Capacity testing?

[Steve Dally]

Hey all,
A few months ago I posted about a build (2p4s, 560ah) that suddenly showed diminished capacity. You guys helped me by making me realize my cells were really out of balance. I finally got the chance to tear it down, top balance and build them back in to 2 separate 4s banks. I'm now trying to capacity test, but my new capacity tester shows very different info than my bms. While running the capacity test, the tester is showing 13 volts and 13.8 amps but my BMS is showing 13.2 volts and 16.2 amps. That's almost a 20% difference in wattage.... Do I just ignore the BMS info and trust the capacity tester?
(I'm using properly gaged wires, I've brushed and cleaned connections, etc.)

Thanks in advance for any thoughts or suggestions on this!

 

[chrisski]

What is the wire gauge from the capacity tester?

I made mine 10 gauge with no problem.

What does a multimeter say? That would be truth data. Try the measurements at the battery terminals and at the BMS terminals if possible.

I can’t say which amps is more accurate. I have a clamp ammeter to double check. If you can keep it under 5 amps, the multimeter may be able to measure without blowing a fuse.

I find BMSs to be notoriously bad at measuring SOC. My capacity tester was good at amps and volts, but I only tested a single cell, not a whole battery.

 

[MattMan119]

BMS should be calibrated … I used this method .. for JKBMS .. not sure what kind of BMS you have .. after calibration it is much much better ..

 

[Wattson25]

The mismatch between your capacity tester (13V, 13.8A) and BMS (13.2V, 16.2A) is likely due to one of these:

• Voltage Drop: Even with good wires, slight resistance at connections can cause the BMS to read higher current/lower voltage than the tester. Double-check tightness and maybe measure voltage at the cell terminals directly.
• Calibration Drift: Your BMS or tester might be off. If the tester is high-quality and recently calibrated, it’s probably more accurate.
• BMS Sensing Issue: Some BMS units misread current under load. Compare with a clamp meter if you have one.

For now, trust the capacity tester if it’s a reputable model, but verify with a multimeter or clamp meter to be sure. Don’t ignore the BMS entirely—it’s still your safety net.

 

[frankrudis65]

Hi there,instead of worrying too much of differences in your equipment,you could buy yourself a smart shunt which gives you quite exact details of energy going in and out of your battery pack without stressing the cells charging and discharging them to their limits(they love to call it capacity test)Those statistics at the shunt can be reset,from time to time, to give you daily/monthly aso detailed views of what's happening!Another advantage: no time is consumed