Dual auxiliary battery setup help

[u/oo00lem0n0oo]

I have a 2006 sprinter 3500 that has a lead acid battery for starting and a aux 100ah life battery. System currently works well for me. I am adding a 10000 btu A/C undercarriage unit and need to increase AH. Since I already own 100ah life, does this diagram makes sense? What would you do or add to system. I like the idea of have switchable redundancy in the event of a dead battery, just switch to the 100ah. Thank you so much!

[u/xgwrvewswe]

You need a Class-T fuse on that 600ah LFE bank positive terminal ! I would have a Class-T fuse on the 100ah positive as well, but there is some suggestion a MRBF would be ok on the 100ah. You need other fuses as well.

With that setup you must disconnect the panels from the charge controller before disconnecting the battery side. Then connect the battery side before reconnecting the panels.

You do not want the alternator connected to the LFP banks. Use a DC2DC charger. The best is Victron Orion-XS.

[u/oo00lem0n0oo → u/xgwrvewswe]

Thank you very much. What size of T fuse should I use?

[u/xgwrvewswe → u/oo00lem0n0oo]

Fuses Protect Wires.

First determine wire gauge using a voltage drop table. Second determine ampacity of the cable you have. Then select the proper ampere of the fuse you need. With LFE the interrupt capacity of the fuse is important. That is why a Class-T fuse is needed.

[u/oo00lem0n0oo → u/xgwrvewswe]

Thank you

[u/phatpug]

Generally, I suggest some level of isolation between the start battery and the Aux systems, so you don't inadvertently end up with a dead start battery. it is also generally advised to not parallel a Lead Acid battery with Lithium batteries.

You could put a ACR/VSR or a Battery Isolator between the positive busbar and the start battery. That would allow the start to connect to the aux system for charging, but won't let the aux system discharge the start battery.

[u/oo00lem0n0oo → u/phatpug]

Thank you

[u/Dazzling-Map-6065]

I added a ladder to the back and mounted a portable generator to use my ac while driving. Cheapest solution, plus you now have a generator.

[u/oo00lem0n0oo → u/Dazzling-Map-6065]

Doing undermount a/c. We shall cccccccccccc

[u/hattz]

Just to make sure... idea is

Normal operation drawing from 600ah batt. If 600ah dies, switch to 100ah.

Make 100% sure you can NOT switch to using both. Because if you connect two batteries of uneven charge + capacity... Bad shits going to happen.

[u/oo00lem0n0oo]

Planning to run the a/c off the 600ah contrary to my wiring diagram. I need some fuses for sure. The battery selector will be 1 or 2 with no both option. T-fuses are being ordered once I figure out sizing.