Best way to determine 18650 capacity, resistance and condition ?

I'd like to get idea on the best way or process to check the capacity, resistance and usable condition of used 18650 batteries ?

I plan on buying several hundreds of these batteries for an up coming project .

Thank you, Oscar.

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pctechnocrat over 4 years ago +4

I have done this for larger battery packs, but the same general method applies for measuring the internal resistance of the cell. Without buying special battery test hardware, you will need an electronic…
cookmesomekai over 4 years ago +3

Reliably determining the state of health of a lithium ion cell is pretty hard to do unless you have a reference cell. If time is not an issue and you have a source, a sink and a power meter available then…
jw0752 over 4 years ago in reply to o.thornwall +2

The problem is defining exactly the point of total discharge, When I test them I fully charge them and then time how long it takes for them to discharge at a specified current to a lower voltage point…

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o.thornwall over 4 years ago

Is it better to test for resistance when the cell is fully charged or fully discharged ? Anyone know ?

Thank you.
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jw0752 over 4 years ago in reply to o.thornwall

The problem is defining exactly the point of total discharge, When I test them I fully charge them and then time how long it takes for them to discharge at a specified current to a lower voltage point. I can then rank the batteries relative to one another and choose the ones that took the longest to discharge from full charge to the chosen voltage point. To do this test I used an electronic load and a piece of test equipment that was built specifically for timing long duration events.

John
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o.thornwall over 4 years ago in reply to jw0752

Ok John, by doing this test, does this tell you which cells have higher resistance and lower resistance ?

And what is the device your using for this test ?

Oscar
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jw0752 over 4 years ago in reply to o.thornwall

This test does not directly measure the internal resistance but there is a correlation between the amount of time that a current can be maintained and the internal health of the battery. If you search "Process Duration Timer" on this site you will get the list of the series of blogs I did when I initially built my tester 3 years ago.

https://www.element14.com/community/search.jspa?q=process+duration+timer

Many of the better electronic loads can also time operations and be programmed. It may take several hours for a good battery to discharge depending on the load current so it is difficult to manually sit and measure the time.

John
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jw0752 over 4 years ago in reply to o.thornwall

Hi Oscar,

I wanted to also mention that if you do set up this experiment it is important to have a way to disconnect the load when the batteries get to their discharged voltage. If you pull a Li-Ion battery too low you can damage it.

John
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jw0752 over 4 years ago in reply to o.thornwall

Hi Oscar,

I wanted to also mention that if you do set up this experiment it is important to have a way to disconnect the load when the batteries get to their discharged voltage. If you pull a Li-Ion battery too low you can damage it.

John
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