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Is there a way I can test the capacity of a 3.7v rechargable lion battery like this at home? All I really have is a multimeter.

Image of batteries originally from eBay

(Image source)

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  • \$\begingroup\$ Ask goolge :( techwalla.com/articles/how-to-measure-mah-with-a-multimeter \$\endgroup\$ – narzan Jun 29 '17 at 23:11
  • \$\begingroup\$ @narzan Don't think I have batttery check \$\endgroup\$ – TheMidnightNarwhal Jun 29 '17 at 23:14
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    \$\begingroup\$ Put a known load on it and start a timer? ;) \$\endgroup\$ – DigitalNinja Jun 29 '17 at 23:42
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    \$\begingroup\$ Fake batteries: A full answer would require more time than I have at the moment. I just wanted to warn you that the claimed 9900mAh capacity on those batteries is fake - and since that tells you that the manufacturer is prepared to lie, you cannot rely on them including any other safety feature including over-pressure vent etc. either. Do not test them assuming a 9900mAh capacity; in fact, without a trustworthy datasheet, you don't know what discharge current is suitable for testing. Google 18650 9900mAh for results of other people's tests. Good luck, stay safe, don't use them indoors. \$\endgroup\$ – SamGibson Jun 30 '17 at 12:52
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To really test a battery capacity requires a timed test with a constant current load on the battery. Since the battery voltage changes as the battery gets depleted trying to use a fixed resistor type load leads to the load current getting smaller as the battery voltage goes down. The constant current load varies its effective resistance dynamically to so as to keep the current the same over time.

For low current levels it is relatively easy to build your own constant current load device using an opamp, power MosFET, a few resistors and a capacitor or two. Power the constant current circuit from an external power supply. There are plenty of circuits on the web to go by.

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  • \$\begingroup\$ How can I test a 12v 170Ah battery with an output of 50Amps? I just want to see if its giving me the advertised output of somewhere in the regions of 170Ah in 10hours? (13.8v to 10.2v) It seems to me it is not and I do not know how to prove if its the battery or the equipment fault \$\endgroup\$ – Piotr Kula May 13 at 9:38
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A lithium ion cell is considered to be discharged when it reaches 3.0 volts. The amp-hour capacity is over a one hour period. Your picture shows 9900 mAh cells which means they should supply 9.9 amps for one hour at which time the voltage of the cell should be 3.0 volts or greater.

You can approximate a capacity test by connecting a resistor of 0.34 ohms across a cell. You can make this value by paralleling 20 6.8 ohm resistors for example. The resistor will need a power rating of at least 40 watts. It will get quite hot, so take care. Monitor the voltage of the cell during the one hour test. If it drops below 3.0 volts, stop the test. If it makes it to one hour, you can be reasonably sure of its capacity.

This is only an approximate test since as the test progresses, the voltage of the cell is dropping. This means the resistor is not a true common current load. But for some simple home testing, this should be close enough.

When testing batteries, always put everything in a fireproof enclosure. Anticipate that battery will explode so wear appropriate personal protection gear and have appropriate fire fighting equipment at the ready.

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    \$\begingroup\$ It is worth mentioning that it is rare to realize the full capacity (C) of a battery when discharged at the full rating. It is most common that the capacity of a battery is specified based upon a discharge rate of 0.1 * C. So for a 9900mAh cell you would want to discharge at 990mA over a 10 hour period. Of course many folks use batteries at full capacity discharge rate and greater but it is usually doubtful that you will ever realize the full rated capacity when they are used this way and the real capacity of the battery typically goes down when they are cycled at rates like this. \$\endgroup\$ – Michael Karas Jun 30 '17 at 12:04
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    \$\begingroup\$ @MichaelKaras Or by extension, perhaps the best discharge rate to test is that which matches the intended application of the cell. \$\endgroup\$ – Glenn W9IQ Jun 30 '17 at 12:14
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JUWEI 2in1 Digital Type-C USB tester. Has a built in countdown timer and reset switch. Tells watt hours as wells as mAh. It's under $20. Use a simple usb fan(one on a stand) and let it run until it goes off. This will give you an accurate reading of usable the current needed to power a device from the battery!

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    \$\begingroup\$ Welcome to EE.SE, Leon. We avoid product recommendations on this site as they go out of date. If you haven't already done so, please take the site Tour to learn how it works. \$\endgroup\$ – Transistor May 28 '18 at 19:26

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