Leisure battery keeps dying overnight even with solar panel connected

Been through this exact headache with my static caravan setup a few years back. Nine times out of ten it's one of three culprits:

1. Parasitic drain
Something's pulling current overnight even when you think everything's off. Fridge, alarm, gas detector — they all add up. Stick a cheap clamp meter on the negative terminal after dark and see what you're actually drawing.

2. The battery itself is knackered
If it's an older flooded lead-acid, it might be showing 12.6V but have virtually no usable capacity left. Voltage means nothing on a tired battery — do a proper load test. Fogstar do decent lithium options if you're thinking of upgrading, and they hold up much better in partial state of charge situations.

3. Solar isn't actually charging properly
A 100W panel in UK conditions, especially autumn/winter, might only push 10-20Ah on a decent day. That's not always enough to recover a battery that's being hammered overnight. What controller are you running — PWM or MPPT? MPPT makes a noticeable difference with smaller panels and low-angle winter sun.

Worth checking:

• What's your overnight consumption (roughly)?
• How old is the battery and what type?
• Any recent changes to the van's electrics?

Victron make a decent battery monitor (the BMV-712) that'll show you exactly what's going in and out — honestly one of the best tenners-well-spent upgrades I ever made. Takes all the guesswork out of it.

What panel and battery size are you working with? Others here will have more specific experience on the motorhome side of things.

@OldSailor79 makes solid points but I'd add one thing nobody's mentioned yet — check your solar controller isn't the drain itself.

Had this exact scenario on my boat last summer. Victron MPPT showed zero input overnight (obviously) but was still pulling ~50mA through a dodgy connection on the load output terminals. Tiny figure, but over 10-12 hours on an already-stressed leisure battery? You're looking at 0.5-0.6Ah minimum, and that compounds fast if the battery's already degraded.

Stick a cheap clamp meter on each wire individually after isolating your known loads. Takes 10 minutes and rules it out properly rather than guessing.

Also worth knowing — most PWM controllers (Renogy's budget range especially) are notorious for this. MPPT units less so, but not immune.

@DODGuy makes a fair point about the controller. Had exactly that with a cheap PWM unit on my cabin — was backfeeding through the panel at night, draining about 0.3A constant. Swapped to a Victron SmartSolar and sorted it overnight.

Also worth checking: what's the actual battery capacity vs your overnight load? People massively underestimate their consumption. Grab a DC clamp meter and measure everything running after dark. Bet you'll find something stupid like a 12V fridge compressor cycling all night.

What battery are you actually running? Age matters too — if it's a flooded lead-acid over 3-4 years old, it's probably lost 40-50% of rated capacity regardless of what the label says.

Worth adding — have you actually load tested the battery itself? Solar and controller can be perfect but if the battery's knackered internally it'll show full voltage sitting idle then collapse the moment you draw anything from it.

Had this exact thing with my shepherd's hut. Fogstar 100Ah looked fine on the Victron app, showing 12.6v no problem. Put a proper load on it and it was dropping to 11.8v almost instantly. Battery was toast.

A multimeter reading voltage tells you very little. You need to see how it behaves under load. Most decent auto parts places will do a free battery health check, or grab a cheap load tester off Amazon.

How old is the battery and do you know its history?

@Ben1968 nails it on the battery test — worth doing that first before chasing anything else tbh.

One thing nobody's mentioned: check your battery's state of health vs state of charge. A knackered cell can show 12.6V sitting on charge but collapse the moment you put load on it. Seen this loads of times with older leisure batteries.

If you've got a Victron SmartShunt or similar, the "time remaining" figure tanks instantly when a cell's gone — dead giveaway.

Also worth checking: what size is your panel vs battery? Running a 100W panel into a 110Ah battery through a cheap controller is often right on the edge — any shade, any inefficiency and you're just not replacing what you use overnight.

What battery are you running and how old is it?