Anyone else keep a "lifeboat battery" completely separate from their main bank?

Running a 200Ah Fogstar Drift LiFePO4 as my primary on the boat, but after a BMS hiccup last winter left me without navigation lights in a tidal channel, I've been paranoid ever since.

Now I've got a dusty old 100Ah AGM tucked under the V-berth, wired to nothing except a trickle charger and a single 12V socket — basically Schrodinger's battery, I never know if it actually works until I need it.

Curious whether anyone runs a proper isolated backup with its own Victron MPPT and panel, or whether I'm just hoarding dead weight?

SaltyViking | Posts: 847

@OhmsLaw7 Mate, that tidal channel scenario gives me the cold sweats just reading it. After a similar wake-up call I keep a completely isolated 20Ah sealed AGM tucked under the chart table - nothing connected to it except a trickle charger on a separate solar panel and a dedicated fuse block for nav lights, VHF, and bilge pump. Absolutely nothing else touches it.

The key for me was genuine isolation - different charging circuit, different cabling run, even a different fuse colour so I can identify it in a panic at 3am in the dark.

AGM rather than lithium for the lifeboat bank deliberately - simpler chemistry, no BMS to potentially misbehave, and it'll sit at 80% charge happily for months without sulphating badly. Sometimes boring is exactly what you want when things go sideways.

TrevorRoberts75 | Posts: 112

@OhmsLaw7 That BMS hiccup scenario is exactly why I keep a dedicated 20Ah AGM completely isolated from my main Victron/Fogstar setup — nothing shared, no clever charging integration, just a simple trickle maintainer on a separate circuit.

Curious though — what triggered your BMS fault in the first place? Was it a temperature cutoff or an over-discharge event? I've been wondering whether running a small Bluetooth BMS monitor on the lifeboat battery itself defeats the purpose, or whether it's actually worth knowing its state of health at a glance rather than assuming it's always ready.

Also, what's your isolation strategy — a physical rotary switch or are you using something like a Victron Battery Protect to keep it genuinely separated from the main bank's charging circuits?