Re: Adding a second battery........
Right away we lead him down a path he doesn't want to go down. Yes - you can do what you want to dio. No it does not make it 24 volts. Two batteries in parallel make 12 volts but the capacity of the system is double. Two atteries in series make 24 volts.
But the long and short of this is that you don't need a second battery. Your all around light draws so little current it would likely run all night and the battery would still start the engine. You can certainly add the switch in the hot lead between the two batteries but be aware that it must have a very high current rating and the wires must be battery cable size because the same current that passes through the large battery cables going to the engine must also pass between the two batteries. You can charge the START battery alone (switch open) or both of them (switch closed). You do not need to rewire the light. When you night fish, leave the switch OPEN. Yes, the accessories run off the START battery but so what. You have a fresh backup which all you need to do is close the switch.
