Mixing marine & starting batteries with a switch

Question

I have a truck equipped with some emergency equipment that I'd like to be able to use without running the engine the entire time, and without draining the main starting batteries.

There is a 1/2/both/off switch in the cab, and it has two 12V batteries wired in parallel connected to post #1 in the switch. I'm thinking of getting a pair of marine/deep cycle batteries and hooking them up to the #2 post, so I can switch to that when parked.

I've been doing some reading and generally know what I'd need to do, but haven't been able to find a good answer to two things:

1. Is it safe to use the engine to charge the marine batteries? For example, I use #1 to start, then switch to "Both" when it's running. Would that be OK, should I switch to #2 so it only charges the marine, or should I just use a battery charger when I get home?
2. Assuming I use fully sealed batteries and put them in a plywood box, is that safe to mount under the seat in the cab? If so, would I need to add heat/vent holes?

Note: Question 1 assumes that the switch doesn't cut power when switching between batteries

Answer 1

Using a switch is possible, but has some disadvantages.

1. it's a manual operation, and if you forget to put the switch to the correct position you can end up in the situation you want to avoid.

2. When the #2 battery is in a low charge state, the charge current can get quite large.

There is another way to accomplish what you want: A battery isolator diode. This allows you to connect the 2 batteries permanently, the diode will allow current flow in one direction only so the #2 battery gets charged while driving, but #2 can't discharge the #1 battery when parked. This way you have no manual operation, and no switch that can get damaged by arcing.

Answer 2

1. Depending on how discharged they are and the size of your alternator this may be hard on the alternator.

2. If they are glass mat batteries it would be ok inside the cab, yes vent the box for cooling. Conventional lead acid batteries vent a lot of gasses when charging or discharging so inside the cab is a bad idea.