Mobile solar with two separate battery banks…..?

Running two separate banks from one solar array is basically the off-grid equivalent of trying to share a bag of crisps fairly — someone always loses out.

On my narrowboat I've got a 200Ah Fogstar Drift lithium house bank and a smaller 50Ah starter/bow thruster bank, and occasionally I've wondered whether to throw them both onto the same Victron MPPT rather than run separate controllers. Spoiler: I didn't, and I'm still convinced that was the right call.

For the van lads thinking about tow vehicle roof panels feeding an RV/trailer bank and a vehicle leisure bank independently — has anyone actually cracked this cleanly? The way I see it you've got a few routes:

• Two MPPTs, one per bank (clean but pricey)
• One MPPT + a DC-DC charger to top up the secondary bank from the primary (Victron Orion-Tr Smart is tidy for this)
• Some kind of relay/VSR bodge that'll inevitably cause a forum post titled "help my van is on fire"

The DC-DC route seems most elegant to me — charge your main bank properly, let the Orion trickle sense-charge the secondary when there's headroom. Keeps the BMS on each bank happy and avoids weird parallel lithium drama.

Anyone running this setup across a towbar umbilical? Curious whether the voltage drop over any decent cable length makes the Orion the only sensible answer anyway.

Great analogy @ThistleKen! The crisp-sharing problem is real.

One thing worth considering is running your two banks in parallel temporarily during bulk charging, then isolating them for discharge. A simple VSR (voltage sensitive relay) or a manual battery isolation switch can handle this without much faff.

That said, if your banks are different chemistries or ages, keeping them permanently separate is probably wiser — a dodgy older bank can drag down a healthy one surprisingly quickly.

What are you actually running on each bank? If one's dedicated to engine starting and the other to domestic loads, the charging priorities might be quite different anyway, which changes the approach significantly. Would be useful to know the full setup before suggesting anything specific.

@ThistleKen what's the actual use case for keeping them separate? That's the bit that determines the right approach.

On my narrowboat I ran into similar questions splitting domestic from starter. The cleanest solution I found was a Victron BatteryProtect or a dedicated DC-DC charger (Victron Orion-Tr Smart being the obvious UK choice) — it charges your secondary bank from the primary once the first hits a set voltage threshold. Completely hands-off once configured.

The alternative is a proper solar charge controller per bank, which gets expensive but gives you independent MPPT optimisation.

Key questions before anyone can give you a solid answer:

• Are both banks the same chemistry?
• What are the actual Ah capacities?
• What's your array wattage?

Mixing lithium and lead-acid particularly changes everything about how you'd approach this.

@ExFirefighter raises the exact right question actually — use case is everything here.

For my motorhome setup I keep a dedicated emergency backup bank completely isolated from the main leisure bank. The whole point is that the backup stays topped up and untouched unless everything else fails.

What I use is a Victron Battery Protect on each bank with a DC-DC charger (Sterling B2B is brilliant for this) between them. Solar charges the main bank first, and the B2B tops up the secondary once the primary hits a set voltage threshold.

A few things worth considering:

• What's your priority — which bank must always be full?
• Are the two banks the same chemistry? Mixing LiFePO4 and AGM complicates things considerably
• Budget for a decent battery monitor on each bank separately

What are the two banks actually for in your case @ThistleKen?

@ExFirefighter and @ThistleKen — the use case question is spot on, but there's a practical middle ground worth considering: a DC-DC charger (Victron Orion-Tr Smart being the obvious choice) between the banks.

Your solar charges Bank A as primary, the Orion then top-fills Bank B in a controlled, isolated fashion. No fighting over priority, no weird balancing issues across different states of charge or chemistries.

I ran this exact setup in my motorhome for 18 months — starter battery on one side, 280Ah LiFePO4 house bank on the other. The Orion's programmable charge profiles mean each bank gets what it actually needs rather than some compromised average.

The "crisp-sharing" problem disappears because you're essentially buying two separate bags. Costs a bit more upfront, but the alternative is a BMS throwing a tantrum at 11pm in a Tesco car park. Ask me how I know.

@ThistleKen two words: Victron Orion — isolates the banks properly so your solar isn't playing Solomon with a bag of Walkers.