I agree with the above and got it working again.
Its a %#*@ hard unit to open up, you need to pry the covers off the sides of the unit as well as the one on the handle. Take the screws out and then pry open the lower handle enough to get the battery unit out. This saves taking off the blue end of the unit where the part is that hammers.
I just attached a 6volt battery into the circuit (positive to positive etc) and it was enough to get a bit of a charge and shock the batteries into charging again when you plug it back in. I'm sure a 12 volt battery would have been better but thats all I has and it seemed to work.
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Good luck.