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Old 03-05-2014, 10:36 AM   #1
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Battery management on a slightlydifferent kind of conversion

Hey all,

Forgive me the slight OT, but I'm a longtime skoolie owner looking to turn 1988 Cadillac into an undercover 2 person boondocking camper. This involves chiefly (1) modifying the big blue leather seats to fold into a bigger blue leather bed and (2) house power, which is the subject at hand.

As my needs can be limited to lights, stereo, and whatever auto accessories can be plugged into a cigarette lighter, I plan on simply powering the stock electrical system with a pair of 6v series-wired golf cart "house" batteries in the trunk when parked. For simplicity, economy, and robustness, if not elegance, I was thinking of controlling which battery bank the car is using with a heavy marine switch - e.g.

These switches have an setting to put both battery banks on the same circuit, which is what I would use to charge all batteries off the alternator when running the car on long road trips. I have heard rumor and innuendo that putting a fully charged starting battery and a drained deep cycle on the same circuit can result in the starting battery flash charging the deep cycle, causing battery damage and general mayhem. True? Anyone else had or not had problems charging their house and starting batteries on the same circuit? Any other comments, suggestions, invective?

Thanks in advance!
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Old 03-05-2014, 11:37 AM   #2
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Re: Battery management on a slightlydifferent kind of conver

Well, I have a battery isolator in my bus that automatically sends the charge to both once I start the bus. I haven't noticed any issues, but that doesn't mean for certain that I'm not causing any damage.
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Old 03-05-2014, 03:12 PM   #3
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Re: Battery management on a slightlydifferent kind of conver

A lot of hi-buck RV rigs use the same charging setup that Jake mentioned.

I've read the same stuff about deep cycle batteries needing different charging than starting batteries. As I understand it, starting batteries are meant to pump out a big surge of power to start the engine then be recharged quickly. Deep cycle batteries are built to output a little juice for a long period then be recharged for extended periods because they've usually been drawn down by 50% or more and need a lot more juice to recharge.

The main problem with charging starting and deep cycle batteries together is that the deep cycle bank can keep the alternator at peak output for a much longer time which can do two potentially nasty things: First, alternators aren't designed to work at full output for extended periods and (may) burn their windings out, particularly if the alt is kinda small to begin with. The second problem is that the starting batteries can become over charged and boil out a lot of battery water before the house bank gets a full charge.

If both the starting and house banks haven't been discharged much, the alternator will do fine but if there's a big difference in charge states there could be trouble. The best way is to charge starting batts. with the alternator and charge the house battery with a good three stage battery charger. If you have access to shore power when parked charging them separately isn't too much of a problem.

That said, I have our banks cabled together so I can use either charging method in a pinch if needed, but I do try to charge the banks separately.

The Progressive Dynamics line has some nice chargers. ... s&kw=bb074
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Old 03-05-2014, 06:21 PM   #4
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Re: Battery management on a slightlydifferent kind of conver

So it's a bad idea AND lots of high buck RV's do it. Got it.

I suppose I can sidestep the problem by just switching back to the house battery with the motor already running, so it's the only one on the circuit. Then, the alternator's voltage regulator should prevent overcharging the deep cycles, right?
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