Used li ion cells...

Discussion in 'The Projects Forum' started by Externet, Jul 28, 2013.

  1. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
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    57
    What circuit to build to evaluate 18650 cells ?

    Lithium cells salvaged from used battery packs do have an use in hobbies and general applications.
    As some cells may be weak, I would like to build a circuit to evaluate their health state.
    For a given -say 2.0 Amperehour- rated cell, what load resistor value should be convenient to use for how long and measuring what curve of voltage drop during the discharge test in order to qualify the cell as 'good' or 'useable' or 'bad' ?

    (+)---------------V(+)-----------------R
    Cell
    under
    test
    (-)----------------V(-)------------------R

    [ Voltmeter connected at V; resistor connected at R ]
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    I'd just use a lightbulb and a stopwatch. Close enough.
     
  3. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
    758
    57
  4. wayneh

    Expert

    Sep 9, 2010
    12,094
    3,033
    You said the magic word, "arbitrary". The suitability of a cell depends on the application you want to use it for. If you can, use a test and a hurdle that relate closely to the intended use. Anything else leads to mistakes, where the test said it was a good cell, but it fails in the real world application.

    Another method might be to estimate the percent of a cell's expected life that remains. You might then choose to toss out any that fall below, say, 10%. You don't want to keep wasting time on the same bad cells.
     
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