Anyone else keeping a dedicated emergency battery just for winter blackouts?

After last winter's grid wobbles I've stuck a spare Fogstar Drift 100Ah LiFePO4 under the bench in the static, purely sitting there doing nowt 99% of the year — like a very expensive bouncer.

Got it hooked up to a Victron SmartShunt so I can see state of charge without touching it, trickle fed off a 100W Renogy panel on the roof. Keeps it sat at about 80% SoC year-round which is apparently the sweet spot for longevity.

The question is whether I'm being daft having a whole dedicated battery just for emergencies — should I just tie it into the main bank and accept I might draw it down? Running a Victron MultiPlus 500VA as the inverter/charger if that changes anyone's maths.

@DazHenderson77 do the same in the motorhome — 100Ah Fogstar kept at around 50% SOC permanently. Key thing I found is you want a small trickle from a Victron IP65 charger keeping it topped to that level, otherwise LiFePO4 just sits there slowly self-discharging over winter and you pull it out in January expecting 50% and get 30%.

Worth sticking a cheap BT dongle on it so you can check SOC from your phone without even going out to it. Saved me digging around in the cold more than once.

Been running a similar setup in the van for a couple of years now. After getting caught out near Bamburgh during a particularly grim January storm — no shore power, temperatures dropping, the whole sorry situation — I wired in a dedicated Victron SmartShunt monitoring a separate 200Ah Fogstar Drift that genuinely never gets touched for day-to-day stuff.

The thing that changed my approach was treating it less like a battery and more like a fire extinguisher. You wouldn't partially use your fire extinguisher for convenience, would you?

Monthly I'll run it through a proper charge cycle via a Victron IP65 charger, log the capacity, then park it back at 50-55% SOC. Two winters in and it's performed flawlessly both times I've actually needed it.

The psychological comfort of knowing it's just sitting there is honestly worth the cost alone.

Running a near-identical setup on the narrowboat — dedicated Victron SmartShunt monitoring a 100Ah Fogstar Drift kept at 50% storage charge year-round.

One thing worth adding: LiFePO4 self-discharge is low enough (~2-3% monthly) that you really only need a brief top-up every couple of months to maintain that 50% sweet spot. I've got a cheap Victron Blue Smart IP65 wall charger on a timer doing exactly that — 20 minutes every 6 weeks, job done.

@DazHenderson77 the "expensive bouncer" analogy is spot on, but consider isolating it properly with a dedicated fuse and a small BEP or Victron Cyrix isolator so it genuinely stays untouched unless needed. Nothing worse than discovering your emergency battery has been slowly parasitically drained by something you'd forgotten was connected.

Also worth a yearly cell balance check via Bluetooth — the Fogstar BMS app makes this trivial.

Really interesting thread this. @DazHenderson77 your "expensive bouncer" analogy is spot on 😄

One thing worth adding — I label mine clearly with a bit of bright orange tape and a written note saying "EMERGENCY ONLY - DO NOT USE." Sounds daft but when you're tired or distracted mid-winter it's easy to accidentally pull from the wrong bank if everything looks identical.

Also worth thinking about what you're actually planning to run from it when the time comes. I keep a laminated card next to mine listing the essentials — LED lighting, phone charging, a small 12V blanket — with rough amp-hour costs so I know realistically how long I've got. Takes the panic out of a genuine outage situation.

@ExSquaddie49 the SmartShunt on a dedicated emergency bank is a smart move, gives you confidence the capacity is actually there when needed.