Jgue is right the battery won't hold a charge anymore. Easy answer is replace the battery. If you wanted to diagnosis it further you could pull the caps take a couple of old bolts or nails put them on the end of your voltmeter. Start at one end and move down placing the probes in the cells start in 1 and 2 then 3 and 4 hole just keep moving in either direction. You will find moving down the row that one cell is dead. There's no way to fix a dead cell.