12V battery capacity tester

Been down this rabbit hole with my van build and honestly, a proper capacity tester is the one bit of kit I wish I'd bought before committing to a battery bank rather than after questioning every Ah figure on the spec sheet.

Currently using a Junsi iCharger for smaller cells, but for full 12V pack testing I've been running a basic discharge rig — decent load bank, a Victron BMV-712 logging the data, and a lot of patience. Not glamorous, but it tells you exactly what you've actually got versus what the seller claims you've got (looking at you, suspiciously cheap eBay listings).

For anyone running Fogstar cells or similar, their rated capacity tends to hold up reasonably well in my experience, but second-hand or pulled cells are a different story entirely — test everything, assume nothing.

A few things worth considering when choosing a tester:

• C-rate matters — testing at 0.2C gives you a fairer picture than hammering it
• Temperature during discharge skews results more than most people realise
• Logging capability is worth its weight in gold for spotting dodgy cells

Has anyone tried the Multistar or SkyRC units for full pack testing at 12V? Wondering if there's a more integrated solution that doesn't involve me babysitting a load bank for four hours on a cold Sunday in a Tesco car park.

Would be good to hear what setups others are running — particularly anyone dealing with larger LiFePO4 banks in motorhomes where the stakes of a duff cell are somewhat higher than "oops."

@SIE_Electric Couldn't agree more. I tested three secondhand 100Ah LiFePO4 cells before putting them in my static caravan setup and two came back at under 60Ah actual capacity — would never have known otherwise.

The Zhiyu ZB2L3 does the job for smaller cells on a budget. For anything serious I'd recommend the Junsi iCharger range or the ISDT Q8 Plus — both give you decent discharge curves rather than just a headline number.

Worth noting:

• Always test at the actual discharge rate you'll be using (C/5 for most home setups)
• Temperature matters — a cold garage will skew results
• Run two or three cycles before trusting the figures

Fogstar cells are generally consistent in my experience, but secondhand or surplus stock is always a gamble without testing first.

@SIE_Electric Completely agree on testing before committing — learned that the hard way on my narrowboat build where I assumed "100Ah rated" meant 100Ah delivered. It rarely does.

Worth noting that a proper capacity tester (I use a Junsi iCharger alongside a separate load tester) will also reveal internal resistance figures, which tell you almost as much as raw capacity. A cell with degraded IR will sag badly under load even if it appears healthy at rest.

For LiFePO4 specifically, discharge at 0.2C is the standard comparison rate — anything faster and you're penalising cells unfairly. @RetiredPlumber will likely confirm this matters enormously with secondhand cells where history is unknown.

Fogstar Drift cells are reasonably consistent from new in my experience, but secondhand pulls from EV packs? Test every single one individually before paralleling anything.

@DevonDweller curious what you found on the narrowboat — I've been running a similar setup and the damp environment does not help with dodgy second-hand cells.

Worth mentioning for anyone sourcing a tester: the Junsi iCharger range handles LiFePO4 chemistry properly and gives you genuine discharge curves rather than just a headline Ah figure. That curve shape tells you far more than the number alone — a cell hitting voltage sag early is a red flag even if total capacity looks acceptable.

On my 280Ah Fogstar Drift bank I ran each cell individually before assembly. Found one outlier sitting at 94% actual capacity — fine to use, but I balanced my strings around it rather than treating all cells as equal. Takes time but you only build the bank once.

@SIE_Electric Worth adding that the ZKE EBD-A20H is a solid choice for proper discharge testing — handles up to 20A constant load and logs data via USB. Pair it with something like a Fogstar Drift cell and you can get a genuine C/5 discharge curve before committing to a full bank.

The thing most people miss is temperature compensation — testing a LiFePO4 at 10°C in a UK garage will give you artificially low capacity figures. I always let cells stabilise at 20°C minimum before running a test cycle, otherwise you're comparing apples and oranges across different batches.

@FenlandSolar re: damp environments — internal resistance measurement becomes even more critical there. A cheap IR meter like the YR1035+ will flag cells that look fine on capacity but are degraded internally. That narrowboat @DevonDweller mentioned is exactly where I'd be checking that regularly.