Valence U27-12XP Internal SOC accuracy issue. Can I fix it?

[TheWanderingJackalopes]

I have a bank of six Valence wired 2S3P all wired to busbars with identical cables. I top-charged all batteries individually, and they are showing full, but the Valence diagnostic software is showing one battery at 22% and another at 46%. The individual cell voltages are similar to the ones showing 96%-99%. Previously they were reading correctly, so now I'd like to know what happened, and how do I fix it? I appreciate any direction you can offer. I am also posting a pic of my 6 diagnostic screens. the 2 in red are the ones that are off.

 

[ljwobker]

I'm just getting my dataloggers up and running, but I've noticed that through a couple of sweep tests (discharge to inverter cutout at ~10.6v and then recharge back to full) the battery reported SoC values are all over the place. I have no idea what algorithm the internal BMS uses to compute this, but I'm skeptical at best that it's a reliable value...

 

[RCinFLA]

Chances are you have not charged to point where Columb counter resets to 100%.

Some BMS's are really stupid at this. Some require you hit cell overvoltage charge voltage to trigger Columb counter reset to 100%.

If not reset to 100% periodically the BMS Columb counter is running on 'dead reckoning' just keeping track of goes into and goes out of current verse time increments. No current measurement is accurate over the wide current range and errors will accumulate over time unless reset to 100% by a full recharge trigger periodically.

 

[TheWanderingJackalopes]

Chances are you have not charged to point where Columb counter resets to 100%.

Some BMS's are really stupid at this. Some require you hit cell overvoltage charge voltage to trigger Columb counter reset to 100%.

If not reset to 100% periodically the BMS Columb counter is running on 'dead reckoning' just keeping track of goes into and goes out of current verse time increments. No current measurement is accurate over the wide current range and errors will accumulate over time unless reset to 100% by a full recharge trigger periodically.

That could be, I hadn’t heard of a columb counter before. I’ll need to figure out how to do that on the U27-XP internal BMS. The one battery at 23% I started topping off at 1amp and it drove one of the cells over 4v pretty quickly, way too fast for a battery at 23%. I’m surprised that didn’t trigger the counter. Any suggestion on how to best go about it without cooking my cells?

I should add that most of the time these batteries are just sitting, as the inverter-charger is on shore power and solar panels most of the time.

 

[RCinFLA]

I should add that most of the time these batteries are just sitting, as the inverter-charger is on shore power and solar panels most of the time.

Cell self-leakage discharge is not captured by battery monitor Columb counter. Expect 0.5 to 2% of cell capacity per month depending on ambient temp and cell aging condition. This adds to monitor representation inaccuracy, unless Columb counter reset to 100% by a full charge periodically.

Just like navigation by dead reckoning, need to find a known accurate reference point periodically to wash out any cumulative measurement inaccuracies.

 

[TheWanderingJackalopes]

I'll try running off the batteries for a few days and then charging to full. In the meantime I hope for more information on the U27-XP batteries specifically.

If you had to take a guess (or if you know) what would be the conventional way of resetting the columb counters in a 2s3p LifePo battery bank? Run the bank down and charge back up? Or run each battery down to some point and charge back up before putting back in the bank?

 

[TheWanderingJackalopes]

Problem Solved! I shut down the solar panels and shore power and just ran the batteries overnight. They're all reading 83.1% SOC now. So it looks like I just need to use the batteries more while building out our school bus conversion.