New Heltec Battery Capacity Testers for Solar Off-Grid Battery Test

Been keeping an eye on the Heltec capacity testers for a while now and finally picked one up last month to test a batch of second-hand 280Ah EVE cells I sourced for my cabin build.

Genuinely impressed with how straightforward the discharge testing is. Set your cut-off voltage, connect your load bank, and it logs everything cleanly. Compared my results against what I'd previously done manually with a Victron BMV-712 and the figures aligned within about 1-2%, which gave me decent confidence in the readings.

A few things worth noting if you're considering one:

• Discharge current accuracy seems solid up to around 20A, above that I'd want to verify with a clamp meter
• The fan is surprisingly loud at higher loads — not ideal if your battery space is enclosed
• Software interface is functional but the English translation is rough in places, takes a bit of guessing

My main use case was weeding out underperforming cells before building my 24V LiFePO4 bank. Found two cells sitting at roughly 240Ah actual capacity versus the rated 280Ah — worth knowing before they ended up buried in a Fogstar-style pack configuration.

Would be curious whether anyone's compared these directly against a proper Neware or Chroma tester. Obviously those are professional-grade and priced accordingly, but for DIY off-grid use the Heltec seems like reasonable value.

Anyone else been using these specifically for grading second-hand cells rather than new stock? Interested in what cut-off voltages others are using for LiFePO4 discharge testing — I've been settling on 2.8V but wondering if 2.5V gives a more accurate true capacity figure.

@WezWhite72 second-hand EVE cells are where I'd want proper capacity data too — sellers' figures are basically fiction half the time.

Couple of things I'd want to know before I trust any tester blindly though:

• What discharge current are you running the test at? C/10 vs C/5 gives noticeably different results on aged cells
• Are you accounting for ambient temperature in the cabin? Cold UK winters will skew readings lower than what you'd actually get in summer

I used a basic West Mountain Radio CBA IV on my boat build years back and the accuracy was decent enough, but consistency across multiple cells matters more than absolute numbers when you're matching a pack anyway.

What were the actual capacities coming out vs the rated 280Ah? That's the bit I'm more curious about. Genuine cells from reputable batches usually still hit 270+ even used.

@WezWhite72 what discharge current were you running the tests at? I've got a batch of used cells I'm evaluating for my van build and I'm wondering whether testing at 0.2C gives meaningfully different results compared to 0.5C for cells that may have some age on them.

Also curious — did you test each cell individually before assembling, or did you run the tester across a completed pack? Trying to work out the most practical workflow before I commit to buying one of these units.

The cell matching side of things is what interests me most. I've read that variance of more than 2-3% between cells in a pack causes the Victron BMS to flag imbalance warnings fairly regularly. Is the Heltec data granular enough to sort cells into matched groups reliably?

@WezWhite72 used these on my shepherd's hut build last year — cracking little testers. One thing worth noting: run your cells at 0.2C discharge if you want figures that actually match real-world solar use. Higher currents skew the results noticeably on older cells.

Also grabbed a set of cell spacers and a decent IR meter alongside — the internal resistance readings on second-hand cells tell you as much as capacity tbh. Anything over ~0.3mΩ variance across a batch and I'd be pulling the worst offenders out.

@VanKev for a motorhome application I'd be a bit stricter on cell matching than a static build — vibration doesn't help a weak cell.

@WezWhite72 great shout on the Heltec — I've been running one for about eight months now on various cells for my tiny house build and it's been invaluable. One thing I'd add that nobody's mentioned: temperature matters enormously during capacity testing. I lost a whole afternoon wondering why my Fogstar cells were testing low, then realised my workshop was sitting at around 8°C. Same cells retested at ~20°C came in bang on spec.

Also worth logging your internal resistance readings alongside capacity figures — a cell can pass capacity but show elevated IR, which tells you something about its remaining cycle life. I use a simple spreadsheet to track both metrics across the full batch before committing them to the bank.

For second-hand EVE cells specifically, I'd do at least two full cycles before trusting the numbers.