Before you take out a personal loan to buy new batteries...check the soleniods/resisters.
When you step on the GO pedal, you should hear a "click" when the soleniod sends juice to the motor. As you press further down on the pedal, you should hear more clicks as each resistor kicks in....they kinda act like gears.
My suggestion....jack up the rear end, press pedal, listen for "click", probably be easiest with the seat removed.
Soleniods/resistors are much cheper than batteries....you will just have to raid the grand-kids piggy banks.