Leisure battery keeps dropping overnight even with solar topping it up during the day

Had almost the exact same situation with my Hymer last summer — solar was doing its job beautifully during the day, hitting absorption by early afternoon, but by morning I'd lost 20-30% overnight with barely anything running.

Turned out to be two separate issues ganging up on me:

1. Parasitic drain from the habitation relay — it was staying partially energised even when everything was switched off. Caught it with a cheap clamp meter left overnight.
2. One duff cell in a knackered AGM — battery was accepting charge fine but couldn't actually hold it. Voltage looked healthy at dusk then just quietly collapsed.

The second one is the sneaky villain that catches most people out. A battery can read 12.8V fully charged and still be rubbish at holding capacity. The only real way to know is a proper load test or running it through a Victron BMV-712 and watching the actual amp-hours rather than just voltage.

Ended up replacing with a pair of Fogstar Drift lithium cells and a Victron SmartSolar controller — night and day difference. Genuinely now wake up at 98% even after running the diesel heater and a 12V compressor fridge all night.

Before you spend anything though — what's your current setup? Age of the battery, what charge controller you're running, and crucially, have you got a proper battery monitor or just relying on the panel gauge? That panel gauge is basically decorative.

Would also be worth checking if anything in the cab circuit is bleeding across — some motorhomes have quirky wiring that surprises you.

What's your overnight draw roughly? Anyone else tracked down something unusual causing this?

@DontPanic25 that overnight drop with healthy daytime solar is almost always a parasitic drain issue rather than battery capacity degradation — worth ruling out before anything else.

Grab a clamp meter around your negative cable with everything switched off (including the habitation master switch). Anything above ~20mA is worth investigating. Common culprits in Hymers specifically:

• Alarm/tracker modules drawing 40-80mA continuously
• Truma heating control boards with "standby" modes
• Faulty relay not fully dropping out on the 12V habitation circuit

Also worth confirming your battery monitor (if fitted) is actually zeroing correctly — a miscalibrated Victron BMV will report losses that aren't real capacity drops.

What battery chemistry are you running? AGM, lithium? If AGM and over 4-5 years old, self-discharge alone can account for 15% overnight at cooler temperatures.

@ThistleTel nailed it but also worth checking if your battery is actually as healthy as it thinks it is — my Fogstar 100Ah was "fully charged" according to the BMS yet had the capacity of a damp sponge after two winters 🧽

@MrBodge73 raises a good point about battery health, but before you dive down that rabbit hole — have you actually logged the consumption overnight with nothing running?

I did exactly this with my shepherd's hut setup: killed every circuit, left a Victron SmartShunt recording, and the "nothing running" baseline was pulling 0.8A constantly. Turned out to be a cheap 12V socket with a USB charger left plugged in, plus the Renogy MPPT sat in standby drawing more than its spec sheet claimed.

Total overnight on an 8-hour sleep? Nearly 40Ah gone before I'd even run the lights.

Pull everything you can identify, then shunt-log the remainder. The culprit almost always reveals itself pretty quickly once you have actual numbers rather than guessing.

@DontPanic25 one thing nobody's mentioned yet — what's your resting voltage actually sitting at when you wake up? There's a big difference between a battery that's genuinely discharging overnight versus one that's just showing surface charge bleed-off from the previous day's solar cycle. If you're waking up to 12.4-12.5V on a 100Ah battery with minimal loads, that's actually fairly normal settling behaviour rather than a true drain. Chuck a clamp meter on your negative terminal before bed with everything switched off — if you're seeing more than about 20-30mA draw you've got a genuine parasitic issue worth chasing. Fridges cycling on thermostat are a common culprit people overlook too.

@CraigLamb makes a fair point about resting voltage, but I'd go one step further — are you sure the solar was actually hitting absorption, or just claiming to? 😄

On my narrowboat I spent three weeks convinced my Victron MPPT was doing its job beautifully. Turned out I'd been reading the controller