Dual JBD 24V LifePO4, different currents

I have 2x24V (8s) batteries, 75ah each
Both are on JBD BMS', monitored and historized data in home assistant. I have also made the settings on both JBD's identical, and confirmed that at rest (no load connected) the current meter reads 0.

I have them wired in parallel with the same length wiring to my inverter, with the + and - on separate ends of the battery.

Although I would accept some difference over time, the %SOC and current push/pull varies by ~30% between the batteries (leaning to more load on JBD Pack1, which is about 3 months older than the newer pack). The individual cell voltages are extremely similar across both batteries at all SOC, and both have either active balancers (currently unplugged), in addition to the crappy passive balancing from the BMS above 3.4V.

I can provide tons of data if needed, but any early ideas on the potential issue (or is there one?).

End of the day, one battery is reading at 8%, and the other at 29%. I charge them both to ~95% (or 28V) nightly.
Today, one was at 3%, the other at 30%

Could just be non-calibrated current shunt in one of the BMS? or is there something else I should check or care about?

Have you checked all your connections? A thermal camera is a popular way of doing this by looking for hot spots under load, but you can also just do some (careful) preventative tightening?

Can you provide a photo of the wiring?

I'll check the internal connections again (and tighten), but need to turn it all off as the batteries are stacked in plastic boxes.

I have a thermal IR detector, but not likely sensitive enough to get this.

You can also use a multimeter - while the system is under load - to find bad connections by putting it in DC voltage mode and measuring the potential across a given connection. If you get a big voltage difference it tells you you have a bad connection.

See https://www.underhoodservice.com/tech-tip-find-poor-connections-by-measuring-for-voltage-drops/

Symbioquine said:

You can also use a multimeter - while the system is under load - to find bad connections by putting it in DC voltage mode and measuring the potential across a given connection. If you get a big voltage difference it tells you you have a bad connection.

See https://www.underhoodservice.com/tech-tip-find-poor-connections-by-measuring-for-voltage-drops/

Click to expand...

great idea.. and simple!

Hmm... 5.4mV across the breaker on the lower performing battery, and total 6.2mV from + post to +post on the battery (through breaker + T-class fuse. Doesn't seem much, but probably is the culprit.

I'll take apart and tighten + compare to the other battery tomorrow

from @740GLE : Are you wiring batteries to a bus bar then inverter to the bus bar? Or are you double stacking lugs on the inverter?

Pack 1 on top, 2 on bottom
Single leads to inverter/charger.
SCC connected the opposite way across the batteries

Inside, T-Class on the + of the battery, and breaker on both + and - as a switch. JBD BMS on the neg side

edgecrusher said:

Hmm... 5.4mV across the breaker on the lower performing battery, and total 6.2mV from + post to +post on the battery (through breaker + T-class fuse. Doesn't seem much, but probably is the culprit.

I'll take apart and tighten + compare to the other battery tomorrow

Click to expand...

And the 5.4mV drop is on a 10A charging load, so resistance of 0.0054 ohms?

So under a higher load (15-20A) the voltage drop is right at the breaker (not at the fuse connection). Since I just have stranded wire into the breaker (no ferrule, or crimped connection), it's probably that is isn't tight enough, or loosened off once under higher load/temperature.

Can I assume that this 5-150mV drop is the culprit of 30% delta between the batteries, or should I continue the search after fixing this?

Tightened all connections, and checked the other battery.

About 150mV drop from the + post on the battery to the inverter (via the T-class and breaker). Most of the drop is across the breaker... and this is the same on both batteries... so, probably crappy breaker, but do I care?