Anyone else finding their 12v system struggling in this recent cold snap?

Been a bit worried this past week — my two 100Ah AGM batteries (Victron SmartShunt keeping an eye on things) have been sitting at around 70% SoC by morning even with my 200W of roof panels. Normally I'd be back to full by early afternoon but with these grey skies and the temperature barely creeping above 4°C, I'm lucky to see 80% by 3pm before the light goes completely.

I know AGMs aren't brilliant in the cold compared to lithium, but I wasn't expecting quite this much drop-off. I've got a small 240v immersion heater running off my Honda EU22i for about an hour each morning to heat the water tank, and I'm wondering if that combined with the reduced charging is just pushing things too hard. The Victron is showing my average daily consumption at around 85Ah which feels high, though I haven't sat down properly to audit everything yet.

Has anyone got good tips for stretching things through winter without going down the lithium rabbit hole just yet? Particularly interested in whether it's worth getting a proper battery blanket or insulating the battery box better — they're currently in an uninsulated compartment under the van. Would love to hear what others are doing to cope.

MarshHermit | 847 posts

@RetiredNurse43 completely normal for this time of year, don't panic! Two things working against you simultaneously — AGMs lose a significant chunk of their usable capacity in the cold (roughly 20-30% at near-freezing temps), AND your panels are generating far less with those low sun angles and shorter days. Your SmartShunt is also likely reading a warmer battery temperature than actual, so the SoC figure might be slightly optimistic too.

Honestly 70% by morning through a cold January night isn't terrible depending on your consumption. Worth checking what your SmartShunt shows for average daily draw — that'll tell you whether it's a generation problem, a consumption problem, or simply the batteries not performing to their rated capacity in the cold.

Have you got any supplementary charging? Even a small mains trickle charger on bad weeks makes a real difference.

BirchRunner | 312 posts

@RetiredNurse43 something that caught me out my first winter at the cabin — AGMs genuinely lose a meaningful chunk of usable capacity when temperatures drop. We're talking 20-30% less available capacity at around 5°C compared to a balmy summer's day.

Your 200W array is also doing very little right now. December sun angles in the UK mean you're lucky to harvest 30-40% of rated output even on a clear day.

What's your overnight load like? Running a 12V fridge or anything resistive (heating elements are brutal) will accelerate that morning dip considerably.

I eventually swapped to LiFePO4 — Fogstar cells — partly because they hold capacity in the cold far better than AGMs. Not suggesting you do that immediately, but worth knowing why the chemistry matters when budgeting for the future.

PanelSteve | 1,204 posts

@RetiredNurse43 welcome to winter on a narrowboat — where "off-grid living" quietly becomes "off-grid surviving".

Two 100Ah AGMs in the cold is basically asking two pensioners to run a marathon. They can do it, they just