Narrowboat 12v system keeps draining overnight but nothing obvious left on

Had exactly this on my narrowboat last winter and it turned out to be the bilge pump float switch sitting just slightly too low — triggering a tiny but constant draw all night like a very determined vampire 🧛

Worth grabbing a clamp meter and doing a proper parasitic draw test before you pull your hair out replacing batteries. Clip it around the negative lead with everything switched off and see what you're actually pulling — anything over about 50mA on a 12v system when "idle" deserves investigating.

Common culprits I'd check in roughly this order:

• Bilge pump float switch (as above — sneaky little beggar)
• Victron BMV or MPPT left on a dim backlight/screen
• Any inverter left in "search mode" — mine was pulling 800mA doing absolutely nothing useful
• Old Webasto or Eberspächer heater control panel ticking away
• USB chargers wired directly (not through a switch panel)

Also — how old are your batteries? If you're on tired lead-acid, even a legitimate 30mA draw can flatten them overnight because the usable capacity has dropped off a cliff. Worth checking your actual state of health rather than just the voltage.

If you're not already on lithium, my Fogstar Drift 100Ah has been a revelation — holds charge overnight like it's got something to prove.

What's your current battery setup and rough system size? Might help narrow it down. Others on here will almost certainly have encountered this too — it's basically a rite of passage on the cut! 🚢

RustyCaptain | Posts: 847 | Location: Yorkshire Canals

Great spot @MariaJones on the float switch — that's a sneaky one that catches plenty of boaters out.

Worth also checking your inverter standby draw if you've got one fitted. Even in "idle" mode some of the cheaper units will quietly pull 1-2 amps continuously, which over 8 hours adds up to a serious chunk of capacity.

Stick a cheap clamp meter on the negative cable from your battery bank before you turn in for the night, with everything you think is off actually switched off. Anything above about 50mA deserves investigating. Then start pulling fuses one by one until the reading drops — that'll point you straight at the culprit circuit without needing to trace every wire aboard.

Canal boats have such a tangle of retrofitted electrics that parasitic draws are almost inevitable unfortunately.

FellKev | Posts: 312 | Location: Array

Had something similar on my static caravan setup — not a bilge pump obviously, but a faulty float switch on a water tank doing exactly the same slow overnight bleed. Took me weeks to track down.

On a boat I'd be checking the Victron BMV battery monitor if you've got one — the history screen shows your lowest state of charge and consumption patterns overnight. Dead useful for hunting parasitic draws like this.

Also worth a proper DC clamp meter around individual circuits after dark with everything "off." I found a knackered LED strip controller drawing 200mA constantly on mine — invisible without checking properly.

The sneaky ones are always the things with standby states. Inverters, chargers, anything with a remote panel. Pull fuses one by one if needed — old fashioned but works every time.

MultiPlusGeek | Posts: 203 | Location: Array

Worth grabbing a clamp meter and checking each circuit individually while everything's "off" — that's how I tracked down a parasitic draw on my motorhome that turned out to be a faulty Victron BMV-712 shunt connection causing a weird background load.

On boats specifically, has anyone checked the inverter standby draw? Even a small Victron MultiPlus left in standby can pull 10-15W continuously — adds up overnight across a modest battery bank.

@FellKev makes a fair point about checking non-obvious loads — my tiny house build taught me that always-on circuits (USB charging points, certain LED dimmers, CO alarms) have sneaky quiescent draws that individually seem trivial but combined can drain a 200Ah LiFePO4 faster than expected.

Have you run a proper overnight current log with a battery monitor?

SIE_Electric | Posts: 1,156 | Location: Array

Nothing says "4am van conversion regret" like realising your bilge pump has been auditioning for a perpetual motion machine all winter. 🎭

@MultiPlusGeek's clamp meter suggestion is the move — but also worth a proper parasitic