Anyone else had grief with bilge pump wiring draining their leisure battery overnight?

Ran into exactly this rabbit hole last summer on a friend's 32ft cruiser I was helping rewire. The culprit wasn't the pump itself — it was the float switch sitting there drawing a phantom load through a dodgy connection, basically creating a partial circuit that bled the 110Ah AGM flat by morning. Took us an embarrassing amount of time to find it with a clamp meter.

Since then I've become a bit evangelical about a few things on boat electrics:

• Always wire bilge pumps through a proper fuse, ideally with an inline blade fuse as close to the battery as possible
• Float switches are notorious for this — worth checking resistance across them when "open"; should be infinite
• A battery monitor is non-negotiable — even a basic Victron BMV-712 will show you parasitic drain in real time rather than you discovering it at 6am with flat batteries

I've also seen this happen with automatic bilge pump controllers that have indicator LEDs — those tiny draw figures add up over 12 hours.

For anyone chasing a ghost drain, the simplest method is still the old ammeter-in-series trick. Disconnect the negative, put your meter across the gap, and start pulling fuses one at a time until the reading drops. Boring, but bulletproof.

Anyone else found that the automatic vs manual switch position makes a difference here? I've seen a few setups where leaving it on "auto" creates a leakage path that "off" or "manual" simply doesn't.

Not my area really — I'm more shepherd's hut solar than bilge pumps — but phantom loads are phantom loads regardless of context.

On my hut I track down every little parasitic draw with a clamp meter before it eats my Fogstar battery overnight. Same principle should apply here surely?

@Drift_Geek what float switch make was it? Curious whether it's a known dodgy brand or just the wiring setup creating the path. I'd imagine isolating it on its own fused circuit with a decent inline switch would at least let you confirm whether it's the culprit before pulling everything apart.

@Drift_Geek a float switch slowly murdering your leisure bank overnight is genuinely one of those "why didn't I just fit a proper relay and be done with it" moments — slap a Victron BMV-712 on there and watch in horror as you discover exactly which gremlin is sipping your amps at 3am.

Been there on my narrowboat — the float switch is basically a permanently-live gremlin unless you wire it through a proper relay with a Victron BatteryProtect or similar downstream, so the switch signal triggers the relay rather than carrying the full circuit load itself.

Also worth checking: some cheaper float switches have a sneaky resistive bleed even in the "off" position — swap it for a quality Whale or Rule unit and you'll likely halve the problem before touching the wiring.

@Drift_Geek the phantom load through a dodgy switch can be small enough that your battery monitor doesn't flag it as alarming — until three weeks later when you're flat and confused at 6am.

@RetiredChef the "permanently-live gremlin" is a perfect description. Did something similar on my motorhome build — nothing nautical, but the principle bit me with a water pump pressure switch drawing just enough to flatten a 100Ah Fogstar overnight without the pump ever actually running.

The fix that sorted it for me was running everything through a Victron Battery Protect set with a low-voltage cutoff, so even if something's sitting there sipping quietly, it can't take the bank below a safe threshold.

On the float switch specifically — I've read that wiring it through a relay rather than directly to the bank means the switch circuit itself carries almost nothing. The relay coil draws a few milliamps rather than whatever the switch leakage is allowing through. Might be worth investigating whether that's achievable on the cruiser refit?

@RetiredChef @DontPanic25 worth noting the phantom draw mechanism varies depending on float switch type — the cheap reed-style ones (common on Chinese pumps bundled with Whale or Rule-branded knock-offs) can exhibit resistive leakage across the contacts even when nominally open, especially once they've accumulated bilge grime on the internals. Measured 4–6mA continuously on one last year. Trivial in isolation, but compound that with any other parasitic draws on the same unprotected circuit and you're in trouble overnight.

My fix on a friend's Beneteau was routing everything through a Victron BatteryProtect set with a low-voltage disconnect threshold — belt and braces alongside the proper isolation relay. Means even if the float switch misbehaves, the bank can't be taken below a recoverable SOC. Adds maybe £25–35 to the job but protects a potentially expensive AGM or lithium leisure bank.