SOK 12v/314 uneven discharging Multipluss II 12/3k

Bit of a rabbit hole this one, and I've been down a similar path with my motorhome setup so thought I'd chip in.

Running a pair of Multiplus II 12/3000s myself, originally with four 12V batteries daisy-chained. The uneven discharge problem nearly drove me mad — one battery doing all the heavy lifting whilst the others sat there looking smug. Took me an embarrassingly long time to realise the issue wasn't the Victrons at all.

The root cause in my case came down to cable lengths and connection points. When you daisy-chain, the batteries closest to your busbars see the load first. Physics, innit. I switched to a proper star topology — equal length cables from each battery to a central busbar — and the difference was immediate. Resting voltages across all four banks evened out considerably.

Worth also checking:

• Are all your SOK units running the same firmware? Mixed firmware can cause BMS communication quirks
• Are you using the Ve.Bus to monitor individual battery contributions, or just total bank voltage?
• What's your cable cross-section between batteries? Undersized interconnects cause resistance differences that compound over a discharge cycle

The SOK 314s are solid units — I nearly went Fogstar Drift before settling on a different chemistry — but they're still only as good as the installation around them.

Curious whether anyone's tried active balancers between units in a non-lithium-to-lithium config? I've seen some chat about it but nothing conclusive for this sort of parallel bank arrangement.

Would love to know how others have tackled this.

@PennineCamper - daisy-chaining 12V batteries with a Multiplus II setup is almost always where the trouble starts with uneven discharge. The MP II is quite sensitive to voltage differentials across the bank, and if your cabling isn't perfectly symmetrical you'll get one battery doing the heavy lifting every time.

Worth checking your bus bar connections first - equal cable lengths and gauge to each battery makes a surprising difference. Also, have you confirmed the SOKs are all running the same firmware? Had a situation where two packs from different batches had slightly different cell balancing thresholds which caused all sorts of grief.

What are your charge voltage settings in VEConfig? Sometimes the absorption voltage needs nudging up slightly with SOK to ensure all cells top-balance properly before the BMS cuts off. What does Victron Connect show for the individual battery voltages at rest?

Really interesting thread this one. @PennineCamper I'd be curious whether you've checked the internal resistance readings across all four SOKs individually? Even brand new cells from the same batch can vary enough to cause one battery to carry a disproportionate share of the load. With the Multiplus II pulling serious current during inverting, that imbalance gets amplified pretty quickly.

Have you got the VE.Bus Smart Dongle fitted? Worth keeping an eye on the DC voltage right at the battery terminals versus what the Multiplus is seeing - a voltage drop across your cabling or connections can fool the charge algorithm into cutting off too early on some cells whilst others are still working hard. Busbar quality and cable length matching between batteries makes a surprising difference at these current levels.

Great thread, had almost identical grief with my own 12V bank last year. @PennineCamper one thing worth checking that hasn't been mentioned yet - are your battery cables all the same length and gauge running to a common busbar? Even a small difference in resistance between cables can cause one battery to shoulder more of the load, which shows up as uneven state of charge across the bank over time. I ended up redoing all mine to equal lengths and it made a noticeable difference. Also worth confirming your Multiplus II firmware is current - there was a patch a while back that improved battery balancing behaviour. @AshDawn raises a fair point about internal resistance too, that's definitely worth logging over a few cycles to spot any outlier cells early before they cause bigger headaches.

Hey @PennineCamper, jumping in here as I've wrestled with almost exactly this.

One thing nobody's mentioned yet - have you checked your busbar connections specifically under load? I found my uneven discharge was actually being masked by a loose negative busbar connection that only showed resistance when drawing serious current. Looked fine on a visual inspection, dead tight with a multimeter probe resting on it, but dynamically under 2kW+ draw it was causing one battery to shoulder far more than its share.

Worth sticking a clamp meter on each battery's positive lead while the Multiplus is working hard and comparing the actual current draw per unit. If they're wildly different rather than roughly equal, that's your culprit right there rather than anything in the BMS or the Victron config.

What firmware version are you running on the Multiplus IIs out of curiosity?

@PennineCamper the Multiplus II is basically a very expensive way to discover that your batteries have opinions about sharing the workload 😄

Seriously though — on my narrowboat I had two SOK 206Ah units behaving like bickering siblings until I realised the BMS trip thresholds were ever so slightly mismatched between the two