Invest in a battery charger that will do both (Harbor Frieght) sells one at a decent price that I have had good luck with. Buy one with the amp gauge on it the gauge will tell you if it needs a charge or when it is charged. Sears sells similar models for around $100 cheap when you consider what blowing one up might cost. Cautionary tale my neighbor years ago hooked one up no safety glasses, sparked, battery blew up in his face and he eventually lost one eye. Be careful, use good equipment and follow safe practices they can be very dangerous.
