Using house battery graduall discharges chassis battery?

Time to start measuring. If you can get some amperage NUMBERS discharging from the chassis battery it would help immeasurably as a clue to figure out what is going on.

Amperage discharge if any everything just sitting there grinning at you...

Amperage discharge when your complaint, the heater blower is operating.

Of course I am talking about measuring the chassis battery, here.

Been looking for simple drawing for you but hate to pass along something that is different than what you really have their...

This is probably the most simplest dual battery setup...



This setup when engaged will discharge both batteries from your accessories being used.

Where things are different is where the +12VDC from the ignition switch or manual over-ride comes from to operate your solenoid.

The smart systems control the +12VDC going to the solenoid and switches it in when needed. The grand idea is to NEVER run down your truck start battery when you start pulling current with your extra accessories. The smart solenoid comes into play where the truck alternator is running the system and is producing charge DC voltages (i.e. 14.4VDC) and then tells the solenoid to combine the two system so that the auxiliary batteries get the charge DC voltage the same time the truck start battery is being charged.

This is what you really want to happen and just have beefed up batteries on the camping side to handle your bigger load.

There is also another issue with just paralleling the camper batteries to the truck alternator system is if you exceed the current drain rating of the alternator it may destroy your truck alternator. You don't want that to happen... You want your camping battery isolated from the truck start system when pulling all of your high currents like when using a high wattage Inverter etc...

The diagram above is very prone to blowing your alternator if high enough DC current is pulled from it when you have the truck system running from the motor.

Hope this makes some sense in what you are wanting to do.

This is another diagram showing a simple smart mode solenoid relay installed. Ignore the boat motor shown in the pictorial. In our discussion this would be a truck alternator instead.


MEX is the expert here on vehicle alternators and high current producing system...

I personally would rather run down my camping batteries to their 50% charge rate and not bother the truck battery system at all...

All of this is really intended to determine what kind of dual system you have there. Your description of events sort of points to a smart mode system.

Roy Ken

Mex: that's the strange part, they aren't in lockstep, there is a large delay in the response. If I put a small battery charger on one battery, only *that* battery increases in voltage.

The issue seems to only arise in high-draw situations, like using the heater blower, or recharging at 15+amps from the genny. And then, the chassis battery will charge/discharge at a MUCH slower rate and the voltage will not rise/drop like the house battery.

As to the battery studs, the chassis battery connects to one side, and the other side has the house battery cable and the cable to the generator starter motor. Should that be wired differently?

I'm not sure I understand on the coil wire. It is just a 12v manually switched wire that throws the solenoid. When I power it, the solenoid clunks and everything behaves as expected.

Thanks for the replies. It seems like it has to be something with the relay (I guess it would be considered an isolator) it definitely isn't anything of the "smart" variety, you just apply +12v to the small terminal and a relay clunks in to connect the two large battery terminals.

It's in a rather inaccessible spot so I wanted to see if there was anything else that might be causing it. There isn't any way that the house draws can pull voltage backward through the shared ground of the chassis battery or something odd like that right?

Disconnect one battery's wire from solenoid. If both batteries keep marching in lockstep the solenoid will have been eliminate as a suspect. I assssssume you did check for the presence or no of voltage to one of the solenoid coil terminals?

Also, a charge solenoid power studs should have only TWO battery wires connected to it - not three or thirteen. If you have more than two wires connected to the vig studs, someone screwed up. One oe more wires is fitted to the wrong stus.

Check that someone hasn't put in an extra shunt to run an accessory like the chassis radio so it will run off the house battery.

A friend had just such a situation. Someone had added a light wire, about 18 ga., so that the radio would operate with the key off. It only allowed a very slow drain on the chassis battery.

Is there any chance that the solenoid is actually a smart relay originally configured to only charge the auxiliary battery when the truck system is charging the start battery...

There are two popular smart mode solenoids on the market that do this replacing the older generation DIODE BASED battery isolator/combiners that do not work well with smart mode DC charging voltage due to the very large DC VOLTAGE drop across the high current diodes...





These smart relays monitor the battery voltages and switches the solenoids accordingly to keep your auxiliary battery charged without discharging the start battery.

You may have defeated something with your re-wiring...

Just thinking out loud here...

We camp off the power grid alot here and one of our rules is NEVER mess with the truck start battery system including the truck alternator as this is the only way to get home haha...

We use a 2KW Generator to power up the on-board smart mode converter setup to recharge our camper battery bank back up to its 90% charge state in a quick three hour generator run time after we have discharged the battery bank to its 50% charge state.

We can do 12-14 of these 50% to 90% charge states cycles to our battery bank before doing damage to them by not having a full 100% charge state which takes a good 12 hours or so of smart mode charging..

We very seldom stay out off the power grid more that a week...

Roy Ken

one thing people have over looked for years is the fact that if there is dirt on the top of the battery the battery will lose voltage because of it. you can put a meter on the top of the battery, not the terminals, but the top and you can see the voltage dropping.

I think tenbear is right on. If the solenoid has collected moisture over the years things could get strange.

Replace the solenoid and then take the old one apart to get a good picture of the issue. The solenoid is not an expensive item in the rv world.

Maybe your solenoid has a high resistance (compared to a dead short) when open. With this high resistance the chassis battery would, over time, try to be at the same level as the house battery. This leakage across the solenoid could have some qualities of a diode, thus the .1v difference in the level.

Pretty much a guess.