Issues after battery replacement.

I recently bought up a batch of 4th generation iPod nanos that would not hold charge for battery replacement and resale. The battery installs went beautifully, and the iPods initially took charge and worked as they should. But I found that after they sat for a few days, they no longer took a charge, and on some of them the hold lock was switched to being constantly on all the time and could not be unlocked. I am stumped as to how they would work fine at first and then suddenly present problems. Does anyone know what could be the cause of this issue? I have had to do several returns on these due to this problem....

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