determining rebuild battery pack amp hour settings off of capacity tested values for my jk-bms

Really interesting area this, and something I've spent a fair bit of time wrestling with on my narrowboat build.

When I did capacity testing on my rebuilt pack, I found the tested values came in noticeably below the nameplate figures — sometimes 8-12% lower depending on cell age and how rigorously you're testing (proper 0.2C discharge down to the BMS low voltage cutoff, not a quick zap).

For the JK specifically, I'd strongly recommend setting your pack capacity to your actual tested value, not the theoretical nameplate. Here's why:

• The JK uses the capacity figure to calculate SOC via coulomb counting
• If you inflate the number, your SOC percentages become fiction fairly quickly
• This affects any downstream devices (Victron Cerbo, inverters, etc.) that rely on that SOC reading

My approach was:

1. Run three full discharge cycles and average the results
2. Round down slightly (another 3-5%) to give yourself a conservative buffer
3. Enter that number into the JK as your pack capacity

The JK's coulomb counter will drift over time anyway, but starting with an honest baseline means it recalibrates more sensibly at the top and bottom of charge.

One thing worth noting — if your cells are relatively fresh Fogstar or EVE cells, your tested values should be quite close to rated. Older salvage cells are where the discrepancy really bites.

Has anyone else found the JK's SOC accuracy drifting significantly after a few months? Curious whether others are doing periodic recalibration cycles or just letting the BMS self-correct. Would be good to compare notes on what workflows people are using.

Saw exactly this on my narrowboat setup. When my tested capacity came in below nameplate, I set the BMS to the actual tested value rather than the theoretical spec — otherwise your SOC percentages are completely meaningless and the JK will miscalculate when to trigger low voltage warnings.

Worth noting:

• Run 3+ capacity test cycles first, as cells often improve slightly after conditioning
• Use the average of those runs rather than the best result
• Some people set it 5% conservative of tested capacity as a buffer

What cells are you running? With second-life cells especially, tested capacity can drift over time, so I'd revisit the setting every 6 months or so. My pack dropped another 8Ah on the second round of testing compared to initial figures.

Does your JK model let you adjust that setting easily via the app, or are you on an older firmware?

Great point from @MasterProject — one thing worth adding is that the JK-BMS has both a "Battery Capacity" setting and separate over-discharge protection parameters, and they interact in ways that catch people out.

Your capacity figure primarily drives the SOC% calculation — so if you enter nameplate rather than tested capacity, your displayed percentages become meaningfully inaccurate. I found on my Fogstar 280Ah cells (which tested around 271Ah after reconditioning) that setting the actual tested value kept the SOC tracking honest throughout the discharge curve.

Also worth considering cycle-to-cycle variation — I'd recommend running three capacity cycles and averaging before committing to a figure in the BMS. That average gives you a stable baseline rather than chasing a one-off result.

The protection thresholds (low voltage cutoff, etc.) should be set independently on their own merits regardless of your capacity entry.

Something I noticed on mine — the JK also uses that capacity figure to calculate your SOC percentage, so if you're off on that number you'll get skewed readings throughout the whole charge cycle, not just at the limits.

Set it to your actual tested figure every time. I rechecked mine after a few months because cells had settled and capacity crept up slightly — worth retesting periodically rather than just setting and forgetting.

Also if you're running parallel packs, test each one individually. Don't just average them or assume they'll match — mine varied more than I expected.

Really useful thread this — went through exactly the same head-scratching moment with my van pack last winter, parked up in a Tesco car park of all places trying to make sense of the JK app on my phone.

One thing nobody's mentioned yet: if your tested capacity lands somewhere awkward — say 94Ah from a nominal 100Ah pack — I'd resist the urge to round it. Just punch in 94Ah exactly. The JK's coulomb counting is doing proper maths behind the scenes, and rounding introduces a compounding error over every cycle.

Also worth noting: retest after 5–10 cycles. Fresh cells (or rebuilt packs) often improve slightly as they settle. Mine crept up about 3Ah over the first fortnight, so I updated the BMS setting accordingly. Treat it as a living number rather than a one-time verdict. 🔋