My electric Hyster has a good battery and charges to 100% and gradually drops in percentage numbers during use as it should. Then all of a sudden, the battery indicator digital gage drops to a minus number such as -15 and shuts down like it overloaded and resets after fifteen minutes or so only to go -15 and shut off again. If you mess with the throttle, sometimes it goes back to a high plus number like 70 and works perfectly for awhile only to unexpectedly go minus at any time and shut down. Any ideas?