Parallel/Serial 12V battery BMS solution needed

Running a similar setup in my garden office — four 12V Fogstar Drift 100Ah cells in a 2S2P config and it gave me a right headache sorting the BMS situation properly.

Honestly the cleanest solution I found was ditching the individual battery BMSs and going with a single Victron SmartShunt paired with a dedicated multi-cell BMS that treats the whole bank as one unit. Less points of failure, much easier to monitor via the Victron app.

The problem with relying on each battery's built-in BMS in parallel/series configs is they can fight each other during balancing — especially if the cells aren't perfectly matched. Seen a few threads on here where people have had unexpected shutdowns because one BMS tripped and the others couldn't compensate cleanly.

Few options worth looking at:

• Daly BMS — cheap, works, community support is decent
• JK BMS — active balancing, probably the one I'd go with if starting fresh
• Batrium — overkill for most but bulletproof

What voltage are you actually running the whole bank at? And are these lithium or AGM? Makes a big difference to the approach.

Would be good to get a few more people's thoughts on this — there's no single "right" answer and it depends massively on your use case.

@CE_Builds mate, four 12V Fogstars in 2S2P is basically asking the universe for a balancing nightmare — just wire them all in parallel and let a single 12V BMS have a quiet life. 🔋

@CE_Builds the cleanest approach I've seen (and what I run myself) is to treat each parallel pair as a single unit before going series. So balance your two 12V pairs independently first, then series them up to 24V.

Each parallel pair should have its own BMS ideally — Daly or JBD units are cheap enough. Then a proper 24V BMS or the Victron Smart BMS handles the series side.

Mixing parallel and series with a single BMS is where people go wrong — the BMS can't see cell-level differences across both dimensions simultaneously.

@RussHunt82 is hinting at the balancing issue which is real, but it's manageable if you structure it properly rather than bodging one BMS across the whole lot.

What's your current discharge current requirement? That'll determine whether a Daly is sufficient or you need something beefier.

@CE_Builds what voltage are you actually running at the load end — 24V I'm assuming from the 2S2P? Because that changes the BMS options considerably.

I've got a similar headache in my static caravan with Fogstar cells and ended up going with individual BMSs on each battery before paralleling — messy wiring but it's worked solid for 14 months now. Only downside is the Victron Cerbo doesn't see them as one unit which irritates me.

Have you looked at the Daly Smart BMS range? Decent Bluetooth monitoring and not bank-breaking. Genuinely curious whether a single 24V BMS rated high enough would be cleaner here — anyone run that config without balancing grief?

Good thread this. @MarineGaz is on the right track treating pairs as units first — that parallel-before-series approach really does save headaches.

One thing worth adding: with Fogstar Drifts specifically, I'd strongly recommend letting the parallel pairs sit connected but unloaded for a good 24-48 hours before you finalise your BMS setup. Let the cells self-balance passively first. I did something similar on my narrowboat install and it made a noticeable difference to how settled the whole bank was from the off.

@LisaStewart71 raises a fair point about load voltage too — worth confirming your inverter/charger is properly configured for 24V nominal, as I've seen folk catch out by leaving equipment on 12V settings.

What BMS models are you actually considering? Some handle 2S topologies more gracefully than others in my experience.

@CE_Builds had exactly this grief on the narrowboat before I sorted it — the bit nobody mentions is that your parallel cells need to be balanced to within ~10mV of each other before you ever connect them in series, otherwise your BMS is fighting two problems at once.

Worth grabbing a decent cell internal resistance meter too; mismatched IR across your Fogstar Drifts will cause one pair to shoulder more current than the other under load, which no BMS topology will fully compensate for.

Victron's SmartShunt on the 24V output side at least gives you visibility of what's actually happening at the load end once everything's wired up properly.

@CE_Builds yes, 24V from the 2S2P — @LisaStewart71 has the right idea there.

The bit I'd add: each of those Fogstar Drifts already has an internal BMS, which sounds helpful but actually complicates things. You're essentially stacking BMS logic on top of BMS logic.

My current setup runs four Fogstar 100Ah in the same config and I ended up treating each parallel pair as a single logical unit before connecting them in series — so the parallel connections go in first, let them balance, then series the two pairs together.

Also worth checking your Victron MPPT settings if you're charging through one — the absorption voltage needs to account for the full 24V string, not just a single 12V unit. Caught me out badly first time round. 🙄