VMN wrote:skirex wrote:Just Put a DC DC charger before the third battery so it is regulated and there will be less strain on the alternator.
By adding a DC-DC charger in the jack off you won't have the ability to use the vehicle mounted AUX battery in the jack off as the DC-DC charger will isolate the two batteries. You want to be able to charge both the vehicle AUX and the jack Off AUX at the same time. If you ever add solar panels to the jackoff this will also let the trucks AUX battery to be charged from solar which is desirable.
What is rarely mentioned is that a DC-DC charger places a constant load on the alternator whereas a conventional isolator does not.
Umm what is a jackoff? You absolutely will be able to charge the batteries at same time, the alty does the starter, the DC-DC does the auxillary battery. You can bridge them if ever in an emergency (such as accidently running your starter flat while engine was off).
As to load ... a DC-DC charger will in theory place a little longer load on the alternator, becvause its putting more charge into the battery (ie charging it to 100% instead of 70%) ... not sure how this is percieved as an issue?
Once battery is charged and in float (or later stage of absorption mode), it doesn't put jack all load on the alternator.
With that said, the alternator is *always* under some form of load, just running the vehicle will suck between 20 - 40amps being ON (fuel pumps, computers, fans, AC, dashboards, ABS, ESP, headlights, foglights, spotlights, UHF, blah blah blah) ... so the notion of there being a bit longer of a load on alternator is moot. (besides, that's what the alternator is there to do ... service load ... and DC-DC charger is seem as a load device ... exactly as the other items are).
Another benefit of DC-DC is that IF you're having a shortfall in the alternator output (at idle they drop to around half of their peak rating) ... a DC-DC will actually switch itself off (thus meaning you get no charge to AUx battery, but allowing the alty to still dump power into the starter and/or prevent discharge scenario). ... though with that said, if you're having a regular short-fall scenario, then somehting is wrong and you have too much load (whether its 2 batts off the alty and not getting enough charge to them, or a DC-DC scenario with it cutting out ... you will still have a shortfall situation to address).
It all boils down to that for my mind, the starter is the critical battery, it needs priority, and needs to be left alone when engine is off. The Aux battery is there for being cycled a lot.