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Battery Isolation - Is it working as it should?

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My new-to-me 1993 Itasca Suncruiser has the normal battery arrangement of 1 regular automotive battery for the chassis and two deep cycle house batteries connected in parallel.  The power converter seems to be working fine, and there are a couple different 12V-related switches on the dash.  The "Aux Battery" switch appears to work - when I flip it to "off", there's no power for any of the coach's 12V systems as it should be.  The radio power switch works properly, allowing the radio to be powered when the ignition is off.  The Dual Batt switch, however, is confounding me a bit. 

According to the operator's manual, the Dual Battery switch has a momentary position for starting help as well as a "dual" position for connecting the house batteries to the alternator to charge up while driving.  It isn't explicitly stated, but my assumption is that the middle position represents a disconnected state where the automotive 12V system and the house 12V system are isolated from each other.  When I operate that switch, I hear the click of a relay coming from somewhere in the back, so it's doing *something*, I just don't know if it's the right thing.

What makes me question this system is that I was getting a very low reading on my volt meter in the gauge cluster.  Early 90's GM products aren't really world renowned for their accurate readings, so I pulled out my multimeter and did some testing of my own before determining that the alternator had failed.  I do not believe that the Dual Battery switch was in anything other than the "off" position during the time between when the alternator failed and when I started troubleshooting.  With the ignition off, I got the exact same voltage reading on all three batteries.  I would certainly expect that the coach batteries would read identically to each other, but if that switch was in the off position and the automotive battery and house batteries were isolated from each other, shouldn't the house batteries have been reading their normal 13.75 volts while the automotive battery was reading around 10-11 volts since the alternator wasn't charging it?  When I swapped the alternator, I disconnected the negative terminal from the automotive battery only and the engine's systems were dead as a doornail while the house systems continued to work fine.

I'm not familiar with how the isolation system is supposed to work or even where they've physically installed it, so I'm not sure if I have a problem or not.  I'd like to have the peace of mind to know that in the event that I run down my house batteries, the engine will still start without having to fire up the generator or plug the power converter in to shore power to charge the system.  Other than just turning on all the coach 12V systems and waiting for the battery to die, is there a good way to check if the isolator is working as it should?  Will checking continuity between the negative terminals on the house battery and the auto battery tell me anything?  Any good places to look for the isolator so I can verify that it hasn't been bypassed?  The wiring diagram says "bottom leg of front electrical box", but I'm not 100% clear on where that would be.  Thanks in advance!

Kevin Means:
Hi wae. If your battery isolation switch disables your house 12 volt systems, it seems like it's working as advertised. The switch controls a solenoid, which connects/disconnects power. It also sounds like your combiner switch is working properly. Its purpose is to momentarily tie the house and chassis batteries together, so you can use the house-batteries to assist with starting the engine if the chassis batteries die.

I'm also interested in why the house and chassis battery-banks would have identical voltage readings. I'd expect to see that when the combiner switch is pushed, but not otherwise. That 13.75 volts you saw is the charger's voltage - not the battery's voltage, so you were either plugged into shore-power, running a generator or idling the engine when you took the readings. A fully charged 12 volt battery will indicate 12.6 to 12.7 volts when at rest. That 10 to 11 volts your chassis-battery is putting out means it's, essentially, dead.

When you're plugged into shore-power, your converter powers your 12 volt systems and charges your house-batteries. It probably doesn't charge your chassis battery, which isn't unusual for a coach of that era. The alternator, on the other hand, charges both the house and chassis batteries.


Charlie 5320:
It only charges the house batteries if the switch is in the duel mode. Chassis battery most likely isn't charged by the converter.

I put together a pretty complete testing regimen and had little charts all ready to fill out as I walked through the steps but it was all for naught because I confirmed the isolation is working with a quick continuity test between the battery posts.  I don't know why the voltages were all the same before, maybe I had the switch flipped for a bit and didn't remember doing that, but I have continuity on the positive side of both systems but not on the negative side, so the isolator is functioning.  I've also got different voltage numbers on the two battery banks now as well.

TL;DR:  No idea what I was doing before to get wonky readings, but everything is working as it should.

Kevin Means:
Good news! Thanks for the update.



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