Anyone else had their Fogstar Drift cells sag badly under load in cold weather?

Pulled 40A from my 280Ah bank last week when it was 3°C in the garden office and the Victron MPPT was throwing a fit over the voltage drop — down to 48.1V from resting 52V, which seemed excessive for a relatively fresh pack.

BMS is a JK 2A active balancer, cells were all within 0.02V before the load hit, so I don't think it's a balancing issue. More like the chemistry just hates the cold.

Anyone running LFP in an uninsulated outbuilding through winter — is a bit of livestock heating mat under the battery box actually a viable fix, or am I being daft?

@QuietHiker 48.1V in 3°C? Your cells are basically doing what I do on a cold Monday morning — technically functional but deeply unhappy about it 😂

LiFePO4 internal resistance genuinely does climb when it's cold, so some sag is expected, but that does sound steeper than it should be. Worth checking a

@QuietHiker that voltage drop isn't surprising unfortunately — LiFePO4 internal resistance can roughly double below 5°C, so you'll see noticeably more sag under heavy loads. 40A from a 280Ah bank is only around 0.14C which should normally be fine, but the cold is doing the damage here.

Worth checking whether your BMS has any temperature compensation settings, and if the cells are actually sitting outside exposed to the air. Even a bit of insulation around the battery enclosure makes a meaningful difference — I wrapped mine in 25mm PIR offcuts and it helped considerably.

What does your BMS show for individual cell voltages under load? If one or two cells are sagging harder than the others you might have a balancing issue compounding the cold weather problem.

@QuietHiker had almost identical drama last winter on the boat — 280Ah Fogstar bank, bitter morning, pulling current for the inverter and watching the Cerbo GX display numbers that made my stomach drop.

What sorted it for me was adding a bit of pre-warming before hitting the bank hard. I ran a small heating pad (thermostatically controlled, tucked between the cells) powered from a tiny separate lead-acid buffer just to bring the LiFePO4 up past 8°C before any serious load.

Night-and-day difference. The sag went from embarrassing to barely noticeable at the same draw.

Also worth checking your BMS settings — some units will throttle discharge current aggressively in cold as a protection measure, which looks like cell sag but is actually the BMS doing its job. Might explain the Victron throwing a wobbly too.

@QuietHiker worth checking whether your BMS has any temperature compensation settings — some will actually throttle max discharge current automatically in the cold, which can make voltage sag look worse than the cells actually are. Also, is your bank in the garden office itself or outside? Even getting the cells up to 10°C makes a noticeable difference. I've got mine insulated with a bit of closed-cell foam in my shed and it's transformed cold weather performance. The sag you're describing isn't necessarily a sign anything's wrong with the cells themselves — more just physics being awkward. What's your resting voltage looking like once they warm up mid-morning? That'll tell you a lot about actual cell health versus just temperature effects.

@QuietHiker I'd also look at what your Victron MPPT is configured to do when it sees that voltage sag — if you've got the low voltage disconnect set too conservatively it'll start backing off charge current at exactly the wrong moment and make the whole thing look worse than it is.

Worth having a poke around in VictronConnect and checking your absorption/float voltage settings haven't drifted, as some people find the defaults need nudging for cold weather operation. Also, are all your cell interconnects properly torqued? Loose busbars become surprisingly significant at higher currents and low temps combined — I've seen a single dodgy connection add noticeable resistance to an otherwise healthy bank.

@QuietHiker this is pretty normal LiFePO4 behaviour once you drop below about 5°C — internal resistance climbs noticeably and you'll see that sag even on a healthy bank. On my narrowboat I've learned to just accept a bit of voltage drop on cold mornings rather than panic about it.

What I'd actually check is your cell-level voltages rather than pack voltage — if one cell is lagging significantly behind the others under load, that's the worry. The Fogstar Drifts themselves seem reasonably consistent in my experience, but a weak cell will always show itself more dramatically in the cold.

Worth logging a few charge/discharge cycles with the Victron VRM portal if you're not already — the data over time tells you far more than a single voltage reading on a 3°C morning.