I have a 1999 3700SCR. That battery switch on the dash is a momentary switch that operates a soleniod mounted on the aft port side wall in the engine compartment. When activated the soleniod connects the starboard side batteries (house batteries) to your portside Battery (Starter battery). The Starboard side engine is wired to start off the house batteries. Should they go dead then the starboard motor will not turn over. Push the battery switch on the dash then the starter battery on the port side is connected in line and will turn the starboard motor. If you find it does not work then the solenoide most likely has failed. The port side battery starts the port side motor and the generator. The generator when running has a 12volt charging system that recharges the starter battery while it runs. This is above and beyond your 110V battery charger.