George, is the 70% line charge from a vehicle alternator always the case, or is there a method by which the charge may be increased to approx full?
I ask because I have an old 1985 Ford diesel with dual batteries & an (I believe) 105amp alternator. For about 10yrs, late 80's thru mid 90's, it hauled a very large (approx 3000#, 12' box, side entry door) cabover camper. When I first mounted the camper, I had a charge wire going directly to the power converter/charger from my truck alternator. The camper had a Group 27 deep cycle battery. Had no problems until I spent a week at hunting camp in relatively cold weather once. The furnace cycled frequently, along with normal lights, water pump, fridge spark, etc. At the end of several days, I found I'd nearly drained my truck batteries (they wouldn't crank the engine) but the RV battery still contained a weak charge of around 12v (it would still cycle the fridge spark, furnace blower, & lights). Had to drive my 'towed' vehicle home with my truck batteries for a charge (the battery in a '77 Datsun P/U won't 'jump' a 6.9L diesel when it's 18*f).
Next day I had my son bring me a Dual Battery Isolator from his NAPA store. After wiring it into my truck elect/charging system, I replaced the single deep cycle battery in the camper with a pair of Group 29 RV batteries wired in parallel. From that time on I never experienced any problem from the truck batteries, & have spent as many as 7 days 'boondocking' in the camper without starting the truck to charge the RV batteries. After a couple of checks, I found that the first battery in parallel would discharge a little quicker than the second. I made it a habit to 'rotate' their position every couple of months, & even with frequent use of the camper (every couple of weeks or so, for at least a weekend) the batteries lasted about 5yrs before needing replacement.
Think I installed the DBI about 1989/90 so it's been a while & I no longer have the box or instructions. It is quite a large, solid state DBI & the only visible mark on it other than the connector post ID's is "CG-130". Seems I remember something either on the box or in the instructions about it being an auxillary charge controller as well as isolator?? In any case there are 3 contact posts, one wired to the alternator, one "MAIN" connected to the three wires that originally came from the alternator, & one "AUX" to the camper converter.
Guess my question is, did the DBI provide a more complete charge to my camper batteries, or was I just fortunate with my aux battery choice/power usage? I did, quite often check the RV batteries' charge level after a day's drive & they most always showed in excess of 13v on my digital voltmeter. I'd think that was pretty close to a full charge.