Anyone else using secondhand leisure batteries from car boot sales or is it just me?

Been doing this for a couple of years now on the motorhome and picked up some decent finds, but also some absolute duds. Wondering what other people's experiences are?

My approach has become a bit more systematic lately — I always bring a basic load tester now rather than just relying on the seller's word that it "works fine." Caught a battery that was showing 12.6V open circuit but collapsed the moment you put any load on it. Would have been a wasted tenner even at that price.

The best one I've found was a pair of 110Ah AGMs from a caravan dealer's yard clearance — got both for £40 and one of them is still doing decent service 18 months on. The other gave up after about six months, which felt like a fair gamble at that price.

A few questions I've been mulling over:

• Is there a reliable way to estimate remaining cycle life without proper equipment like a Victron battery analyser?
• Do flooded lead-acid tend to be better bets than AGM at car boots, since you can at least check electrolyte levels?
• Has anyone tried reconditioning methods like desulphation — does it actually work in practice or is it mostly snake oil?

The risk is obviously that you're gambling a bit each time, and for the boat I'm more cautious since I really don't want to be stranded somewhere without reliable power. But for the motorhome parked on the drive as a guest room, a cheap gamble feels acceptable.

What's everyone else's strategy for sorting the keepers from the hopeless cases before handing over any cash?

@MarineFrank done this a few times for the van. Hit rate is pretty poor honestly — maybe 1 in 3 worth bothering with.

Big thing I'd add: bring a load tester, not just a voltmeter. Resting voltage tells you almost nothing on an old battery. Had one sitting at 12.6v that collapsed to 10v the moment I put any draw on it. Complete waste of a fiver.

Also worth knowing roughly what you're buying — a knackered AGM isn't worth the same gamble as a flooded wet cell where you can sometimes recover them a bit with a decent conditioning charge cycle.

Wouldn't rely on car boot finds for anything critical. Fine as a cheap extra buffer maybe, but I wouldn't trust my main bank to them.

@LuckySkipper 1 in 3 sounds about right from my experience too. I've grabbed a few at car boots for the static and honestly the only way I've found to not get burned is bringing a basic load tester with me — not just a voltmeter. Resting voltage tells you almost nothing about actual capacity.

Biggest red flag I've learned to spot: internal resistance. Even a cheap conductance tester will weed out the real dogs before you hand over cash. Anything showing badly sulfated plates I walk away from regardless of price.

That said, even knackered batteries aren't worthless if you're just using them as overnight buffer storage with solar top-up during the day. Mine don't need to hold charge for days on end, so I've got away with stuff that'd be useless in a motorhome context.