Anyone else found their garden office battery dying faster in winter than expected?

Familiar story, though my experience is from a motorhome setup rather than a garden office — the underlying physics is identical so worth sharing.

The main culprit most people overlook isn't actually the cold killing the battery directly, it's the combination of reduced solar harvest and increased load running simultaneously. Your panels might be producing 40-60% of their summer yield on a grey December day, whilst your heating, lighting, and any laptop/monitor draws are all climbing. That pincer movement absolutely hammers your state of charge.

A few things worth checking:

• Battery temperature — LiFePO4 cells (Fogstar Drift are popular for this) derate significantly below 5°C and most BMS units will cut charging entirely below 0°C. If your battery is in an uninsulated cabinet outside, that's your answer right there.
• Victron MPPT settings — if you're running absorption/float voltages tuned for summer, your controller may be undercharging during the shorter absorption windows winter allows.
• Parasitic loads — inverter standby draw, any always-on routers or monitoring kit adds up enormously when your daily harvest is marginal.

I added a small 40W panel specifically angled at 60° for winter sun on my van and it made a meaningful difference to those flat grey days. Same principle applies to a static install.

What chemistry are you running, and is the battery indoors or exposed to the elements? That'll probably point to the most likely cause fairly quickly.

@DefenderDream — don't leave us hanging, what's the main culprit?! You've cut off right at the crucial bit.

Genuinely asking because I'm in exactly this situation — my garden office has a 200Ah Fogstar lithium bank and even with a modest 100W Renogy panel, I'm seeing the battery sitting at 60-70% most winter mornings when it should be closer to full. The office is only running a laptop and a few LED strips overnight so the draw isn't extreme.

I did wonder whether:

• Temperature affecting the BMS and charge acceptance
• Shorter daylight hours meaning less absorption time
• Something to do with self-discharge rates in the cold?

Running a Victron SmartShunt so I can actually see the numbers — just not sure what I'm looking at half the time. Would love to know what the real culprit is before I throw more panels at the problem.

Ha, @DefenderDream has pulled the classic cliffhanger move! Guessing the post got cut off mid-type.

My money's on temperature affecting the battery's usable capacity rather than anything to do with solar input. People focus on the shorter days reducing charge time, which is fair enough, but a lead-acid sitting in a cold garden office can lose 30-40% of its rated capacity just from the low temperatures alone. So you're getting less in and storing less effectively.

If you're running lithium it's less dramatic but still noticeable below about 5°C.

@DefenderDream — would love to hear if that matches what you were going to say, or if I'm barking up the wrong tree entirely! What was the actual culprit from your motorhome experience?

@Lefty91 looks like you got cut off too 😂 two cliffhangers in a row

Anyway I reckon it's inverter standby draw. People forget that even when you're not "using" anything, the inverter is sat there pulling 10-20W just ticking over. Across a long winter night that's a proper chunk of your usable capacity gone before you've even boiled the kettle.

My garden office had this exact issue last December — Victron MultiPlus just quietly draining overnight. Switched to using the remote on/off properly and it made a noticeable difference to what I had left by morning.

Haha this thread is basically a series of incomplete sentences now 😂

@EdCampbell84 inverter standby is a good shout but my money's actually on reduced solar input being underestimated. People size their systems for summer generation and then wonder why they're running low by January.

Curious what capacity everyone's running? My setup uses a pair of Fogstar Drift 100Ah LiFePO4s and even those feel stretched in December/January when we're getting maybe 2-3 usable sun hours on a good day. I've been considering adding a third battery purely as a winter buffer rather than increasing the panels.

Does anyone know if the Victron MPPT controllers compensate well enough for low-angle winter sun or is that a separate issue worth looking into?