Anyone else running a used leisure battery from Facebook Marketplace? Share your wins and horror stories

I picked up a 110Ah AGM leisure battery off Facebook Marketplace last month for £25 from a bloke who said it "came out of a motorhome, barely used." Ran it through my cheapo capacity tester and it came back at about 67Ah actual usable — so roughly 60% of rated capacity. Not exactly stellar, but for £25 I'm not crying about it. Running it alongside a 100W panel on the roof of my Transit and it's keeping my 12V fridge and phone charging ticking over fine most days.

The thing is, I've seen people on here swear by secondhand batteries and others who've been absolutely burned. I've heard the rule of thumb that anything under 50% capacity is basically landfill, but I'm curious where everyone draws the line. Also wondering if the brand matters much — mine's a Varta, which I'd consider decent, but I've seen knackered Optimas going for daft money just because of the name.

Has anyone got a solid method for testing these before handing over cash? I've got a basic multimeter and that capacity tester, but I know a proper load tester tells you a lot more. Just trying to work out if it's worth buying one for around £30-40 or whether there's a smarter way to screen batteries before committing.

@DanMurray £25 is a decent gamble — what did the capacity tester actually show?

I've been down this road a few times sourcing batteries for my shepherd's hut build. The "barely used" claim is almost always optimistic. My rule now: if the seller can't tell me the battery's age or original vehicle, I assume it's at least 4-5 years old and price accordingly.

Biggest red flag I've learned to check before buying — bring a multimeter and measure resting voltage. Anything below 12.4V after sitting overnight suggests it's been deeply discharged repeatedly. Below 12V and I walk away regardless of the asking price.

Picked up a decent 100Ah AGM last year for £30 that actually tested at 87Ah — perfectly usable for emergency backup duties. But I've also bought complete lemons that wouldn't hold charge past an hour.

What's your capacity tester showing?

Bought a "barely used" 85Ah off Facebook last summer for my narrowboat — turned out it had the internal resistance of a soggy digestive biscuit and could hold about 14Ah on a good day 🍪

@OffGridJulie go on then, don't leave us hanging — internal resistance of what? A potato? A retired politician? The suspense is killing me.

Right, my two penneth: bought a "perfect condition" 100Ah AGM off Marketplace two years ago for the narrowboat backup bank. Looked immaculate. Ran it through my Victron BMV-712 and it sulked its way to about 34Ah actual usable capacity before voltage collapsed like a depressed soufflé.

The golden rule nobody mentions — always ask for a photo of the battery date code, not just the "hardly used" fairy story. Most punters haven't the faintest idea what those stamps mean, which is precisely why they can't lie convincingly about them.

Sub-£30 is lottery ticket money. Sometimes you win. Usually you don't. Factor in your time and you've already lost economically, but the occasional genuine find does exist. Just don't bet the boat on it. Literally.

@Jess1972:

Ha, @Boycie25 I'm going to guess @OffGridJulie is about to say something unprintable 😄

Anyway, I've got a proper mixed bag of experiences with Facebook batteries. Picked up a 100Ah AGM last year for £30 — bloke swore blind it was barely used. Capacity tester said 67Ah which honestly wasn't bad for the price, been running my shed setup off it for eight months now with no dramas.

The horror story was the one before that — looked absolutely mint externally, but it was pulling down to 10.8v under any decent load. Classic sign the cells have gone uneven. Complete write-off.

My advice: always bring a basic load tester with you when you collect, don't rely on the seller's word or photos. Takes two minutes and saves a wasted journey. The good ones are out there, you just need patience.

Got a 100Ah "like new" AGM off Marketplace for £20 last spring — Victron Battery Monitor said it was delivering a heroic 34Ah before giving up, which is basically the off-grid equivalent of a politician's promise.

@TID_Electric that Victron monitor catching it out is exactly the kind of thing I need to sort on the boat — currently flying blind with just a voltage gauge which I know is basically useless for actual state of charge.

Quick question for the thread: is it worth buying a cheap internal resistance tester before going to collect one of these batteries, or is that overkill for a £20-30 punt? I've seen some on Amazon for around £15-20 but no idea if they're reliable enough to be worth the trip.

Also — does anyone know if AGMs behave noticeably differently to flooded cells on these testers? Got a mix of both I'm considering picking up locally.

@MasterProject:

@MuddyTinker same boat situation here (no pun intended) — running a couple of suspect second-hand leisure batteries as emergency backup aboard and honestly flying blind without proper monitoring. What Victron monitor are you running or looking at? I've been eyeing up the BMV-712 but wondering if it's overkill for what's essentially a backup bank rather than primary house power.

On the Marketplace stuff — has anyone got a reliable way to quickly test a used battery before handing over cash? I'd rather not drag a full tester down the towpath to meet some bloke in a pub car park.