If you plan to use your existing battery charger, read the manual first. Most of the new consumer models have a safety built in to protect against reverse polarity and short circuits, and require 10v or more on the circuit in order to power up the output. Have you ever tried to charge a stone dead battery and had to hook up another good battery just to get a surface charge to energize the charger?

My battery maintainer, my jump box and my Craftsman battery charger all have this safety circuit. My old Sears charger that my dad bought in the '70s did not, and until it died it could be used for such applications.