Anyone else using a leisure battery as emergency home backup — worth it or am I wasting my time?

So I've been messing about with a setup in my garage for the past few weeks. Got a 110Ah AGM leisure battery off eBay for £65, wired it up to a 400W pure sine inverter I already had knocking about, and I'm using it to run a few essentials during power cuts — router, a couple of LED lamps, and my phone/laptop charging. Nothing fancy, just want to keep the lights on and stay connected when the grid goes down.

Trouble is, I'm not sure I'm charging it properly between outages. I've got a basic mains-powered 10A smart charger keeping it topped up, but I've read mixed things about whether that's actually maintaining it well or slowly killing it. The battery's probably 18 months old now and I'm starting to wonder if I'm losing capacity. Resting voltage sits around 12.5V most of the time, which I think is roughly 80% for an AGM?

Has anyone else cobbled together something similar as a cheap stopgap before committing to a proper LiFePO4 setup? Curious what chargers people are actually using for this kind of standby/float situation, and whether it's even worth persevering with AGM or if I should just bite the bullet and grab a 100Ah lithium now.

@Brummie88 Been down this exact road with my cabin setup. The honest answer is — it depends entirely what you're trying to power.

A 110Ah AGM at 50% usable depth gives you 55Ah realistically. At 12V that's 660Wh. Fine for phone charging, LED lighting, maybe a router. Forget anything with a heating element.

The bigger issue I found with AGMs is they genuinely don't like sitting partially discharged for weeks at a time. Without a decent trickle charger keeping it topped off, you'll sulphate that battery into uselessness within a year. A CTEK MXS 5.0 transformed how long mine lasted.

For £65 though, you've not exactly bet the house on it. Run it, learn from it, then decide if a proper LiFePO4 from Fogstar is worth the upgrade. That's basically how most of us started.

@Brummie88 That 110Ah AGM will serve you, but temper expectations. From my static caravan setup I learned the hard way — AGMs hate being taken below 50% repeatedly. You're realistically looking at 50-55Ah usable before you start eating into cycle life.

For genuine peace of mind I'd eventually look at a lithium option — Fogstar Drift cells hold up far better over time. The maths changes completely.

One thing worth checking: that eBay battery's actual capacity. "110Ah" from a private seller often isn't. A simple load test tells the truth quickly.

The 400W inverter sounds right-sized assuming you're just covering essentials — a lamp, phone charging, maybe a router. Push a kettle through it and the whole conversation changes rather rapidly.

@Brummie88 One thing nobody's mentioned yet — AGM self-discharge is surprisingly significant if that battery sits unused for weeks between outages. You need a maintenance charger keeping it topped up constantly, otherwise you'll reach for it during a power cut and find it's sitting at 60% capacity. A proper smart charger like the CTEK MXS 5.0 or even a cheap Victron Blue Smart IP65 will pay for itself in battery longevity alone.

Also worth calculating your actual usable capacity — AGM typically you're looking at 50% DoD maximum before you start damaging cycle life. So realistically that 110Ah gives you ~55Ah usable. At 400W load that's potentially under an hour before you're in the danger zone. Know your loads before you need them.

Really resonates with me — I went through almost exactly this process when I was sorting out backup power for my garden office a couple of years back.

The thing that shifted my thinking was moving from a single AGM to a proper lithium setup. I picked up a Fogstar 100Ah LiFePO4 and the difference in usable capacity alone made it worthwhile — you're genuinely getting ~95Ah versus maybe 50Ah from your AGM before you start damaging it.

For a garden office context, the AGM got me through a few short outages absolutely fine — kettle, laptop, LED lighting, no drama. But the moment you want anything like a reliable daily driver for backup rather than occasional emergency use, the maths starts working against AGM pretty quickly.

What are you actually trying to run when the power drops, @Brummie88? That changes everything.

@Brummie88 done something similar myself — 110Ah AGM is honestly fine for emergency backup if you're realistic about it. You're not running your kettle off it, but lights, phone charging, a router, maybe a small 12V fridge? Sorted.

Main thing I'd add: keep it on a trickle charger between uses or you'll kill it within a year. A basic CTEK or similar does the job. Also worth checking your inverter's idle draw — some 400W units sip 15-20W doing nothing, which quietly drains things faster than you'd expect.

For £65 it's a decent starting point. If you catch the bug you'll end up with Victron kit and a proper battery bank within 18 months anyway. We all do. 😄