78 Dodge Glendale Motorhome House Batteries Overcharging

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bainer1290

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Hello

Well we just bought our first RV a few weeks back and I'm in the process of getting it ready for the spring. The seller mentioned that the rear batteries need to be replaced and for some reason they are overcharging. The front battery is fine and is not getting to much voltage, but the back batteries seem to be getting about 16 volts. I think the correct range is about 13-14? Anyways the seller was trying to figure it out and so far he has replaced the alternator and the voltage regulator. Is there just one voltage regulator? The one he replaced is mounted just above the engine on the firewall.

I have been doing some research and I see there is some sort of aluminum block mounted on the upper rad support that looks like it is some sort of distribution block for charging the rear and front batteries separately. What is this thing called? Could it be the problem? What else could it be?

Thank you,
Rob
 
The house battery is charged by the converter when plugged in and the old converters didn't regulate charge well.  He didn't check his water on a regular basis while plugged in and burnt the battery up.  If it boild dry while driving then its got a dead cell and needs to be replaced.  If you don't mind the exspense (around $250.00) it may be best to replace the present converter with a PD9245 by progressive dinamics.  It is the best charger on the market right now.
 
I see, so the house batteries are charged by the converter when plugged in and by the alternator while driving. So does the converter also control the amount of charge that the batteries receive while being charged from the alternator, for example, when it is not plugged in and being driven? Or does the charge that comes from the alternator bypass the converter and go directly to the house batteries?
 
I did some reading an it looks like the converter in completely separate and has nothing to do with the alternator charging voltage. I see what you mean by upgrading it if I use shore power lots of the time as it would be better for the new house batteries, so I may change the converter down the road. My main concern right now is to figure out why the house batteries are getting 16 volts from the alternator. Is there anything other than the voltage regulator that controls this?
 
On our old motorhome, a 1977 Champion on a Dodge frame, there was an isolation diode used to charge both batteries while the engine was running.  They do go bad.  In our case the part was bad when we bought the unit and it had been rewired to by pass the diode.  We learned that when all batteries went dead after a cold night.  The replacement diode lasted us some 5 years before it again needed replacing.

From the symptoms you are describing, I'd look for this device.  They have 3 or 4 large wires attached to them and have cooling fins on it.  They are usually made out of aluminum. 

Examples of isolators can be seen at: http://www.ase-supply.com/category_s/20.htm?gclid=CL2VtLGZjJACFQ2nGgodqRd_tQ

Various wiring examples can be seen here: http://www.bcae1.com/battiso.htm

There is a chance that you have a set of solenoids that can isolate the 'house' from the 'chassis' batteries.  This is also described in the wiring example reference.  If you have this arrangement, there should be a set of switches on the dash to allow you to turn off either set of batteries.  If  so then the cylindrical shaped solenoids should be near the batteries.  We have this type in our 1989 motorhome.  They started to randomly not operate which I trace back to a broken wire.

Let us know how you make out.
 
I believe it has an isolation diode because that sounds like the part that is mounted on the upper rad support. I will take some pictures later on today. I don't have any switches to control the batteries so probably not the solenoid type.

Thanks for the help so far guys!
 
Here's the pic, the battery isolator looks old, it says 70 amps on it, is that good cause I have seen a few that are 100 amp on eBay. Could I go with the 100 amp?

It also looks corroded pretty bad where it contacted the rad support. I assume it needs to have a good ground source?

I think this may be the culprit. It only makes sense because the front battery is good and gets the correct amount of charge, but the house batteries don't. The alternator and the voltage regulator don't have different outputs for the front and house batteries this is the only part that splits it. Right?
 

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I would remove all 3 wires and clean up the terminals.  Put new nuts on, and perhaps new crimped on connectors on the wires.  The center wire from the alternator looks pretty bad. If you know how to check the diodes with an ohm meter, do so.  As the diodes are in the positive leads only, the ground doesn't matter.  This web site has a diagram of how the isolator is connected and an explanation of how it works.
 
Yeah those wires do look badly corroded. I will clean up the terminal connections and put new ends on the wires. I have a mutimeter that I can use to measure resistance, any idea what I should be looking for? I guess I could also just compare the main battery terminal to the aux battery terminal, using the alternator in for a common input and see if there is a large difference.
 
If you know how to use the ohmmeter, check the diodes.  They should show very low resistance in one direction and infinity in the other.  Any other result and the diode is bad.  If a diode shows low resistance in both directions, then it's shorted and needs to be replaced. If it shows open in both directions, it's also bad and needs replacement.
 
Using the volt meter, check the voltages at the isolator input & at each battery, with the engine running, before & after cleaning all connections (including battery posts).  You should get 13-14 volts at all locations.  I suspect you'll find different voltages at the isolator.  An isolator labeled as 70 amp is the max current rating that it can handle.  Going to a higher capacity will work fine, just the physical size will get bigger.  Most RV, marine & some auto part stores will stock the part, ~$35.
 
I'll dig out the volt meter and do some readings this weekend. I'll post the results. Thanks again
 
Just one point.  The diode isolator cannot increase the alternator voltage.  There is usually a 0.7V drop, which is insignificant.  If the alternator is properly charging the chassis battery, it cannot be putting a higher voltage into the house battery.  I would suspect a failed converter.
Art
 
A .7V drop is significant, but won't cause overcharging.  The alternator voltage is set higher to compensate for the diode voltage drop.  However, if the diode is shorted out, then the .7V drop isn't present and the battery can be overcharged.
 
I will test the voltage from the alternator at the isolator and at the front and house battery connections. Then I will turn off the engine and test the voltage from the converter at the house battery connections. I'll report back with my findings but won't be able to do the testing until Saturday...
 
When you're testing with the engine running, unplug the shore power.  If both the engine is running and the shore power is connected, you will read the higher voltage at the house battery(s)
Art
 
Yep I'll make sure to unplug the shore power then test with the engine running. Then turn off the engine and test it with shore power to see how the converter is doing.
 
Alright here's the numbers...

Engine running with shore power disconnected:

Battery isolator in from alternator 16.9v
Isolator output engine battery 15.8v
Isolator output house batteries 16.5v

Voltage at engine battery 15.8v
Voltage at house battery #1 16.5v
Voltage at house battery #2 16.5v

Resistance at Isolator from input to engine battery 17.9 (multimeter on 20k Ohm setting)
Resistance at Isolator from input to house batteries 18.5 (multimeter on 20k Ohm setting)


Engine off with shore power connected:

Voltage at house battery #1 12.8-13.4v
Voltage at house battery #2 12.8-13.4v


So it looks like the coverter is alright. I guess I was mis-informed from the seller as it looks like the engine battery is also getting to much voltage. I also found it interesting that the isolator has a variance from oun output to the other of about 0.5 -0.7v

So what do you think replace the isolator and get a new voltage regulator?
 
Those voltages from the alternator will destroy your batteries.  The isolator is just diodes, so I would definitely suspect the regulator.  Diode resistance doesn't really mean that much, so don't worry about it.  As long as they read open in the reverse direction, they're OK.
 
Yeah, try a new voltage regulator. However, old automotive regulators were notorious for putting up very high voltages, which is one of the reasons that car batteries used to need replacement every 1-2 years.  If you get an original design regulator it may still be very high, but a newly designed unit ought to do much better regulation. Or maybe replace the old generator with a new alternator with built in regulation - should be a big improvement.
 
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