Anyone else finding their AGM bank absolutely useless in winter cold?

Running a 200Ah AGM setup in the motorhome and the performance drop over the past few weeks has been genuinely shocking. Parked up in Northumberland last weekend, overnight temps around -4°C, and by morning the bank was reading 11.9V under virtually no load — just the Victron BMV-712 and a 12V fan running. That's maybe 15–20W of draw overnight. In summer the same bank handles 50–60Ah no problem.

I know AGMs lose a chunk of capacity in the cold — roughly 20–30% at 0°C and worse below that — but this felt more dramatic than the numbers suggest. The batteries are about 3 years old (Leoch 100Ah × 2, so not premium but should be decent enough). Wondering if the cold is compounding what might already be some age-related degradation. My solar (2 × 175W Renogy panels, Victron SmartSolar 100/30) barely touched them the following morning because the voltage was so low the MPPT was clearly confused about state of charge.

Has anyone actually done a capacity test on their AGMs mid-winter versus summer to get real numbers? And at what point did you make the jump to LiFePO4 — was it purely performance, or did the maths on longevity finally stack up? Genuinely considering a Fogstar Drift 200Ah but not sure if I'm just being impatient with chemistry that has real limitations in cold climates.

@Curly AGMs are brutal in the cold — you lose a significant chunk of usable capacity below freezing, and they also won't accept a proper charge until they warm up a bit, so you end up in a vicious cycle.

Went through exactly this a couple of winters ago before switching to lithium. The Fogstar Drift cells handle cold storage far better, though they do still need to be above about 5°C to actually charge safely.

If you're sticking with AGM for now, a few things that help:

• Insulate the battery compartment (even basic foam board makes a difference)
• Don't discharge below 50% in winter — cold + deep discharge kills them fast
• Prioritise getting them warm before bulk charging

Northumberland in January is pretty unforgiving tbh. Have you looked at what your overnight draw actually is vs what the AGMs are realistically delivering at that temp?

Hey @Curly, really feel your pain on this one! Just to add to what @Mark1978 is saying — the charging side is just as problematic as the discharge. At those Northumberland temps your alternator or solar controller needs to be applying higher absorption voltages to actually get meaningful charge back in, and most standard profiles won't compensate properly. So you're potentially running into a double whammy of reduced capacity and incomplete charging cycles.

Worth checking whether your charger/controller has a temperature compensation setting — even a basic temp sensor lead can make a noticeable difference. Also, where's your battery bank physically located in the motorhome? If there's any way to insulate around it (not seal it completely obviously, venting still matters) that helps considerably. Lithium would solve most of this but that's a whole other budget conversation! 😄

Really sorry to hear this @Curly — Northumberland in winter is no joke! Just to add something the others haven't touched on yet: even if your AGM accepts a charge in the cold, its internal resistance shoots up significantly, so your inverter and high-draw appliances will see a much steeper voltage sag under load. Practically speaking, things like kettles or heating pumps might cut out earlier than you'd expect, even if the battery monitor still shows decent capacity remaining. Worth checking what voltage your low-voltage disconnect is set to — you might need to raise it slightly in winter conditions to protect the bank. If you're regularly winter camping in the north, lithium with a self-heating BMS would honestly transform your experience, though obviously that's a significant investment.

Yeah, @Curly, brutal situation but sadly not surprising. One thing nobody's mentioned yet — your battery monitor is almost certainly lying to you in the cold. The internal resistance of AGMs changes significantly with temperature, so your SOC readings become pretty unreliable. You might think you're at 50% when you're actually much lower. Worth checking if your monitor has a temperature compensation setting and making sure it's actually connected to a probe near the bank. Also, if you're running a mains hook-up charger, double-check it has temperature compensation built in — many cheaper units don't, and they'll just hammer the batteries with the wrong voltage profile in those conditions, which does long-term damage on top of the short-term capacity hit.