Secondary battery for camping

[greiswig]

We have all, I think, had trouble when camping, and opening and closing doors causing the computers to all wake up and draw power from the starter battery. I long ago had the GAP tool shut off the ECO warning, but actually did replace the eco battery with a 100ah LiFePo battery. It currently only runs a set of external lights for me.

What I have done on past vehicles like my Unimog was to make a secondary system with an isolator/combiner (which I gather is basically what the big assembly next to the eco battery does) such that I could run accessories off one batter, and the starter battery had only a single job: starting the truck.

I know there are systems out there that could do this for me, but I’m wondering about whether I could somehow rewire things to where the computers and accessories would instead run off that eco battery instead of draining the main battery? I know 100ah isn’t much, but it’s something.

 

[greiswig]

Does anyone have a wiring diagram that outlines, in particular, the three primary + leads that come off the starter battery terminal? One goes into a black box on the front of the battery box, another seems like it probably heads off to the starter, and the third, I don’t know what all it feeds.

I found the attached online, but the block diagram kind of doesn’t tell me exactly what goes where. If I read it right, it is only showing two wires coming off the positive terminal, and the third one with the bracket is an alternative paired with the lead that feeds the junction box. But I seem to have 3 large wires coming off that post.

I’m interested in trying to figure out if I can isolate the computers and interior lights and such and run them off the secondary battery if there is enough ampacity in it.

Thank you!

 

[ftillier]

Might be simpler adding a toggle switch to the door switch, so that even when the door is closed, the truck thinks it's open? I looked at this a bit for keeping the accessory sockets powered longer than 3 minutes. I think you might be able to change the power source for some of this stuff by removing fuse 18E (50A) in the under-hood junction box and wiring in a remote/inline 50A fuse powered by your secondary battery. The other thing to do is rework the relays for the accessory sockets (R203) and cigar lighter (R132) that are normally controlled by the BCM. You could have a battery monitor shut them off if the secondary battery is below a certain threshold. The only thing you'd want to preserve is some form or resistor so that the BCM still thinks a relay is installed to match the original relay.

 

[greiswig]

Interesting ideas! Do you have any insights as to what the BCM looks for to verify that a relay is present and working? Is it monitoring current draw from the coil, or voltage present on the load leg of the switch, or…?

 

[ftillier]

The BCM is on the ground path of the signal line from the relay, so it will be able to detect if there's no load when it tries to energize the relay to turn those sockets on. I haven't verified that it cares mind you, pulling one of the relays and scanning for codes would tell you.