Anyone else keeping a dedicated emergency battery just for the bilge pump?

After a scary moment last autumn where my leisure bank was too flat to run the bilge pump properly, I've been mulling over a separate small emergency battery just for critical boat stuff. Thinking something like a 20Ah LiFePO4 (maybe a Fogstar Drift) kept on a trickle from a tiny Victron MPPT and a single 50W panel.

The idea is it stays completely isolated from the main bank — only kicks in for the bilge pump, possibly the VHF radio. Never gets cycled for kettles or lighting nonsense. Should stay near full basically forever.

Anyone actually doing this or is it overkill? Wondering if a simple relay/switch setup is enough or whether I need something smarter to stop it accidentally getting drawn down.

@BoatLouise this is exactly what I ended up doing after a similar near-miss. Ran a dedicated 20Ah AGM purely for bilge pump and a couple of nav lights — completely isolated from the main leisure bank so it can't be accidentally drained.

Worth considering:

• A Victron Battery Protect on the main bank to prevent it ever getting that low again
• Small solar trickle (even a 20W panel) to keep the emergency battery topped up whilst on the mooring
• A simple voltage alarm so you actually know when either bank drops

One question — are you planning to keep it fully isolated or allow a managed trickle charge from your alternator? I've gone fully isolated but wondering if that's the best approach long-term for keeping it conditioned.

Slightly off the usual remit for me (shepherd's hut and EV charging are my world), but the underlying principle is identical to what I did for my hut's critical loads — a completely isolated small reserve that nothing else can drain.

Worth considering a LiFePO4 over AGM for this application; a Fogstar Drift 10Ah will hold charge far longer during periods of neglect than AGM will. Couple it with a basic Victron Battery Protect set to disconnect at say 12.8V, so even if something does creep in and draw current, the bilge pump itself always has priority headroom.

Key thing nobody's mentioned: self-discharge rate matters enormously if this battery sits unused for months. AGM can be genuinely flat by spring. LiFePO4 loses maybe 2-3% per month. For an emergency-only application, that difference is significant.

Tiny house person chiming in here because isolating critical loads is just good practice whether you're floating or not — my Victron SmartShunt would have a breakdown if I let everything compete for the same bank. 🔋