Questions about 12 volt wiring in my old class C

Hello Folks,
I have a '73-'74 Beaver Class C RV on a Dodge chassis. I have no manual or wiring diagram and I'm trying to figure out how the 12 volt wiring system is put together. I plugged in 120 AC shore power and verified it is working. The receptacles work as far as I can tell. Only one light is AC, which is the external light with a switch next to the door. All the other lights are 12 volt. In the "electrical center" of the RV (pictures attached) there is a switch labeled CONV (converter) and BATT. Plugged into shore power and switched to CONV, everything works - receptacles and lights. Switched to BATT, the lights go off. Not knowing better, I thought the battery that cranks the motor would engage here as I didn't see another battery anywhere when I initially looked. Looking closer I found a battery compartment on the other side of the RV that had stuffed piled where the "house" battery would have lived. In that compartment there are 3 wires coming in: large, medium and small. I take it the large one goes on the positive terminal, and the medium goes to ground (I was able to trace it to a bold on the chassis). But I'm not sure about the small one. It has an inline fuse on it. Before I buy an RV battery to place here I have a few questions.

1) Does the converter in the electrical center include a battery charger and is that what the small wire is for in the compartment?
2) Or, could that small wire be coming from the alternator? Or is it something else.

Thanks in advance for your help.

The converter should be a battery charger too.
the small wire is probably from the alternator so that you house battery is charged from it.
The 12v systems for the house and engine are separate. 

The photo of the converter area does not appear to include a charger, though most such systems do.

I'm guessing that the small wire to the battery is a charge line from somewhere - it was common back then to have a separate wire for the charge function. Whether it comes from the engine alternator or a separate charger bears further investigation.  I'd be inclined to add a small, modern charger that runs off shore power a=t provide battery charging while parked. Those 70's vintage charge systems were crude and hard on batteries.

WhIle you're plugged into electricity, use a voltmeter or a 12 volt test light to see if there's voltage on the large black wires.  Then see if it goes away when you unplug from power.  If it does, the voltage is coming from the green converter in the first photo and it also charges the missing house battery.  Check for voltage with the Converter/Battery switch in both positions.

Next verify the fuse in the white wire is good.  Now it's just a matter of seeing if there's voltage between it and the ground wire.  If there's voltage, unplug from power again and see if it goes away. 

If it has voltage when the RV is unplugged, it's a charging wire connected directly to the chassis battery.

If it has voltage only when the RV is plugged into power, it's a charging wire from the converter or possibly from a seperate battery charger.

If there's no voltage in either case, start the main engine and see if this puts voltage on the white wire.

If it only has voltage when the engine is running, it's a charging wire connected to the engine alternator.  There may be a battery isolator between the alternator and the white wire to keep the engine and house batteries seperate, so you can't drain the starting battery while boondocking.

If there's no voltage on the white wire during any of the above tests, it may be a power feed to something else in the RV.

In any case, the white wire will connect to the  (+) battery terminal.

Thanks for the mindshare! )

First, some definitions: chassis battery = motor starting battery, house battery = used for everything else in the RV, in this case, missing.

I used my voltmeter to do some tests as per Lou:

1) Plugged into shore power, getting over 12 volts on the 2 black wires in both CONV and BATT positions
2) Unplugged, still getting 12 volts!?
3) Put a charger on the chassis battery, now getting 14+ volts on those 2 black wires. (Aha!)
4) Little white wire has nothing in all above cases.
5) Have not yet started the motor to see if the alternator is hooked up to the white wire

What I want is to purchase a house battery and have it run the 12 volts lights when unplugged from power. I'm not sure how that would work given it did not work earlier with just the chassis battery in the BATT position unplugged and they are obviously connected unless there is an isolating device of some kind.

I guess I'll get a battery and find out.

With the fuse holder right there on the small wire, I'll put my money on some item that was powered from the battery. Might be some extra equipment in the kitchen area, A light over the bed, or just a 12V outlet somewhere.

After more investigation, I have learned a few things, but now have a few more questions.

I learned that the large wire coming into the house battery bay is hooked up directly to the chassis battery, not to any circuits inside the RV like the 12 volt lights. This can't be a good thing, can it? If I leave it on it means 2 different kinds of batteries are "competing" with one another, a deep cell and a cranking battery, and when charging, the deep cell would not be getting the multi-stage charging that it needs. So, I'm leaving it off for now.

The small wire with the inline fuse is indeed turning on the 12 volt lights in the RV, which is what I wanted. The one light I turned on to test seemed a little dim. I think that the small wire is too small for the load?

So, if I could manage it, what if I simply replaced the small wire with the large wire? The two batteries would no longer be connected, meaning the charging battery would be charged by the alternator. I could get a smart charger for the house battery and the 12 volt loads in the RV would now be serviced by a big fat wire.

Is there something I'm not thinking of that could be a problem?  Thoughts?

lancefb said:

So, if I could manage it, what if I simply replaced the small wire with the large wire? The two batteries would no longer be connected, meaning the charging battery would be charged by the alternator. I could get a smart charger for the house battery and the 12 volt loads in the RV would now be serviced by a big fat wire.

Is there something I'm not thinking of that could be a problem?  Thoughts?

Click to expand...

That should work and there may be no need to add a charger as the converter in most cases takes care of this.

I would look at battery isolators. Put one on the end of the cable coming from the chassis battery and you will be charging the house battery as you drive, but not drain the chassis battery when camping.

In my case, the converter does not have an integrated charger or a charger attached (see photo above).

I'm concerned about letting the alternator charge the house battery as it is a deep cell type and needs a multistage charger. Could this damage the deep cell?