Anyone else running two different BMS units in parallel on a split bank setup?

Running a 200Ah Fogstar Drift LiFePO4 alongside an older 100Ah Battle Born on my narrowboat, each with its own BMS (the Fogstar's internal one plus a Daly 100A on the Battle Born). They're wired in parallel through a Victron SmartShunt sitting between the bank and the 12V bus. Overall capacity reads fine on the Victron and the Cerbo GX sees both, but I'm getting occasional cell voltage discrepancies between the two banks that I suspect are causing one BMS to throttle charge before the other is full.

The symptom is the Victron MPPT backing off around 13.8V when I'd expect it to push to 14.2V absorption — I think the Fogstar's internal BMS is hitting its high-voltage threshold slightly earlier than the Daly, effectively dragging the whole parallel bank down. Checked the Daly settings via the PC software and it's set to 3.65V per cell cutoff, but I've no visibility whatsoever into the Fogstar's internal limits.

Has anyone solved this properly — either by rebalancing the banks before paralleling, or by just accepting that mixed BMS setups need separate charge circuits? Considering splitting them onto separate Victron MPPT outputs but that feels like overkill on a boat where I'm already juggling a 40A shore power charger, a B2B from the engine, and two 175W roof panels.

@JohnBaker ran almost exactly this setup in my garden office for about eight months — Fogstar Drift next to an older Renogy LiFePO4, each with their own BMS wired in parallel.

The problem I kept hitting was the two BMS units cutting out at slightly different low-voltage thresholds. One would disconnect first, then the entire load suddenly dumped onto the remaining bank — which immediately triggered that BMS too. Cascade disconnections at the worst moments.

What actually solved it was setting my Victron MPPT absorption and float voltages conservatively so neither bank was ever getting pushed hard enough to trigger high-side protection either. Kept things peaceful.

On a narrowboat your charging sources are probably less predictable than mine, so I'd watch those cut-off thresholds carefully. Mismatched BMS behaviour is the real gremlin here, not the different battery brands themselves.

@JohnBaker this is almost exactly what I had on my sailboat for about eighteen months before I bit the bullet and rebuilt the whole bank.

The problem I kept running into — and it'll likely bite you too — is that the two BMS units have different cutoff voltages and response times. When one trips, the other suddenly sees a massive load spike. Lost a Daly that way, actually.

What saved me short-term was fitting a Victron Battery Protect downstream and letting it act as a referee of sorts, responding to the weakest bank's signal.

Longer term though, mismatched banks like this rarely balance properly. The Fogstar Drift will want to run rings around the older Battle Born chemistry-wise.

If you're also pushing EV charging loads through this setup like I was, the stress on the weaker bank compounds quickly.

@JohnBaker the main thing I'd watch with your setup is voltage mismatch between the two banks causing one BMS to work much harder than the other — especially during bulk charging. The Fogstar's internal BMS is quite conservative with its balancing thresholds compared to a Daly, so you might find the Battle Born bank sitting slightly higher or lower depending on your charge profile.

Worth putting a cheap voltage logger on each bank independently for a week or two just to see what's actually happening rather than guessing. I did this on a similar mixed setup and was quite surprised how much they diverged under load.

Also, have you got a battery-to-battery charger isolating them, or are they genuinely wired straight in parallel? That detail changes the advice quite a bit!

@JohnBaker worth mentioning something nobody's touched on yet — if either BMS trips under load, the surviving bank suddenly has to carry the full current demand on its own. On a narrowboat that could mean your inverter or bow thruster briefly pulling well beyond what the remaining BMS is rated for, potentially triggering a cascade trip.

I'd strongly suggest setting a low-voltage disconnect on your main distribution that's a touch higher than either BMS's own cutoff, so the load drops before either unit is pushed to its limit. A simple battery monitor with a load disconnect relay sorted this for me.

Also keep an eye on charge current sharing — with different internal resistances the Fogstar will likely absorb more than its fair share initially. Not dangerous as such, just worth knowing. 😊