2002 Gem Car battery issue

I just picked up a gem car has good trojans less then 3 year old i load tested them and they all tested fine but when driving it runs great from 100% to 50% then once it hits the 50% falls on its face down to 15% and shuts cart down there no inbetween 50 and 15 anybody had this problem or can give me some pointer to look for?