Anyone actually saved money going second-hand on lithium? What's the risk?

Looking at building out a small off-grid setup for my tiny house and the budget is tight. New 100Ah LiFePO4 batteries (even the Fogstar Drift ones) are still a fair chunk of money once you're stacking a few together. Been eyeing up used cells on eBay and Facebook Marketplace — Grade B prismatic cells, that sort of thing.

The thing I can't get my head round is how you actually verify what you're buying. Sellers quote capacity figures but without a proper capacity tester how do you know? I've seen people mention checking internal resistance with a meter but I'm not sure what numbers are actually red flags for used 280Ah cells.

Has anyone here gone down the second-hand route and either saved a load or got burned? Particularly interested in whether a decent BMS (thinking Daly or JK) can protect you from a duff cell in a used pack, or whether you're just papering over the cracks. What would you actually budget for second-hand vs new if you were starting again?

@PaulMurray I went second-hand on a couple of 100Ah cells for my garden office build — bought from a reputable eBay seller with capacity test results included. Saved about 40% versus new Fogstar pricing.

The risk is really about cycle count history and whether you can get an actual capacity test done before committing. A proper BMS with cell-level monitoring (I use a JK BMS) will catch any weak cells fairly quickly after installation anyway.

Main things I'd check:

• Ask for cycle count evidence — most sellers won't have it, which tells you something
• Grade B cells from known suppliers like NKON can be a decent middle ground
• Avoid anything without provenance — dodgy EV pull-outs are everywhere right now

What total capacity are you aiming for? That changes the maths quite a bit.

@PaulMurray genuine question before you go down this route — do you actually know what you're buying second-hand, or are you just trusting whatever the seller claims about cycle count and capacity?

I've been tempted myself for the shepherd's hut build. The maths looks gorgeous on paper until you factor in that a dodgy cell could take your whole pack down, and you've got no comeback.

What I'd ask any second-hand seller:

• Actual capacity test results (not "oh it's basically like new mate")
• Cell-level voltage readings, not just pack voltage
• Original source — EV pulls are a different beast to purpose-built LiFePO4

Fogstar do refurbished stock occasionally which at least gives you some accountability. Might be worth watching their site rather than rolling the dice on a random eBay punt with zero warranty.

@PaulMurray I've done this twice now and the key thing nobody mentions is capacity testing before you commit. A battery advertised as 100Ah might only deliver 60-70Ah if it's been through hard cycles. Get yourself a basic battery capacity tester (they're cheap enough) and insist the seller demonstrates a full charge/discharge cycle, or at minimum shows you a recent test result.

Also worth knowing where the cells originated — EV pack pulls and solar storage decommissions tend to be far kinder to cells than forklift or industrial use. Calb and Eve cells from solar installs especially can still have loads of life left.

@Robbo raises a fair point though — if you're not confident reading a BMS or interpreting voltage curves, the savings can disappear quickly if something goes wrong. Budget some time for learning, not just money.

@HeatherOllie is right on the testing front.

Did exactly this for my cabin build — grabbed a couple of 200Ah prismatic cells off a Facebook Marketplace seller who was breaking down a motorhome setup. Bloke had receipts and could tell me the full history, which made the difference.

The real risk imo is cells from unknown sources — ex-EV packs especially, you've no idea what BMS abuse they've seen.

If you can physically inspect first and run a proper capacity test with something like a DC load tester before parting with cash, the savings can be massive. I paid roughly 40% of new price and both cells are still pulling solid capacity two years on.

Source matters more than anything else.

Great thread. One thing I'd add to what @HeatherOllie and @Lefty31 have already covered — source matters enormously. Ex-telecom and ex-UPS cells are generally your best bet second-hand because they've often lived easy lives on float charge rather than being hammered through daily cycles. Facebook Marketplace is hit and miss but there are a few specialist resellers (SBS Battery, certain eBay trade sellers) who actually grade and test stock before listing.

Also worth checking whether the cells come with a working BMS — replacing one eats into your savings pretty quickly if not.

@PaulMurray what sort of daily consumption are you working with? Might be worth calculating whether a smaller new pack actually pencils out better than a larger second-hand one once you factor in the uncertainty.

Bought a pair of second-hand 280Ah prismatics off eBay for my shepherd's hut last spring — half the price of new Fogstar cells — and honestly the BMS was the hidden cost nobody warned me about, ended up replacing both units before I trusted them overnight 🔧

Good thread this. One thing I'd add that hasn't been mentioned yet — check the BMS history if you can get it. Some sellers will have logs or at least know whether the cells ever got pushed hard (over-discharged, high temps, that sort of thing). Cells that've lived an easy life in a static solar setup are a very different proposition to ones pulled from an EV pack or a forklift running multiple cycles daily. Ask the seller directly about the application they came from. Most honest ones will tell you, and the dodgy ones tend to go quiet pretty sharpish. That alone can help you filter out the risky listings.

Good thread, this one. I'll add something nobody's mentioned yet — check the BMS history if you can. A lot of used prismatic cells come pulled from e-bike or EV packs where the BMS has been logging cycle counts and temperature events. If the seller can't provide any data on how the cells were managed, that's a yellow flag for me. Cells that've been regularly taken below 10% or charged in freezing temps can look fine on a capacity test but degrade faster down the line. Ask awkward questions before you part with cash. 🙂