Anyone else running a separate small battery just for emergency backup?

Got a garden office with a decent setup — 400W of Renogy panels, Victron MPPT and a 200Ah Fogstar Drift lithium. Works great day-to-day, but I've been thinking about keeping a smaller dedicated battery purely for emergencies (router, a lamp, phone charging — nothing heavy).

Wondering if a 50–100Ah unit isolated from the main bank makes sense, or if it's overcomplicating things. Thought about a basic BMS-protected LiFePO4 and a small trickle charge from the main system to keep it topped up. Maybe a Victron SmartShunt to keep an eye on state of charge.

Has anyone actually done this or is it unnecessary when you've already got a main bank? Curious whether the isolation is worth the faff or if I'm solving a problem that doesn't exist.

@Dizzy70 funny you mention this — been running exactly that setup on my narrowboat for two years now. Main bank handles everything, but I've got a little 20Ah lithium tucked away that's completely isolated, only connected to a small trickle panel and a basic PWM controller. Nothing fancy. It powers one 12V socket and a basic LED light strip, that's it.

The key thing I learnt the hard way: keep it genuinely isolated. First attempt I had it loosely integrated with the main system and it just got absorbed into normal usage, defeating the whole point.

For a garden office I'd probably go even smaller — a 10Ah would do the job if you're just thinking emergency lighting and maybe keeping a phone charged. Fogstar do decent small units without breaking the bank.

@Dizzy70 running a dedicated emergency bank on my boat and it's been worth every penny. I've got a small 20Ah lithium (Fogstar again, unsurprisingly) completely isolated from the main system — feeds only a 12V socket, a couple of LED strips, and my VHF radio charger.

Key thing I'd recommend: don't connect it to your main MPPT. Give it its own tiny panel — even a 50W Renogy flexi — through a basic PWM controller. That way if your main system develops a fault (BMS lockout, inverter issue, whatever), the emergency bank is genuinely independent.

I use a simple manual isolator switch so there's zero chance of accidental cross-contamination between banks. The small bank just trickle-charges passively and sits there waiting.

On a boat this feels essential. In a garden office it's probably overkill — but "overkill" and "peace of mind" are basically synonyms in this hobby.

Great thread @Dizzy70 — I do something similar in my off-grid cabin up in the Lakes. Rather than a separate battery though, I've set a reserved SOC of 20% on my Victron that's essentially untouchable by normal loads. But I've also got a small 30Ah lithium sat completely isolated on a trickle maintainer, wired only to a dedicated consumer unit with lighting and a phone charger. Completely separate circuit, separate fusing. The key thing I'd add that nobody's mentioned — label everything clearly and actually test the failover occasionally. Found mine had a dodgy connection last winter, which rather defeats the purpose! What specific loads are you trying to protect in the garden office?

Really interesting thread this — we've gone down a similar route at our static, though with a slight twist. Rather than keeping the emergency bank completely isolated, I've got mine wired through a simple manual changeover switch. Means if the main system has an issue I can flip across to it quickly without any faff with crocodile clips in the dark! We used a modest 30Ah Fogstar unit which felt like a sensible middle ground. One thing I'd add that nobody's mentioned yet — worth being really disciplined about not dipping into it for convenience. Ours is strictly for the router, a couple of LED lights and phone charging only. The temptation to "just borrow" a bit of capacity when the main bank is low is real, and before you know it your emergency reserve isn't actually there when you need it! 😄

Really useful thread this. One thing I'd add that nobody's mentioned yet — think carefully about what you actually need that emergency bank to power. I made the mistake of oversizing mine initially because I was being vague about it. Once I sat down and listed the essentials (router, a couple of LED lights, phone charging), I realised a 30Ah was more than enough for my purposes.

Also worth considering a small trickle feed from your main system to keep it topped up, rather than leaving it sitting idle. A permanently partially-discharged lithium isn't doing itself any favours. Even 1-2A from a cheap DC-DC charger keeps it healthy without pulling much from your main bank. @Dizzy70 your Victron setup should make that fairly straightforward to configure.