular, I have something that uses a pair of CR123 Li-Ion batteries sealed together in series to make a single rechargeable battery that is supposed to produce 7.2 volts and have a capacity of 500mAh or 3.6 watt/hours. How much resistance should I use to test it and what results should I expect both initially and after waiting a few days? What voltage should I consider to be too low for the battery to be considered healthy? Thanks!

Note added on August 23, 2021: Since I could not get any advice on the right size resistor to use I decided to try the same size that was advised for AA batteries: 10 ohms. It seems like this is providing good information but a word of warning to anyone else who might want to try this method. Li-ion batteries can put out a lot of watts, enough to overload many resistors. Make sure the resistor you use is able to handle the wattage that a Li-ion battery will generate.

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So you probably did not read the answer for the question you mentioned carefully enough.

What you may do is buying a digital multimeter and just check the voltage of battery. If battery is charged the voltage should be arround 7.2V

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  • Thank you. The key is like the original questioner I'm trying to determine not just whether the battery is fully charged but how much of its original ability to hold a charge it still retains. I think this is often referred to as battery health and simply being able to accept a full charge is not a good measure of this. Based on DrMoishe Pippik's answer, the solution appears to be to test the battery under load but the question is given the different battery type I am dealing with, how much load (resistance) to introduce and what voltage when tested under load should be considered too low.
    – Bruce
    Jul 23 at 13:14
  • What? That's not how rechargeable batteries work. Voltage is not the same as capacity.
    – piojo
    Sep 2 at 10:45
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If you know anyone who fits intruder alarms, they use a device that discharges alarm batteries once only to measure their predicted capacity. The more modern devices can test the capacity of a wider range of batteries.

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