What is the purpose of the 2nd battery in a 5.4?

[carringb]

Originally Posted by joey2cool

I'm curious. What's the amperage on the van aux circuit battery?

The draw from the battery is pretty much battery limited if accessories are directly connected. Otherwise, the vans aux circuit is 40 AMPS I believe, which is also what the AUX battery get for a charge. There is a charging relay under the hood, which is controlled with a purple wire. I think the ECU will not charge the AUX battery until the starting battery is adequately charged, or there is otherwise enough alternator capacity.

[j.whitbread]

In the past, I've tried to get specific info on the diagram. On my 07 I have the factory auxiliary battery. They come wired in series so they are both discharge and charge at the same time.

It is my understanding, when the key is in the ON position; engine off, both batteries are connected. If you turn the key backwards to the AUXILIARY position, the 12v trailer wiring harness will draw only from the auxiliary battery.

Theoretically, you could draw of the trailer wiring without drawing down the battery under the hood. I'm not sure about the diesels where both batteries are on the frame rail.

An amp meter and some testing could confirm this.

[jage]

Factory Axillary battery not to be confused with an SMB installation, which is where the "house battery" radio switch comes into play.

Originally Posted by joey2cool

I've seen some chatter on the forum about SMB's house battery circuitry connection via a relay to the radio, but I couldn't understand how it can be a liability to the starter battery if the radio is turned off and the relay is not switched back to the SMB house system.

Depends on how they wired the relay, or if it even is a relay. I haven't been in there to see how it is wire, but the reverse light override switch is a rather interesting solution so I wouldn't be surprised at most anything. Also there is a light involved after a certain year. Best post this question as a separate thread.

I'm having a battery isolation switch installed to avoid the hassel of removing propane and CO detector fuses in order to prevent SMB house battery drain.

Why not just install a 12v switch inline to disable the fuses? That's one of my grand plans that I haven't gotten around to. Reminds me I need to check my voltage since the van is sitting...