Winter, parked, nothing running but a 12V compressor fridge and a small ceiling fan on low. I go to bed at 100 percent and wake up around 20, which is roughly 160Ah for maybe fourteen hours. The fridge is rated 45W. Something doesn't add up and I'd rather find it than add more battery. Shunt is a proper battery monitor, not the panel gauge.
Second thing worth checking: is the fridge actually cycling or running continuously? In a van with poor ventilation behind the fridge, the compressor never gets ahead of the heat it's rejecting and it runs 80% of the time. That turns a 15Ah night into a 45Ah night. Feel the coils, and if the space behind is sealed, cut a vent.
It's in a cabinet with one small gap at the bottom. Adding an upper vent this weekend, thanks.
Also sanity-check the shunt configuration. If the battery capacity is set wrong in the monitor, or the shunt isn't on the main negative with every single load downstream of it, the percentage it shows is fiction. A common build error is running one circuit direct to the negative bus and bypassing the shunt entirely.
This is worth checking before anything else, honestly. I've seen two builds where the starter circuit was on the wrong side of the shunt and the numbers made no sense until it was moved.
In winter you also have almost no solar recovery, so any overnight drain compounds day after day. The fix is finding the drain, but the other half is having a plan to actually recharge — a DC-DC charger off the alternator will put 30-40A back in an hour of driving, which in December is worth more than another panel.
The pull-fuses-one-at-a-time method sounds tedious and takes about ten minutes in practice. Worth doing once a year even if you're sure the system is clean, because every accessory you add afterwards is a new candidate. I write the standing draw on a bit of tape inside the electrical cupboard so I have something to compare against.
@drainfield_dave · 6mo ago · 3 replies
The numbers say you have a parasitic draw, not a fridge problem. A 45W compressor fridge in winter should cycle maybe 20-30% of the time, so call it 15-20Ah over fourteen hours. A small fan on low is 5-10Ah. You should be waking up around 75-80%, not 20.
How to find it: turn absolutely everything off at the switches, then look at the shunt's current reading. Anything above about 0.3A with everything "off" is your culprit. Then pull fuses one at a time and watch the number drop.
Usual suspects, in the order I find them:
Also check the battery is actually warm enough. Lithium below freezing will show odd behaviour, and if your bank sits in an uninsulated space and your BMS is cutting things, capacity readings get strange.
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@litercount_lou · 6mo ago
Inverter standby, 1.4A. It's been on since I built the thing because I never thought of it as a load. That's 20Ah a night doing absolutely nothing.
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@two_stroke_tess · 6mo ago
Put the inverter on its own switch somewhere you'll see it. Mine has a little red light by the bed specifically so I can't forget.
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