Anyone else using second-hand leisure batteries to cut costs on a small off-grid build?

Picked up a pair of 110Ah AGMs from a local caravan dealer for £40 the pair — bloke said they'd done "light use" for a couple of seasons. Tested them with a basic capacity tester and they're sitting around 85-90Ah actual, which is more than acceptable for what I'm running on the shepherd's hut. That's a 12V lighting circuit, a small 12V compressor fridge, and USB charging. Total daily draw is roughly 40-50Ah in summer.

Paired them with a 200W Renogy panel and a Victron SmartSolar 75/15 MPPT I already had kicking about. The Victron app lets me keep an eye on battery health over time, which matters when you're starting with unknowns. So far after six weeks the capacity seems stable — no obvious degradation showing up.

The obvious risk is you're buying someone else's problem. I got lucky here but I've also been stung before on a narrowboat build with a "good condition" battery that was internally knackered. Anyone got a reliable way to properly vet second-hand AGMs before committing, beyond a basic load test?

Good find @Spud79! £40 for the pair is decent if they're genuinely holding capacity. One thing worth doing beyond basic capacity testing is checking the internal resistance on each cell — a cheap battery analyser from Amazon will show up any weak cells that a simple load test might miss. AGMs from caravans can be quite variable; some owners genuinely do use them lightly, others have left them discharged over winter which absolutely kills them internally even if they seem okay on a quick test. I'd also cycle them a few times before relying on them properly — charge fully, draw down to around 50%, recharge, and see if the capacity reading stabilises or drops. If it keeps declining after three or four cycles, you've likely got sulphation issues. Still usable for lower-demand stuff, but manage your expectations accordingly!

Really curious what capacity they're actually showing on that tester, @Spud79 — that's the make-or-break question with second-hand AGMs.

I picked up a couple of used batteries for my boat last year and the internal resistance figures told a very different story to the headline capacity. Worth grabbing a cheap conductance tester if you haven't already; I got a no-brand one off eBay for under a tenner and it's saved me from buying a couple of duds since.

One thing I'd watch with AGMs specifically: if they've ever been deeply discharged and left sitting, they can appear to test okay but collapse under any real load. Do you have a way of pulling a decent sustained current through them to check?

@Spud79 worth noting that "light use for a couple of seasons" from a caravan dealer often means they've been left partially discharged over winter — which is genuinely one of the worst things for AGM longevity. Sulphation sets in fast.

Before trusting that capacity reading, I'd run a proper charge/discharge cycle using something like a Victron BMV to track actual Ah in/out rather than relying on a basic tester. I did exactly this on a set of used Trojans I picked up for my narrowboat build — the tester said ~85% capacity, the BMV told a different story after a real-world 20hr discharge.

Also check resting voltage after a full charge 24hrs later. Anything below 12.7V suggests the cells aren't balancing properly. At £40 you've not lost much, but I wouldn't design your system around that capacity figure just yet.