Do not ever set your battery on concrete.
I don't understand the chemistry/physics behind it, nor do I ever intend to delve into the science of why it happens but something about rebarb, concrete and long half-life radiation
drains your battery if it's in close proximity to concrete. I saw it happen repeatedly at the VA State Fair with our robot and rather than trying to understand why I'll just accept the fact that it happens for now.
What we saw was that the charger would say our battery was fully charged. We'd then transfer the battery to the robot and place it on the field. At the start of the match, the bot would move
extremely slowly, and the OI would indicated ~7V on the battery. About 15 seconds into the match, the voltage would go
UP and the robot would act normally. Who knows why
? What I do know is, we do not place batteries on concrete any more and we haven't seen the problem since.
If you thought your batteries were charged and then you get these results, perhaps this is why.