Anyone else finding cheap 100Ah lithium drop-ins a bit hit and miss lately?

Picked up a pair of "100Ah" LiFePO4 batteries off a well-known marketplace last month — branded ones I'd seen recommended on here, about £140 each. Seemed decent on paper: claimed BMS, low-temp cutoff, the lot. Stuck them in my Transit conversion alongside a 200W panel and a Victron SmartSolar 100/20.

Thing is, my battery monitor (Victron BMV-712) is telling me I'm pulling roughly 85Ah before the BMS cuts out. Not 100. Checked the wiring, checked the shunt, everything looks right. A mate with the same batteries from the same seller is getting about 88Ah. Neither of us is running them particularly hard — maybe 20A discharge max.

Has anyone else noticed this with budget cells recently? I'm half wondering if the genuine capacity has quietly dropped with this latest batch, or if there's something dodgy going on with the BMS cutoff voltages. Tempted to do a proper discharge test with a load bank to get a definitive number. Would love to know if others are seeing similar figures or if I've just been unlucky.

Yeah, I've had a similar experience @BrambleHermit. Bought a couple of supposedly 100Ah cells last spring and when I actually load-tested them properly they were pushing maybe 78-82Ah realistically. Not catastrophic, but not what I paid for either.

The thing that gets me is the BMS quality varies wildly even within the same brand batch. Mine had the low-temp cutoff advertised but it triggered way too conservatively — cutting out at around 5°C which was a real nuisance over winter.

My advice: always run a full capacity test before integrating them into your system. I use a simple DC load tester and log the discharge curve. Takes an afternoon but saves headaches later. If you bought recently you might still have grounds for a return if the capacity's genuinely misrepresented. What actual figures are you seeing on yours?

Honestly this is becoming a pattern isn't it. I tested three different budget LiFePO4 drop-ins last year and capacity ranged from 71Ah to 94Ah — none hit the claimed 100Ah under any reasonable discharge rate. My advice: always do a proper capacity test within your return window, not just a quick charge/discharge. I use a decent DC load tester and log everything. Also worth checking whether the BMS actually does what it claims — I've seen a few where the low-temp cutoff is technically present but set at an unusably low threshold, basically pointless for a UK winter. @BrambleHermit what discharge rate were you testing at? Some of these cells aren't terrible at 0.1C but fall apart at 0.5C, which is more realistic for most of our setups.

Really interesting thread this — I've been down a similar rabbit hole trying to sort charging for my narrowboat and a tiny house build simultaneously, so I've tested a fair few budget cells.

One thing nobody's mentioned yet: has anyone actually stress-tested the low-temp cutoff claims? I got a batch last winter that supposedly cut off at 0°C but were happily charging at -4°C in the boat's engine bay. That's potentially damaging the cells without you even knowing.

Has anyone got a reliable way of verifying BMS specs beyond just trusting the datasheet? I ended up cross-referencing with a clamp meter and a temperature probe, but I'm not convinced I'm catching everything.

Thinking about just biting the bullet on Fogstar Drift cells — pricier but at least there's accountability if specs don't match, yeah?

Really feel for you @BrambleHermit — had almost the exact same experience about eighteen months ago with a pair from a marketplace seller. What finally helped me was getting a proper capacity tester (picked up a basic one for under £20) and just cycling them a few times before committing them to my system. The first cycle you often get misleadingly low numbers, but by cycle three you've got a much truer picture.

One thing I'd add that nobody's mentioned yet — check whether the BMS is actually communicating anything or just sitting there looking decorative. Several of the cheaper units have a BMS that'll protect against catastrophic failure but won't balance cells properly over time. That's where the real capacity drift creeps in with these budget ones.

Been following this thread with interest. One thing I'd add that hasn't been mentioned yet — have you tried putting a proper capacity tester on them rather than relying on the BMS readout? I picked up a cheap DC electronic load off eBay for about £35 and it's genuinely eye-opening how much some of these cells are padded. Did a full discharge cycle on a supposedly 100Ah unit last spring and got 71Ah out of it. Not catastrophic but definitely not what I paid for. Worth documenting it properly too if you're thinking about a dispute with the seller — screenshots of the discharge curve carry a lot more weight than just saying "it feels weak." @BrambleHermit what voltages are you seeing under load?