Connect the Multiplus II to the vehicle electrical system

Been thinking about this setup for a while actually, as I'm planning something similar for a campervan conversion alongside my garden office build.

The idea is to use the Multiplus II as the heart of the system — handling both the leisure battery bank (probably going with Fogstar Drift LiFePO4 cells) and keeping the starter battery topped up when hooked up to shore power on a campsite.

From what I've read, you can use the Victron Battery to Battery (B2B) charger to isolate the starter battery properly rather than wiring it directly into the Multiplus II's DC bus. This keeps the starter battery circuit clean and avoids any weird interactions with the vehicle's smart alternator — those modern Euro 6 engines can be really fussy about what you connect to them.

What I'm less clear on:

• Whether the Multiplus II's charger profile needs to match the starter battery chemistry (standard flooded lead acid in my case)
• Whether a VSR (voltage sensitive relay) is still worth having in this kind of setup, or if the B2B makes it redundant
• Any issues with the VE.Bus configuration when two separate battery banks are involved

Has anyone here actually wired a Multiplus II into a vehicle system like this rather than just a static off-grid setup? Curious whether VictronConnect or Venus OS gives you decent visibility across both banks simultaneously.

Would be good to hear from anyone who's done this — especially if you've used a Cerbo GX to tie it all together.

Watching this thread closely — doing something very similar in my motorhome build.

One thing worth flagging: the Multiplus II's PowerAssist feature is genuinely useful here, but you need to be careful about the input current limit settings if you're pulling from the vehicle's alternator. Too aggressive and you'll cook the alternator over a long run.

Has anyone used a Victron Orion-Tr Smart DC-DC charger in between rather than connecting the Multiplus directly to the vehicle system? Seems like it might offer better isolation and protection for the vehicle's electrics.

Also wondering what @DanPhillips99 is planning for the earth/chassis bonding — that can get complicated when you're bridging vehicle and shore power systems on a Multiplus II. The neutral switching behaviour catches people out.

Good shout on the Multiplus II — running one in my motorhome and it's solid kit.

One thing nobody's mentioned yet: the DC input wiring is absolutely critical. Undersized cables between your alternator/battery and the Multiplus will throttle your charging current badly. I learned this the hard way — was scratching my head wondering why my charge rate was rubbish, turned out the previous owner had bodged it with 16mm² when it needed 35mm² minimum for the current I was pulling.

Also worth considering Victron's VE.Bus integration if you're running a Cerbo GX — gives you proper visibility over what the vehicle system's actually doing vs what the Multiplus thinks it's doing. They don't always agree, especially with smart alternators on modern vehicles.

What alternator are you working with @DanPhillips99? That changes things considerably.

Great thread! One thing I'd add — make sure you're thinking carefully about your cable sizing between the vehicle alternator circuit and the Multiplus II. A lot of people underestimate the current draw when the charger kicks in at full capacity, especially if you're running a larger model like the 3000VA version.

Also worth considering a Battery Protect or properly rated fusing close to your leisure battery bank. Victron's own documentation is actually pretty thorough on this, but it's easy to gloss over.

@DanPhillips99 are you planning to use the Multiplus II's built-in transfer switch to also handle shore power hookups at campsites? If so, that changes the wiring topology a fair bit and might be worth planning for from the outset rather than retrofitting later. 😊