What's everyone's minimum viable backup setup for liveaboards when the main system goes down?

Had a proper wake-up call last week on the cut near Middlewich. Fogstar Drift 200Ah main battery developed a dodgy cell mid-trip, Victron SmartSolar started throwing alarms, and suddenly I'm down to shore power at a marina I'd not planned to stop at. Not the end of the world, but it got me thinking — what's the actual fallback when everything goes sideways at once?

Currently I've got a battered 30Ah AGM shoved under the stern as an emergency start battery for the engine, but that's it. No dedicated backup inverter, no secondary charging path. On a narrowboat you can idle the engine to charge, sure, but what if it's the alternator that's fried? I've been eyeing a small 500W Victron Phoenix and a second lithium bank kept at storage charge, but not sure if that's overkill or just sensible belt-and-braces.

What's the minimum setup you'd actually trust for keeping the essentials running — lights, bilge pump, maybe a phone charger — for 48–72 hours without shore power or engine? Genuinely curious whether other liveaboards and continuous cruisers have thought this through properly or are just winging it like I apparently was.

@KangooAdventure that's a rough one — Middlewich stretch isn't exactly littered with chandlers either.

My minimum viable layer for the garden office setup taught me this: one independent 12V 100Ah LiFePO4 on a completely separate circuit, no BMS sharing, no shared fusing. Fogstar Drift is fine as the workhorse but I keep a basic Renogy 100Ah as the lifeboat battery — dumb, isolated, charged via a dedicated 20W panel with a basic PWM controller.

Key points I'd add:

• Galvanic isolation between backup and main — if the main system has a fault, you don't want it dragging the backup down
• A Victron IP65 12/10 charger running off a leisure battery bank if you're near a marina with shore power
• Physical labelling on every breaker — stress scenarios are not the time to be reading wiring diagrams

What failed first — the BMS comms or actual cell voltage drop?

MoreTeaVicar60 | Posts: 847 | Location: Nr. Ellesmere Port

@KangooAdventure sympathies mate, been there on a different stretch of the Middlewich branch funnily enough — seems to attract gremlins!

My non-negotiables as a liveaboard:

• A modest 20-30Ah AGM tucked away completely separate from the main bank, charged via its own cheap PWM controller off even a single 100W panel
• A 12V compressor blanket pump for water (saves the inverter)
• Decent LED lanterns already charged and ready

The key thing people overlook is keeping that backup battery genuinely isolated day-to-day. Mine's on a manual isolator and I don't touch it unless things go sideways. Temptation to "just borrow a bit" kills the whole strategy.

What actually happened with the Fogstar cell — was it manufacturing defect or had it taken a knock? Curious whether that's a batch issue.

Boycie74 | Posts: 312 | Location: Floating somewhere on the Shroppie

@KangooAdventure nasty situation that, Middlewich's not ideal for sourcing bits quickly.

My belt-and-braces backup is a 20Ah sealed AGM in a waterproof box under the stern deck — nothing fancy, just enough to run navigation lights, VHF and a 12V USB hub for a couple of days. Kept completely isolated from the main system so a BMS fault can't drag it down too.

Also swear by a small 40W folding panel with a basic PWM controller that clips straight onto it independently. Bit agricultural compared to the Victron setup but it just works when everything else decides to have a moment.

Total outlay was about £85 secondhand. Hasn't moved from that box in two years which is exactly what you want from a backup really. 😄

24V_Nerd | Posts: 1,203 | Location: Midlands/wherever the mooring's free

@KangaoAdventure solid reminder that redundancy doesn't have to be expensive. My non-negotiable minimum is a decent 20Ah lithium jump-starter box — doubles as a USB power source for phone/nav for days if you're careful with it. Costs nowt to keep aboard.

Beyond that I keep a small 100W flexible panel with bare MC4 tails and a basic PWM controller in a dry bag — can bodge it onto almost any 12V battery in a pinch, even a car leisure battery borrowed from a marina neighbour.

@MoreTeaVicar60 is right that the controller failing is often more disruptive than the battery itself — people forget that. Always worth having a cheap backup MPPT rather than leaving yourself completely dark.

What was the actual fault code the SmartSolar was throwing?

VanKen | Posts: 156 | Location: Static caravan, somewhere damp

My "backup system" is a £40 Lidl inverter generator, a Fogstar 100Ah I pretend is retired, and the quiet dignity of a man who's already made every mistake you lot are about to make. 🏕️