Tennis tutor charging problem

I bought a used older Jugs Tennis Tutor and the battery does not charge. I can't tell if the batteries or the charger is bad. I opened it up and it has 2 12v7.2 batteries. It does not charge like some I have seen, AC adapter that plugs into the unit. There is a 6in plug that sticks out of the unit so you plug it into an extension cord. Transformer is inside. If I put a meter to the leads coming off the charging circuit board it measures 11v. The is a sticker that says "charge battery when not in use. Battery can not be overcharged". Batteries never get above 6v after a long charge time. Thanks for any input.Jeff