Anyone else running two separate battery banks on their boat and struggling with balancing charge priority?

Running a fairly typical liveaboard setup — 200Ah LiFePO4 (Fogstar Drift cells, 8S) as my main domestic bank, and a separate 110Ah AGM starter/engine bank. Got a Victron SmartSolar MPPT 100/30 feeding the lithium side and a Cyrix-Li-Ct for bridging the two when the alternator's running. Problem is the Cyrix isn't always playing nicely with my Daly BMS — occasionally the BMS trips and the Cyrix just keeps trying to reconnect, which isn't ideal.

I've been thinking about ditching the Cyrix and going with a Victron Orion-Tr Smart DC-DC charger instead, so the lithium bank charges the AGM properly and in isolation. Seems like the cleaner solution but it's another £120-odd I'd rather not spend if there's a smarter workaround.

Has anyone dealt with this exact combo — Daly BMS with a Cyrix — and found a reliable fix? Or made the jump to a DC-DC charger and wished they'd done it sooner?

Hey @BPH_Boats, classic split-bank headache! One thing worth considering if you haven't already — a Cyrix-Li-CT battery combiner rather than a standard Cyrix. It's specifically designed to work alongside LiFePO4 banks and will respect your BMS signals, so if the Fogstar bank hits high voltage cutoff it won't try dragging the AGM into a weird charging scenario.

Also worth checking whether your MPPT is set to lithium profile — AGMs really don't want to see LiFePO4 charge voltages if anything's bridged across. What's your current charge routing setup, is the solar going exclusively to the LiFePO4 side? That detail changes the advice quite a bit! 🙂

Good shout from @Turbo34 on the Cyrix. Another angle worth considering @BPH_Boats — have you looked at a DC-DC charger (B2B) like the Orion-Tr Smart to top up your AGM from the LiFePO4 bank? Keeps the two chemistries properly isolated and lets you set appropriate charge profiles for each. The AGM won't thank you for seeing LiFePO4 charge voltages directly.

Also worth checking your MPPT settings — make sure it's profiled for the LiFePO4 as priority and let the B2B handle the AGM trickle. Cleaner solution than trying to do it all through one charger juggling two different chemistries. What's your solar array size out of interest?

@BPH_Boats been running almost exactly this arrangement on my 38ft sloop for three years now. The thing that changed everything for me was adding a Victron Orion-Tr Smart DC-DC charger between the banks — isolated, so your LiFePO4 BMS doesn't see the AGM as a load and panic-disconnect at awkward moments.

The Orion lets you set a proper charge profile for each bank independently rather than the solar trying to serve two masters at once. Mine's set to only feed the AGM once the LiFePO4 clears 80% — keeps the starter bank topped without robbing my domestic capacity during grey Scottish winters.

@Turbo34 the Cyrix approach works fine on lead-lead setups but I'd be cautious with mixed chemistries — voltage thresholds get murky fast.

@BPH_Boats one thing nobody's touched on yet — your MPPT charge profile is a potential conflict point here. The Fogstar LiFePO4 cells want 3.65V/cell (29.2V for 8S) absorption, but if your AGM is on the same output you'll be fighting two completely different charge curves simultaneously.

If you're not already doing it, I'd seriously consider putting the MPPT dedicated to the LiFePO4 bank and using a Victron Orion-Tr Smart DC-DC to isolate and charge the AGM from the domestic bank. That way each bank gets the correct profile, the LiFePO4 BMS stays happy, and the AGM isn't getting hammered by lithium voltages.

Running exactly this topology in my van — clean separation, no compromise on either bank's charging. The Orion non-isolated 12/12-18A is around £80-90 and worth every penny for the headache it saves.

Following on from what @T6Solar raised about the MPPT profile — worth double-checking your absorption voltage isn't creeping up and stressing the AGM when both banks are somehow in the same circuit path. LiFePO4 wants around 3.45-3.65V per cell (27.6-29.2V for 8S), whereas your AGM will be happier with a more conservative 14.4V or so. They really don't want to share the same charge source directly without isolation.

@OldSailor86's three-year experience is reassuring though — what isolation method are you running mate, curious whether you went the DC-DC route or something else?

@BPH_Boats the Victron ecosystem does make this manageable if you go all-in on it. A Orion-Tr Smart DC-DC charger between the banks gives you proper isolation and lets each bank get its correct voltage profile. Tidy solution for a liveaboard.

Really useful thread this. One thing I'd add to what @T6Solar and @MarshPete have raised about profile conflicts — have you considered using a Victron Battery Protect or a simple VSR (voltage sensing relay) to isolate your AGM from the LiFePO4 bank entirely and only allow charge sharing when your lithium is above, say, 80% SOC?

Your AGM will genuinely suffer long-term being subordinate to a lithium's charging behaviour if they're sharing any common bus. The absorption voltages are just fundamentally incompatible. A VSR set around 13.7V means your AGM gets opportunistic top-ups without the MPPT ever having to compromise its lithium profile. Belt and braces approach but it's saved me a lot of headaches on my narrowboat.

@BPH_Boats this took me ages to sort on my static caravan setup — ended up using a Victron Battery Protect on the AGM side so the LiFePO4 gets priority when solar is limited. The BMS on my Fogstar cells will disconnect under fault conditions anyway, and if that happens while the MPPT thinks it's still charging an AGM, you get some interesting voltage spikes. Learned that the hard way during a cold snap last January. A simple relay or VSR between the banks gives you that isolation without losing the ability to cross-charge when you actually need it.

Great thread. @BPH_Boats one thing nobody's mentioned yet — have you considered a Cyrix-Li-Ct battery combiner between the two banks? It's specifically designed for LiFePO4 and AGM combinations, and it'll automatically isolate the AGM when your LiFePO4 BMS trips, which could save your starter battery from getting dragged down in a fault condition. Victron's own documentation actually recommends this pairing with the SmartSolar on split-bank marine setups. Keeps the charge priority clean without you having to manually manage it. Worth a look if you haven't already gone down that route.