Yes, the CTEK MULTI US 7002 12-Volt Universal Battery Charger part # CTEK56353 can be used to charge two 6 volt batteries in series. This is a 12 volt battery charge yes but when you combine batteries in series it doubles the output. In order to hook up the charger, the main ground and positive terminal of the two batteries that runs to the trailer's wiring would be the pins to use. view full answer...