Charge two battery banks using a single set of solar panels.

Has anyone actually done this successfully with a single MPPT controller, or does it always end up needing two separate units?

My situation: I've got a 400W array on the shed roof and I'm trying to figure out whether I can sensibly charge both a 12V leisure bank (Fogstar lithium) and a separate 12V lead-acid bank for starting my backup generator — without duplicating the whole solar setup.

I've seen people mention using a Victron SmartSolar with a battery-to-battery charger downstream, so the MPPT charges the primary bank and a B2B then tops up the secondary. That seems like the most logical approach but I'm wondering if there's any efficiency loss worth worrying about, or whether it's essentially negligible in real-world use.

The other option I've come across is a DC-DC charger with solar assist input, though I'm not entirely sure how that differs from the B2B route in practice.

A few questions I can't quite get my head around:

• Does the MPPT essentially "see" both banks as one load, or does it only ever respond to the primary bank's state of charge?
• Is there any risk of the lithium BMS cutting out and leaving the B2B with nowhere to push current?
• Would a Victron Orion-Tr Smart be the right tool here, or is that overkill for a relatively small secondary bank?

Would be good to hear from anyone who's running a similar split-bank arrangement. Photos of wiring setups particularly welcome if you've got them.

@AlanPalmer57 Done exactly this on the Ducato — one 400W array, two separate banks (leisure and starter).

The honest answer is: one MPPT can only charge one bank at a time, full stop. Physics isn't negotiable, unfortunately.

What actually works:

• Victron SmartSolar into your primary bank, then a Cyrix-Li-Ct or similar battery-to-battery relay to top up the second
• Or a proper DC-DC B2B charger (Sterling and Victron both do decent ones) fed from the primary bank output

The B2B route is cleaner if your banks are different chemistries — say lithium primary and lead-acid starter, which is exactly my setup. The Cyrix gets confused and starts making expensive decisions when the voltages don't match.

Two separate MPPT units sounds like the logical answer but you'd need to split the array, which halves your harvest and makes the whole thing a headache.

@AlanPalmer57 done something similar in my van build. Ended up using a Victron SmartSolar with a DC-DC charger (Victron Orion-Tr Smart) between the two banks rather than trying to split direct from the MPPT. Cleaner solution imo — one bank charges priority from solar, second bank gets fed via the DC-DC.

Only real downside is you lose a bit of efficiency through the conversion, but nothing dramatic. Works a treat for keeping my starter battery topped up off the leisure bank.

@DucatoDream curious what method you went with — some blokes just use a VSR/relay but I found those a bit basic for lithium setups.

@AlanPalmer57 Worth clarifying what you're actually trying to achieve here — are both banks the same chemistry? Because if you're mixing lithium and lead-acid (common scenario: LiFePO4 house bank + AGM starter battery), then two separate MPPTs is genuinely the cleaner solution, since each bank wants different charge profiles.

That said, I ran exactly this arrangement in my garden office build for about eight months — single Victron SmartSolar 100/30 feeding a Fogstar LiFePO4 house bank, with a Victron Orion DC-DC handling the secondary AGM backup. Works a treat, though the Orion isn't free obviously.

If both banks are identical chemistry and voltage, some controllers support a manual alternating relay trick, but honestly it gets messy fast. What's the second bank actually for — emergency backup, or something running simultaneously? That changes the answer considerably.

Right, so I went down this exact rabbit hole when building out the Kangoo — spent three weeks convinced I'd cracked it before realising I absolutely hadn't.

What nobody mentions is the priority question: which bank gets the love first? Most single MPPT controllers will just blend the output toward whichever bank is hungriest, which sounds sensible until your starter battery is quietly sulking at 30% while your leisure bank is living its best life.

@TracyAllen raises the chemistry point and it's crucial — I nearly cooked a lithium bank trying to share charging profile with an AGM. Different absorption voltages, different float targets. The MPPT can only be told one story at a time.

My eventual solution was embarrassingly simple: Fogstar lithium leisure bank, Victron SmartSolar, then a separate DC-DC for the starter. Costs a bit more upfront but you sleep better.