Good enough is a bit subjective... it doesn't have to be 100% new to be usable.
But Here's my 2 bits.
You want the cells / sticks to be as close to each other (balanced) as possible ... close in all aspects : Ah or Wh of discharge capacity ... Resistance ... Self Discharge Rate ... etc.
In order top 4 sorting criteria :
- Balance of all 20 sticks as close to each other as possible , is better
- less resistance (Ohms) each stick is better.
- more capacity (Ah or Wh) each stick is better... whole pack only as good as the weakest cell or stick.
- less self discharge is better
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Test result values will vary with conditions ... different testing methods and conditions can give different results ... be it Ah or Wh numbers or Ohms of resistance , etc... ( so be as consistent in all testing as you can be ).
to determine resistance ... V=IR --> dV / dI = R
dV = the change in voltage from zero amps to under a load.
dI = the change in current from zero amps to the amps of the load.
R = Ohms of resistance.
A simple method is to use a volt meter before and less than 10 second after the amps change to a steady value ... you can use the superbrain (or any controlled load) to act and the load ( charge amps or discharge amps )... and a basic multimeter to see the change in voltage (dV) before and after that amps change (dI)... more than 10 seconds you are getting a bit too much into the natural voltage change as the battery charges or discharges.
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If you can verify the following minimums you know you'll be fine ... but plenty of people don't weed out as many as the bellow would.
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Originally Posted by retepsnikrep
Cells/sticks with less than say <4500ma capacity at 10A should probably be binned.
Cells/sticks that can't sustain a 70-80A load for at least 15 seconds without dropping below 0.9/0.8V should probably be binned.
Cells/sticks that heat up by more than say 10C during the above high current test should probably be binned.
Cells/sticks that self discharge more than 25% over one week should probably be binned.
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I would say if you can't do the 70+ Amp high load test described above ... do the resistance testing as high of Amps as your setup can do... Brand new 6 cell NiMH sticks (depending on temperature and SoC) can have as low as 18mOhms (0.018Ohms) of internal resistance ... the lower your sticks / cells are the better ... but I would personally give an absolute maximum high of about ~43 mOhms per stick ... more than that , and you can use it for something else ... but I wouldn't put it back into a HEV.