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twistedsistersr's avatar
Sep 29, 2014

chasis battery not charging; house battery cooked

1987 Holiday Rambler Class C - not in working condition when we got it. This was the first trip more than 30 minutes away - long enough for things to heat up.

Starting point: New convertor with charge wizzard, existing isolator, new house battery and new chasis batteries.

Was connected to shore power for 4 days prior to shoving off (with fridge on in elec mode)

We set sail and about 2 hours into the 3.5 hour trip smelt a sulfur smell. Thought it was an exhasust problem. When we arrived at the campground, the house battery had boiled over and the chasis battery was completely dead. Matter of fact - the last 15 miles the RV choked off at every stoplight and we had to jump it every time.

Once at camp... disconnected the house battery - filled it up and re-connected. Connected to shore power for 24 hours. House battery was at 12.4 v but chasis only at 9v.

Day 3 - time to leave - chasis battery at 10v, house at 12.4. Made it from the campsite to the office to check out - choked out and would not start. Now stranded on a Sunday we went to the store and got a new chasis and house battery - installed both. Both started at 12.4v ish. Hit the road. Smelt same smell at 1.5 hour or so and pulled over and let everything cool off for 45 min. Engine then started right up - another 1 hour and we were home. Once home, went to check voltages - to my suprise - the new house battery had overcharged and had leaked!

Any idea what could be causing this? Bad altenator? Bad Isloator? Connections wrong on isolator?

I identified the convertor connections and they are plugged into the house battery properly. When on shore power - it is charging just fine. When we hit the road - something is over charging it. Have no idea how old the altenator is - could be original.

Any suggestions on what to test or replace next?
  • just the drive engine running ?
    no generator running for roof top A/C ?

    Bad alternator.. too much voltage

    pull the alternator and take into parts store like autozone and have it tested
  • I wonder if somehow the alternator's regulator is sensing the chassis battery voltage but the output is going to the house battery—due to incorrect isolator wiring or stuck isolator contacts etc.? It seems strange that if it's just the alternator that's bad that it would selectively undercharge one battery and overcharge the other.
  • Needs further voltmeter testing before any sense can be made of this. Definition of "isolator" needs to be established. Finned box or tin can with studs type?

    If it were me and I had only 5 minutes to diagnose I would measure voltage at alternator output stud then see WHICH battery agrees with the potential. If you have voltage at the alternator with a finned box (rectifier type) isolator, you've found your problem. DrewE's diagnosis makes the most sense. I made the mistake of brushing the back of my hand against the output stud of a Chrysler Squareback alternator that had gone open circuit full fielded. Vertical learning curve about safety.

    Do this first

    JUMPER WIRE across the isolator. Start engine. Check voltage at BOTH batteries. What have you got?
  • Yes - it only happens when going down the road with nothing on. I have no generator and don't run anything inside while camping at this point.

    I have a sure power box with fins isolator with 3 prongs. When I got the RV - wires were already on the posts marked 2 and A so I left them. I connected the house battery (which was not installed when I bought it) onto post marked 1 as it was the only one empty.

    After the issues on the trip - I decided I woudld try connecting the opposite way - Chasis on 1 and house on 2 but the wires are not long enough to do that so I would "assume" that it was never connected that way. There were "jumper" wires connected when I got teh RV so could they have been jumpering posts 1 and 2 to accomplish proper connections?

    I will definately take the RV up to Pep Boys and have the altinator tested. In addition....I was thinking of purchasing a new isolator and new wiring and running it all new so I knew exactly what went to where without question.

    Parked with shore power everything is fine....If I don't run the fridge. I have a 3 way fridge that does not currently work on LP. I don't run it unless on shore power but it still seems to kill the battery - and oddly enough - the chasis battery, not the house battery which is confusing as it is wired directly to the house battery.
  • Check all your connections to make sure phase is correct. Check input and output voltages as Mex suggested.

    Connecting anything electrical by relying on wire length or color of jacket is an invitation for problems. I'd suggest that you take the brand and model number of the isolator and find the exact pinout of the connections. Most brands publish a data sheet of their product that will show connections in a schematic drawing.

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