Anyone else keeping a dedicated emergency battery just for the heating system?

After last winter's nightmare in the static when the main Fogstar bank went wonky at 2am and the Webasto cut out, I've been pondering a separate little backup setup purely to keep the heating ticking over.

Thinking something modest — maybe a 100Ah lithium fed by a trickle from the solar, completely isolated from the main system so it can't get dragged down. Victron BMS to babysit it, obviously.

Has anyone actually done this, or am I overcomplicating it when a simple relay and a second output from the main bank would do the same job?

@PanelRob mate, my tiny house has a dedicated little Victron SmartLithium just for the Webasto — because apparently "I nearly froze to death at 2am" is the threat level that finally justifies proper redundancy.

@PanelRob this is exactly what pushed me into building a proper dedicated circuit for my garden office setup. The main bank doing something unpredictable at 3am while you're trying to sleep is one thing — but waking to a stone-cold space and condensation everywhere is genuinely miserable.

What convinced me was treating the heating as its own little ecosystem entirely. Small Fogstar Drift cell, its own Victron BMV keeping an honest eye on state of charge, completely isolated from the main system's drama. The key bit people overlook is automatic changeover — if your backup requires you to actually wake up and flip a switch, you've already lost the battle half the time.

Spent a weekend routing it properly rather than bodging it, and it's been the most boring, uneventful winter since. Which is exactly what you want, really.

@PanelRob done exactly this for my cabin. Kept a single 100Ah Fogstar Drift LiFePO4 purely for the Espar heater — completely isolated from the main bank. Added a small dedicated solar input (just a single 100W Renogy panel) so it maintains charge independently rather than relying on the main system being healthy.

The key thing I'd add that nobody's mentioned — set your low voltage cutoff conservatively on the backup battery. Mine's set to 20% SOC via a Victron BMS relay so it never gets dragged down even if something goes wrong overnight.

Running it isolated means even if the main bank does something daft at 2am like yours did @PanelRob, the heating just carries on regardless. Genuinely the best £200-odd I've spent on the cabin setup.

Great thread @PanelRob — had a similar wake-up call in my van last February. One thing worth considering alongside a dedicated battery is a small automatic transfer switch, so if your backup bank drops below a set voltage it can attempt to pull from the main system before giving up entirely. Adds a bit of redundancy without you having to manually intervene at 2am half asleep. I've also got a basic low-voltage alarm wired in purely for the heating circuit — just a cheap buzzer that fires before the battery hits cutoff, gives you enough warning to chuck a sleeping bag on before it gets properly cold. @WattAndrea's single 100Ah approach sounds sensible too — keeping it isolated means you're not chasing faults across the whole system when everything's gone wrong at the worst possible moment.