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My university's student electric racing team has to test a large number (250) of lipo cells before using them in our electric car. Our testing methodology will be approximately like this:

  • Discharge.
  • Charge at 1C
  • Let cells rest for a specific period.
  • Discharge at constant current and measure capacity.

I've heard that some bad cells may charge and discharge to the nominal capacity if they are charged immediately, but do not hold up after time. Is this correct?

I've been unable to find any hard data on the interval between charging and discharging required to differentiate good cells from bad ones -- how long do we need to wait to know if the cell is good?

Sources of empirical data would be especially appreciated.

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    \$\begingroup\$ This seems silly. The cells have already been resting for a long time during shipping. Just measure the OCV. Any cells with high leakage will have a low OCV. Set them aside. Don't capacity test every cell. Nobody does that in production. There may be an OCV test prior to assembly, but not a capacity test with rest. \$\endgroup\$ – mkeith May 23 '16 at 6:53
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The statement that "some bad cells may charge and discharge to the nominal capacity if they are charged immediately, but do not hold up after time" it is very slightly correct and it refers to the fact that the internal resistance varies slightly with different cells and after a VERY long time an imbalance will appear.

Just as mkeith said, all you need to do is measure the OCV (open circuit voltage) and match cells which are very close one to another and put them in series. Then put the closely matched series packs in parallel.

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