Sounds good thanks for the heads up. I had same setup in mind, however the DCC50S will isolate access to PCR > N3 hence the startup check might not happen if on the PCR side even with the C2 pulled just because DCC50 might not have a closed circuit to AUX unless it's charging. The chain will be as follows: N3 > DCC50 > PCR > N1+AUX Batt.Yeah, that’s a pretty different application than it’s intended for. Not saying you couldn’t make it work, but just not as straightforward.
If I were you, I would start by completely removing the Aux battery, and in its place, hook up DCC50S “ALT+“ going towards the PCR relay (and ultimately the alternator if/when the PCR relay allows it).
Hook up DCC50S “OUT+“ to both the LiFePo battery and “N1,” to supply the vehicle electronics. You would still need to connect the “IGN” signal wire to Fuse 50 (or equivalent, like just jumping it to the “ALT+“ might work since that’s coming from the alternator anyway).
The only remaining question would be the PCR /Aux battery check when starting up. I don’t know if the voltage is read on the “N1” side or the ”PCR” side as they are normally one and the same with no DC-DC in between (read the @Jebiruph ESS guides for more info). If it’s the former, then you should be good to go. If it’s the latter, then unplugging the PCR “C2” connection should do it, which permanently engages the relay.
YMMV and do all this at your own risk, I take no responsibility if anything goes wrong, big or small. But do let us know how it goes!
If the startup check happens on the N1 side then this becomes less of an issue (most likely since IBS is on negative terminal, probably for that reason :D).
Still, the main question remains that while ALT is running the PCR normally closed passed ALT current to N1 hence powering the electronics besides charging AUX. Injecting the DCC50 will make that "trickle charge" that 30 sec N1 is on battery then switches to BATT+ALT as DCC50 charges, then DCC50 reads batt fully charged and disconnects and the cycle starts over. Weird stuff.
Maybe I will just leave the AGM in AUX and put a solar MPPT only on top of the Main batt and call it a day . TBH, I'm annoyed that on ACC with engine off both batteries are drained, yes technically until main drops to 12.7v then apparently PCR opens (on ACC still) to protect the main and allow AUX to continue to power ACC. Which makes sense for startup check when ECU decides if AUX is too discharged to be reconnected to Main or keep PCR open until engine cranks up then reconnect to charge both batteries.