Anyone else running a second-hand leisure battery bank on a tight budget? Share your setups and gotchas

I've been slowly piecing together a solar setup for my old Transit camper and decided to go the cheap route with second-hand leisure batteries rather than splash out on lithium. Picked up three 110Ah AGM batteries from a local car boot for £25 each — bloke said they'd come out of a motorhome that was being broken for parts. Put them on a proper conditioning charge when I got home and they're holding around 85–90Ah each by my reckoning, which I'm well chuffed with for that money.

Running them alongside a 200W panel I got off eBay and a Victron SmartSolar 100/20 MPPT (the one splurge I allowed myself — couldn't bring myself to buy a dodgy PWM controller). On a decent sunny day in late spring I was seeing around 9–10A charge current, which is keeping the bank healthy enough for an LED lighting circuit, a 12V compressor fridge, and charging phones and a laptop via a 300W inverter.

The bit I'm still getting my head round is balancing the batteries properly. I've read you should charge them individually now and then to make sure one duff cell isn't dragging the others down, but I'm not entirely sure how often folk actually bother doing this in practice. Has anyone got a proper routine for maintaining a second-hand AGM bank, or am I overthinking it? Also curious whether anyone's mixed different age batteries in a bank and lived to tell the tale.

Neil1978 | 47 posts

@CraftyGrafter nice one, I went down a similar road last year with my shed workshop setup. One thing that caught me out with mixed second-hand batteries — even if they're the same rated capacity, if they've got different histories they'll charge and discharge unevenly. The weakest one essentially drags the others down. Worth doing a proper load test on each individually before wiring them together in parallel.

I used a cheap battery analyser off eBay (about a tenner) and was surprised how much the actual capacity varied from what was written on the labels. Two of my "110Ah" batteries were barely pushing 70Ah in real use.

Keep an eye on temperatures too — older lead acid cells can get a bit warm if they're being pushed hard. Not dangerous usually, just worth checking occasionally.

Keith1981 | 134 posts

Great thread this. I've got four second-hand 110Ah batteries in my narrowboat setup, picked up two from a motorhome breaker and two off Facebook Marketplace. Biggest lesson I learned the hard way — always test individual batteries with a proper load tester before wiring them in parallel. I had one weak cell dragging the whole bank down for weeks before I figured out what was happening. Grabbed a cheap Duracell load tester off Amazon for about £20 and it's saved me loads of grief since. Also worth checking the date codes stamped on the casing — anything over four years old I'd personally walk away from regardless of how good the seller says they are. Mix old and new at your peril! 😅

MattJones | 134 posts

Great thread! One thing I'd flag that caught me out badly — always test second-hand leisure batteries individually before wiring them together in a bank. I bought four 100Ah batteries off Facebook Marketplace thinking I was getting a bargain, connected them all up, and one duff cell in a single battery was quietly dragging the whole bank down. Took me ages to work out why my capacity seemed rubbish.

A cheap battery tester or even just a basic load tester makes a massive difference. Ideally you want to charge each one fully first and then check the resting voltage after 24 hours — anything sitting below 12.4V is already suspect. Also worth checking they're all the same age and ideally same brand if you can manage it, mismatched batteries can cause headaches longer term.

StormyViking | 89 posts

Running a mixed bank of reconditioned batteries in my shepherd's hut — learned the hard way that mismatched internal resistance kills your overall capacity faster than a duff cell on its own.

Worth grabbing a basic conductance tester (not just a voltage checker) before you commit to any second-hand unit. Picked one up for about £25 on eBay, saved me binning a whole bank that actually had two perfectly decent batteries in it.

Also — keep your eye on equalisation charging if you're on lead acid. My Victron SmartSolar handles it automatically but I didn't have that on my motorhome previously and the bank sulphated badly within a season.

@CraftyGrafter what controller are you running? That'll determine how much babysitting your bank actually needs.

FX_Power | 89 posts

One thing nobody's mentioned — keep an eye on how they self-discharge when sat idle. Dodgy second-hand units can lose meaningful capacity overnight even with nothing connected. I check mine weekly with a basic multimeter after a full charge and log it in a notebook.

Also worth knowing: mixed age batteries in a bank will always drag down to the weakest cell. If one's properly knackered, it'll limit the whole lot. Learned that the hard way in my shepherds hut build before I went lithium.

Victron's Battery Protect is cheap insurance if you're worried about over-discharge killing them prematurely — saved my bank more than once when I misjudged consumption over a cloudy week.

FormerMechanic15 | 212 posts

Had a shepherd's hut setup running on second-hand batteries for two years before switching to Fogstar lithium last autumn. Biggest gotcha nobody talks about — internal resistance creep. Battery might test fine on voltage but if internal resistance is climbing, it'll sag badly under load and your inverter will cut out at the worst moment.

Grab a cheap battery tester that measures CCA and internal resistance, not just voltage. Picked mine up for about £25 off Amazon.

Also worth knowing: leisure batteries from motorhomes tend to be in better nick than caravan ones. Caravans often sit unused for months, motorhomes usually get more regular cycling which keeps them healthier.

@StormyViking mixed banks are a proper headache long-term — weakest cell drags everything down eventually. Matching capacity and chemistry saves a lot of grief.

StormyHiker | 147 posts

The self-discharge point @FX_Power raises is spot on, but there's another gotcha nobody's mentioned — internal resistance variance across a mixed second-hand bank. I ran three mismatched 100Ah batteries in my static caravan for eighteen months and the weakest cell was quietly dragging the whole bank down. Only caught it when I stuck a basic battery analyser on each one individually.

Worth grabbing a cheap conductance tester before you commit to any second-hand unit. Maplin's gone obviously, but Amazon has decent no-name ones for under a tenner. Saved me binning what turned out to be two perfectly good batteries whilst identifying the actual rogue one.

DorsetCruiser | 147 posts

Great thread this. One thing worth adding — when you're buying second-hand leisure batteries, always ask for the manufacture date, not just the age of use. A battery that's been sat in someone's garage for three years barely used can actually be in worse nick than one that's been regularly cycled properly. Also worth doing a proper load test before committing, not just a resting voltage check — plenty of sellers will show you a healthy 12.6V reading but the battery collapses the moment you draw anything meaningful from it. @CraftyGrafter what charger are you running? That makes a massive difference to how long second-hand cells will last you.

Running a mixed second-hand bank on the boat for two years and the real killer nobody mentions is internal resistance variance between mismatched batteries — your weakest cell drags the whole bank down faster than a soggy biscuit in tea, so grab a cheap internal resistance tester before you commit to any second-hand purchase rather than just checking voltage like everyone does.

CarlBaker | 312 posts

Worth adding a practical workflow for anyone actually buying these: bring a proper load tester, not just a multimeter. Resting voltage tells you almost nothing useful. I picked up a Durite 0-525 load tester for about £40 and it's saved me from buying rubbish twice over. Apply a load equivalent to half the CCA rating for 15 seconds — anything dropping below 9.6V under load is borderline scrap regardless of what the seller claims. Also check manufacturing date codes; most batteries are stamped or stickered. Anything over 4-5 years old I walk away from regardless of price.