Just bought a 2002 GEM 825
Looking for FAQ to get mine to go more than 5 miles!
Fully charge the batteries. Check voltage at each battery, no need to disconnect cables. Then brake torque or drive while monitoring voltage at each battery. This should show any bad batteries. How old are the batteries? May be time for a new set.
I had the sams problem. It was the turf road switch. Previous owner wanted to limit speed and disconnect the wire that would o enable the controller to go into road speed mode.