Picked up a used Fogstar 100Ah LiFePO4 for £120 — worth the gamble?

Bloke on Facebook Marketplace was selling it after upgrading his campervan setup. Said it had done about 18 months of use, maybe 200 cycles. Looked clean, terminals in decent nick, and he had a BMS readout on his phone showing all cells balanced within about 20mV of each other. Seemed legit, so I handed over the cash.

Got it back to the cabin and ran it through my Victron BMV-712 properly. First full charge to 3.65V per cell, then a controlled discharge down to 10% — it came back showing 94Ah usable. For £120 I was honestly gobsmacked. New they're around £280–£300, so I'd effectively got nearly full capacity for less than half price.

The one thing nagging me is longevity. I've no idea how hard those 200 cycles were — whether it was sitting in a warm van getting hammered at high charge rates or gently cycled at 0.2C in a shed. My Renogy 40A MPPT is set to charge it at around 20A max so I'm keeping things sensible from here on.

Anyone else gone down the second-hand lithium route? Curious whether people have had these degrade quickly after purchase or whether a healthy cell reading at the point of sale is a reasonable indicator it'll go the distance.

T5Solar | ⚡ 847 posts

Nice find @Chris1973 — £120 for a Fogstar at that cycle count is decent value if it checks out. First thing I'd do is hit it with a proper capacity test; charge it fully, then discharge at a controlled rate and see what you actually get out of it. LiFePO4 cells hold up remarkably well at 200 cycles so you might be pleasantly surprised.

Worth grabbing a cheap battery tester or running it through a Victron if you've got access to one. Also check what the BMS is doing under load — voltage sag under decent current draw will tell you a lot about cell condition.

Fogstar use decent cells in their packs, so the bones are probably solid. Let us know what capacity test gives you!

ExFarmer90 | 🌱 312 posts

Cracking deal if it tests out well, @Chris1973. I picked up a second-hand 200Ah LiFePO4 last spring — different brand mind you — and the first thing I did was run it through a proper capacity test with a cheap DC load tester from Amazon. Took an afternoon but completely worth it.

With Fogstar specifically, their BMS protection is fairly solid so if the previous owner wasn't abusing it with dodgy charging voltages, 200 cycles is barely broken in for LiFePO4 — these cells are rated for 2000+ typically.

Two things I'd check straight away:

• Resting voltage — should sit around 13.2–13.3V if reasonably healthy
• Ask if he used a LiFePO4-compatible charger, not a generic lead-acid profile

At £120 you've not lost the earth even if capacity's degraded a bit. Could still serve beautifully as garden office backup.

Solid buy if the capacity holds up. Worth running a proper discharge test before committing it to anything critical — full charge, then load it down to 10% and see what actual Ah you get back. Fogstar cells are generally decent quality so even at 200 cycles you'd hope to see 90%+ capacity remaining.

I picked up a second-hand 100Ah for my garden office last year, similar story. Tested fine, been rock solid since.

Only thing I'd check is whether the BMS has any history of over-discharge events — ask the seller if he ever ran it flat accidentally. That's usually where used LiFePO4 gets sketchy, not the cycle count itself.

Rob1963 | 🔧 2,341 posts

200 cycles on a Fogstar is basically run-in — those cells are rated to 2,000+ so you've bought someone else's warranty period for buttons. Hook it up to a Victron BMV-712 and do a proper capacity test; if it's still showing 90Ah+ you've robbed him blind. Main thing I'd check is whether the BMS has any fault history — a BMS that's been tripping repeatedly tells you more about a battery's past life than cycle count ever will. Grab a decent multimeter, check cell balance voltages through the BMS comms if it supports it, and if everything looks tidy, £120 is frankly embarrassing value. Mine cost £240 new and I still feel hard done by.

Trigger | 🔋 1,847 posts

@Chris1973 One thing nobody's mentioned yet — check the cell voltage balance before anything else. Connect a Victron SmartShunt or even a basic cell meter across each cell group and look for any outliers. A pack that's sat partially discharged for any length of time can develop imbalance that won't show up on a surface voltage check but absolutely destroys usable capacity.

Had a similar Fogstar 100Ah come into my shepherd's hut setup last year. Top-balanced the cells manually before first use, lost about 4Ah to a weak cell but the remaining capacity has been rock solid since.

At £120 you've got headroom even if you need to nurse it a bit. Just don't chuck it straight onto a solar controller and assume the BMS will sort everything out — it won't.

@Rob1963 is right about the cycle count, basically brand new.

Got a similar Fogstar second-hand for the cabin backup last year — main thing I'd add is don't just test capacity, give it a few full charge/discharge cycles before you trust it in anything important. Sometimes they've been sat partly discharged for months and the first cycle or two look a bit ropey before they settle back in.

£120 is a cracking price if it holds 85%+ capacity. Run it through a proper load test and if it's hitting 85Ah+ you've done well. Anything under 80Ah I'd want money back tbh.