Connecting Multiplus trickle charger to LA starter battery, while using LFP domestic battery?

Been wrestling with this exact scenario on my own build and figured it was worth opening up for discussion.

Running a Victron Multiplus 12/800 with a 200Ah Fogstar Drift LFP bank as my domestic setup. Everything's great when I'm plugged in or the alternator's spinning — but stick the van in storage for a few weeks and the starter battery quietly sulks its way down to nothing. The LFP doesn't care, it just sits there at 3.3V per cell minding its own business, completely indifferent to the lead-acid's suffering.

The obvious fix is using the Multiplus's built-in PowerAssist or a dedicated trickle output to float the starter battery, but here's where it gets interesting: the Multiplus is configured around the LFP chemistry (absorption/float voltages set accordingly), so you can't just blindly tie the LA starter battery across the same bus without potentially cooking it or — worse — confusing the BMS.

What I'm currently considering:

• Victron Orion-Tr Smart DC-DC isolated charger between the LFP bank and the starter battery, set to a proper LA float profile
• Alternatively, a standalone trickle charger (something like a CTEK MXS 5.0) on a timer fed from shore power

Anyone actually wired an Orion or similar into a Multiplus-based LFP system cleanly? Curious whether you've triggered any BMS oddities during low-current trickle phases, particularly with the Smart Shunt miscounting small parasitic draws.

Feels like there should be a cleaner, more integrated solution here — maybe something configurable in VE.Configure that I'm missing. Thoughts?

@GoldenNomad ran almost exactly this setup in my tiny house on wheels for about eight months before I sorted it properly.

The bit that caught me out — the Multiplus doesn't natively split charge profiles between two battery types. What actually worked for me was using the Victron Battery Isolator (the 1+2 model) combined with setting the Multiplus bulk/absorption to suit the LFP bank, then letting the isolator trickle whatever's left across to the LA starter.

The LA will happily accept a lower voltage tail — it doesn't need the full AGM profile to stay healthy, especially if it's just cranking duties.

Worth checking your VE.Configure settings though. If you've got the charge current wound up for fast LFP cycling, the isolator path can occasionally push more than you'd expect into a small starter battery.

Been through this exact headache on my motorhome build. The bit that caught me out was the Multiplus's charge profile — even on the trickle setting it'll push a voltage that's fine for the LFP but slightly aggressive long-term for a LA starter battery if your alternator isn't also doing its bit to regulate things.

What actually worked for me was fitting a Battery-to-Battery charger (I used a Victron Orion-Tr Smart) between the two banks rather than relying on the Multiplus to babysit the starter battery at all. Keeps everything isolated and the LA gets a proper dedicated charge curve.

@Chippy curious what your "sorted it properly" solution ended up being — eight months is a decent trial run before committing to a fix!

Really interested in this thread as I'm currently planning a garden office build with almost identical components.

@Stacey could you expand on the charge profile issue? I've been wondering whether the Multiplus needs to be configured specifically for LFP (absorption voltage, float settings etc.) and whether that then causes problems for the LA starter battery sitting alongside it.

Is the usual approach to use a Cyrix or similar battery-to-battery charger to isolate the two chemistries properly? Or are people running a dedicated trickle charger for the starter battery entirely separate from the Multiplus?

Also wondering whether anyone has experience with this in a static installation rather than a vehicle — presumably the starter battery sees less regular use which might complicate things further?

The key thing nobody's mentioned yet: the Multiplus's absorption/float voltages are tuned for LFP, so your LA starter battery will be chronically undercharged if you run charge sharing directly from the DC bus.

What actually works: fit a Victron Cyrix-Li-Ct between the two banks. It's specifically designed to handle this LFP/LA split — it reads the BMS signal and only bridges the batteries when conditions are safe, then drops out cleanly.

On my narrowboat I run almost this exact topology. The Cyrix does the heavy lifting; the starter battery tops up when the LFP bank is in bulk/absorption, and disconnects before float kicks in.

@Stacey is right to flag the charge profile — absorption voltage on LFP (14.2V) is too low to fully charge a 12V LA. The Cyrix sidesteps this entirely.