There is not enough public info about flat batteries..if a battery sits long enough and discharges on it's own it will go flat.
sometimes, specially with newer batteries and lithium, you have to get some juice in them to get them started charging normally.
I sometimes can do this by plugging and unplugging the charger many times then leaving it plugged in for a while and doing it again.
power tools will seem to have bad batteries after sitting a winter etc. they just need a jump start