If the plates only show a little add distilled water, but make sure the low electro level was not the result of a spill. If it doesn't hold a charge and you can't buy a new battery, use a trickle charger going all the time to start it if you have one. I remember back in the early 1950s batteries would short out when the flaking from the plates accumulated in the bottom and finally shorted the plates. A good shake got some of them going again for a while. Maybe that won't work with recent batteries. That was a long time ago in a Model A I had.