Anyone else keeping a dedicated emergency battery separate from their main bank?

Been thinking about this after my Victron 200Ah LiFePO4 bank went into low-temp cutoff during a cold snap last winter — woke up to nothing at all. No lights, no heating controller, no way to even check what was wrong. Felt pretty vulnerable.

Wondering whether it makes sense to keep a small, completely separate 12V battery (maybe a modest 20–30Ah AGM or a cheap Fogstar Drift cell) wired independently, just for a few critical circuits — think USB charging, a small 12V light, and maybe the heating ignition. Totally isolated from the main system so even if the BMS trips or the inverter dies, you've still got something.

Has anyone actually built this into their setup? Curious how you'd handle keeping it topped up without it just becoming another thing to manage — trickle charger off the van alternator, tiny solar panel, or just check it manually every few weeks?

@WonkyRigger yeah, exactly why I keep a separate small AGM tucked away specifically for emergencies. Nothing fancy — old 100Ah unit sat on a trickle charger. AGM handles the cold way better than LiFePO4 and it's isolated from my main Fogstar bank entirely.

Mine powers just the essentials:

• A couple of LED circuits
• The heating controller
• Phone charging

Means if the main bank throws a tantrum (low-temp cutoff, BMS fault, whatever) I've still got light and heat while I diagnose the issue.

The deliberate isolation is the key bit — same bus and you risk dragging the backup down with the main system.

Good thread this. @WonkyRigger your low-temp cutoff situation is exactly the scenario most people don't plan for until it bites them. I've got a dedicated 20Ah AGM on a completely separate trickle charger — totally isolated from the main bank. Powers just the essentials: a small LED strip, the heating controller, and crucially my phone charger so I can actually diagnose what's gone wrong. Key thing for me was making sure it's on its own circuit with a clearly labelled isolator switch, so there's zero chance of accidentally draining it during normal use. AGM handles the cold far better than lithium too, which is the whole point in winter. Bit of overkill maybe, but after a similar experience a few years back I sleep much better knowing it's there. Worth every penny of the £40 it cost me.

@WonkyRigger this is exactly why I run a dedicated 20Ah AGM on a completely separate charge circuit — it only powers the heating controller, one LED light, and my phone charger. Deliberately kept isolated from the main LiFePO4 bank so whatever takes down the main system can't touch it.

The key thing most people miss is testing it regularly. Mine sits on a Victron IP65 5A trickle charger year-round. I drain and recharge it every few months just to confirm it'll actually deliver when needed.

Also worth considering — AGM handles low temps far better than LiFePO4 for an emergency reserve, which sounds directly relevant to your situation. Not glamorous technology, but for a backup that genuinely works at -5°C when your BMS has shut everything else down, it's hard to argue against it.

@WonkyRigger this mirrors what I ended up doing at the cabin after a similar wake-up call. My main bank is a 200Ah Fogstar Drift LiFePO4 — brilliant in normal conditions, useless below about -10°C if the BMS decides to protect itself.

My dedicated emergency bank is a 20Ah Yuasa AGM, completely isolated from the main system, trickle-charged via a tiny Victron Blue Smart IP65 on its own solar input. Critically, I've wired it to power only:

• Heating controller
• A single LED circuit
• The Victron Cerbo GX (so I can actually diagnose what's happened)

The key thing nobody mentions — keep the emergency bank in a slightly warmer location than your main bank. Mine sits inside the caravan wall void rather than the external battery box. AGM handles cold far better anyway, but insulation helps regardless.

Great thread. @WonkyRigger that low-temp cutoff situation is a proper wake-up call (no pun intended).

I've gone a step further than most here — my emergency battery is a 12Ah sealed AGM mounted inside the living space rather than in the battery compartment. Keeps it above the thermal threshold even on the nastiest nights, and being inside means it's not subject to the same conditions that might have knocked out the main bank in the first place.

One thing worth adding: I put a small LED voltmeter on it permanently so I can glance at its state of charge without having to wake anything up. Costs pennies and means you're never flying blind. Trickle charged off a dedicated 5W panel through a basic PWM controller — completely isolated from the main system.

Redundancy through separation is really the key principle here.

Really glad this thread came up — I had almost the exact same situation last February. What I'd add is to make sure your emergency battery is on a completely independent charging source too, not just a separate bank. Mine is topped up via a small 10W solar panel wired directly to a dedicated PWM controller — completely isolated from the main system. Even on a grey winter day it keeps that 20Ah AGM ticking over. @WonkyRigger the other thing worth considering is a low-voltage alarm running off it, so you get early warning before the main bank hits cutoff again.

Really good thread this. @WonkyRigger your situation is almost exactly what pushed me to rethink things too.

What I've settled on is a small 15Ah AGM tucked under the bed, completely isolated from the main system except for a trickle feed through a basic PWM controller off a dedicated 50W panel. No BMS to fall over, no low-temp cutoff, just reliable old lead-acid doing what it's always done.

Crucially I keep it on a totally separate consumer unit — just the essentials: a 12V LED strip, the propane heating igniter circuit, and a USB socket for the phone. Nothing fancy.

The key thing people overlook is testing it periodically. Mine gets a proper discharge check every few months otherwise you end up with a flat emergency battery when you actually need it, which rather defeats the point!

@WonkyRigger been running a knackered old 20Ah AGM under the stern deck purely for the bilge pump and a cheap 12V lamp — basically my "well at least I won't drown in the dark" battery — completely isolated from the main Fogstar bank so no BMS drama can touch it, trickle charged off a £15 Halfords maintainer and honestly it's the most boring reliable thing on the whole boat, which for emergency kit is exactly what you want.