Electrical Issues

Can someone help me im from the Bahamas and i have a customer that has a Gem e825 4 seater car the problem I’m having is all batteries are charged the volt meter will read 100% but within mins of moving the percentage startes to drop right away. And with in about 10-15 mins the battery indicator starts 2 beep. I would turn it off and in 10 secs restart it and the indicator goes back up but ot fully may go around 75%. The same thing will keep happening until the indicator reads nothing. Can someone tell me where my problem could be either its the motor controller or the charging system not staying charged… thanks in advance