Originally Posted by garbonz
Cmon, you lose valuable capacity and service life if you keep them separated. and if you buy them and use them together, they will equalize capacity very quickly. ALL of the good reference books recommend a single large house battery bank. I have had sailboats for 40 years in all kinds of situations, and been crew on long time full timers and none of them every kept separate banks, except for having a starting battery dedicated to starting the engine. If you want an emergency spare get a small separate 12 volt and keep it unconnected from anything unless you need it.
IMO, of course.
Yes, I am normally operating on the "spare" battery concept, except that the spare is the same as the regular. Whether it's a good idea or not depends upon loads relative to battery capacity. If the load is small enough that the single size 24 is adequate 90% of the time of better, then the spare concept works out well. Yes, one battery will likely out-live the other - not necessarily a bad outcome to me.
Using 2 size 27s wired through a selector switch on my outboard-powered (9 amp alternator) 25ft sailboat also worked well. Of course, a single battery could go a week or two (and did) when the only draws were cabin lights, and during an occasional night sailing, compass and running lights (6 amps draw there). Anchor light used separate 6 volt lantern battery. Quite successfully cruised the northern Bahamas for 4 weeks with only 3 nights with dock power and 3 nights sailing all night.
As has been pointed out, you cannot know a battery's charge level from a voltage measurement unless the battery has been idle for a while. In my scenario, I can measure the state of the backup, but I cannot accurately measure the battery in use.
But what would be a lot more helpful is to know the current draw of the furnace when running, the anticipated duty cycle, and the normal (furnace not running) parasitic load of the camper. Then I can make a good assessment of whether or not I need to combine the batteries for a weekend boondock. Hope to get some of these measurements on an outing in 2 weeks - our first trip was lost to a leaking propane fitting where the lines connect to the trailer plumbing.
If I truly normally need 2 size 24 batteries paralleled to handle the load for a 3 day weekend, then I should be looking at wiring upgrades to more efficiently charge the larger battery bank. Or, switch to a catalytic heater to drive the load way down.
Fred W (keeping things simple so we use the camper more)