Anyone else's cabin battery bank absolutely giving up the ghost in January?

My 200Ah Fogstar lithium setup in the Welsh borders cabin has been reading a suspiciously optimistic 47% SoC all week despite barely running a 12V fan and a couple of LED strips — the Victron BMV-712 is basically gaslighting me at this point.

Panels are 2x 200W Renogy on a 30° pitch, and with the sun barely clearing the treeline before disappearing again, I'm pulling maybe 8-10Ah on a good day. Meanwhile the MultiPlus is just sat there judging me.

Anyone running a small cabin setup through a British winter without losing the will to live — what's your secret, because mine clearly isn't working?

Hey @MultiPlusNerd, that stuck SoC reading is a classic sign your BMV-712 needs a full synchronisation. The monitor loses its reference point over time, especially if you've had any partial cycles through the winter.

What you want to do is run the battery down to a known low point, then do a proper full charge until the absorption stage fully completes and the tail current drops right off — the BMV will resync its 100% reference from there.

Also worth checking your Peukert exponent and charge efficiency factor in the BMV settings. Fogstar cells are pretty capable but if those figures are set for a generic profile they'll throw your readings right out. January cold doesn't help either — even lithium loses a bit of usable capacity below about 5°C. What temperatures are you seeing at the battery itself?

GarySmith58 | 47 posts | East Midlands

@MultiPlusNerd before you go diving into settings, worth checking whether your shunt connections have any corrosion on them first — had exactly this with my Fogstar 200Ah last February and the culprit was a slightly loose negative terminal at the shunt rather than anything software-related. Even a tiny bit of resistance there throws the BMV's current readings completely off and you end up with a SoC that just sits stubbornly wherever it fancies.

Give all the connections a proper torque-check and look for any white powdery residue around the terminals. Once I sorted mine and did the resync @CraigDavies mentioned, it was spot on again. January cold doesn't help either — lithium cells show reduced capacity in low temps regardless of what the monitor reckons.

DucatoCamper | 134 posts | Peak District

@MultiPlusNerd worth checking your Fogstar cells individually if you've got a way to measure across each one. January cold can mask a dodgy cell that's quietly dragging the whole bank down — the BMS might be papering over the cracks and your BMV is just reading what it's told.

Also, what's your charged voltage set to in the BMV settings? If it's slightly out from what your charger actually hits, the sync point never triggers properly and the SoC just drifts into fantasy land. I had almost exactly this on my Duато last February — turned out my absorption voltage was set 0.1V too high in the BMV. Five minute fix once I spotted it.

@MultiPlusNerd worth considering whether your Fogstar cells have drifted out of balance over winter. If the BMS is reading a mid-pack voltage and one or two cells are sitting significantly lower, the pack will hit low-voltage cutoff well before your BMV-712 expects it to — which often manifests as that "frozen" SoC reading you're describing.

Pull the actual cell voltage data if your BMS exposes it (some Fogstar units do via Bluetooth). A spread greater than ~50mV under load is a red flag.

Also, January ambient temperatures in the Welsh borders will be suppressing your usable capacity noticeably — lithium loses meaningful capacity below 5°C even when technically operational. If your battery is in an uninsulated space, that 47% might be representing far less actual energy than it did in September.