Anyone actually saving money with a second-hand lithium setup or is it a false economy?

Picked up a used 100Ah LiFePO4 battery off eBay for £140 a few months back for the narrowboat. Seller claimed it had "light use" — no BMS data, no cycle count, nothing. Took a gamble. So far it's holding charge fine but I've no idea what I actually bought or how many cycles are left on it.

Tempted to grab another one to pair with it but then I start wondering if I'm building on sand. New Fogstar Drift 100Ah is what, £180-ish? Not a massive jump from what I paid used. At least you know what you're getting.

So what's the actual verdict from people who've gone down this route? Is there a reliable way to test a used lithium before committing, or are you just rolling the dice every time? Anyone had a dud that cost them more in the end?

@BurnWalker the missing BMS history is the real gamble — capacity tells you nothing without cycle data. Before trusting it fully, run a proper capacity test: charge to 100%, rest 2 hours, discharge at 0.2C through a known load, log the Ah out before it hits 2.8V/cell. If you're getting 85Ah+ from a claimed 100Ah you've done well. Anything under 70Ah and degradation is already significant.

For £140 it could still pencil out — a comparable new Fogstar Drift 100Ah is around £185, so your margin for disappointment is tighter than it looks. The real risk isn't today's capacity, it's whether an unknown-history cell has had thermal events that compromise long-term cycle life. No external test catches that reliably.

I'd run it in parallel with something known if your application allows it rather than relying on it as a standalone bank.

@HelenPhillips is absolutely right about the BMS data gap — but I'm curious, is there a reliable way to actually test remaining capacity yourself before committing to a second-hand cell?

I'm considering a similar route for my narrowboat setup and wondering whether something like an active capacity tester would give a realistic picture. I've seen a few Chinese units on Amazon that discharge at a fixed current and log the result — anyone actually used one of these, or are they wildly inaccurate?

Also — does it matter who manufactured the cells inside the pack? I've heard Fogstar use quality cells but with second-hand units you've no idea what's inside the casing. Is there any way to identify cell origin without cracking it open?

@HelenPhillips is right about BMS data being the crux of it, but there's a practical workaround — run a proper capacity test yourself before trusting the battery with anything critical.

What I did with a secondhand 200Ah unit I picked up for my van build: discharged it at a known load (I used a 12V immersion element), logged the time, calculated actual usable Ah. Mine came back at 178Ah — perfectly acceptable degradation for the price paid.

The real hidden cost people miss is the BMS replacement risk. Generic BMSs on cheap second-hand cells fail silently, and you won't know until something expensive happens. Budget £30–50 for a quality replacement BMS upfront.

For narrowboat applications specifically, the float/absorption cycles are gentler than van use, so second-hand LiFePO4 should hold up reasonably well long-term if the cells themselves test healthy.

@Lefty28 and @GeoffHenderson80 — the capacity test is a solid starting point, but worth adding that temperature matters a lot when you run it. Do it in a cold garage in January and you'll get a pessimistic reading that doesn't reflect summer performance. Aim for 15–20°C ideally.

Also, @BurnWalker, one thing nobody's mentioned — check the cell balance. A battery can pass a rough capacity test but have one weak cell that'll drag the whole pack down under load. If your BMS has a balancing display or you can get a cell-level voltage reading, do it at both full charge and near empty. Significant divergence between cells is a red flag that no headline capacity figure will show you.

£140 is a decent price if it holds up — just don't trust it for critical loads until you've properly stress-tested it.

@BatteryDoug makes a fair point on temp. One thing nobody's mentioned — check the cell balance after a full charge. If you've got a Victron SmartShunt or even a basic cell voltage tester, wildly unbalanced cells are a red flag that the BMS has been struggling or cells are degrading unevenly. That'll tank your usable capacity way faster than headline Ah figures suggest.

Picked up a couple of Fogstar Drift cells second-hand last year. Capacity tested fine but balance was all over the shop. Took a good few cycles with active balancing to sort it.

Second-hand LiFePO4 can be decent value but you're buying blind without that data. Budget an extra £20-30 for proper testing kit before committing to anything load-critical.

Following on from @Wez1961's point on cell balance — is there a reliable way to check that without pulling the battery apart or needing specialist kit?

I've got a used Fogstar 200Ah sitting in my motorhome setup and I'm trying to assess it properly before I trust it for longer runs. Currently just monitoring via a Victron SmartShunt, which shows overall voltage but I'm not sure it gives me individual cell data.

• Does the SmartShunt surface cell-level info or do I need something like a Bluetooth BMS to see that properly?
• Are there any visual indicators of cell imbalance I should be looking for during charging?

Genuinely trying to build a systematic checklist for evaluating second-hand lithium before relying on it. Feels like there's a lot of tribal knowledge scattered across threads but no clear process.

@BevJackson64 — most modern BMS units expose cell-level data via Bluetooth. If the battery has a JBD or JK BMS (extremely common on eBay LiFePO4 packs), download the respective app and you'll see individual cell voltages in real time. I do this routinely on my van build using a £12 JBD Bluetooth module. What you're hunting for is cells diverging beyond ~20mV at rest — anything wider suggests a weak or damaged cell. Victron's Battery Monitor can flag overall capacity drift too, though it won't show individual cells without a compatible smart BMS talking over VE.Direct or CAN.

@CamperGeek makes a good point on JBD/JK BMS units — dead easy to interrogate with a phone. Worth adding though, if you've got a battery with no Bluetooth BMS or a locked-down one, a decent capacity test under known load tells you a huge amount. I use a simple resistive load and log the discharge time. If a claimed 100Ah only delivers 75Ah to your cutoff voltage, you know exactly where you stand before committing further. @BurnWalker — that's honestly the first thing I'd do with any second-hand purchase regardless of what the seller claims.