Uneven charging and discharging

[piper18o]

On my solar system, I just added three more 48 volt batteries for a total of six, and one of the new batteries charges slowly and discharges slowly. (on the picture, it is the top dark blue bar.) All six are lifepo4 eve 3.2 cell x 16 batteries, giving me two banks of three 16 cell batteries. (that is a total of 96 3.2 cell batteries for clarification) My inverter is a Sol-ark 15K. before adding these last three, one of the first three was a little faster charging and faster discharging than the other two but not that far off. That one (Red on the graph) still charges a little faster and discharges faster than the other two on it's bank but at least it is closer. In the attached picture, the top three are the new batteries, and the bottom three are about 4 months older. My bus bars are 4 position bars with the main cable on position 2 while the battery cables are on positions 1,3, and 4. You would think that it is the 4 position battery that might be charging and discharging slower since it is the farthest from the main battery cable, but that is not the case. It is the 1 position on it's busbar. On the other bank, the faster charge and discharge battery is in the 3 position. All cables are the same size and length. What would cause the discrepency? By the way, this picture is at night after heavily discharging for three hours.

 

[sunshine_eggo]

Step one on all oddities is to check that all connections are properly torqued, crimps are of high quality, etc.

You have to eliminate any potential for bad contact. If you're stacking ring terminals on the same post, you have to make sure they are perfectly flat.

Did you confirm that the BMS for each battery was true and accurate via fully charging and synchronizing the individual batteries?

You likely need to top balance the bank at your peak absorption voltage and linger there for a couple hours. This is needed to ensure that the BMS are individually synchronized to 100%.

 

[piper18o]

Step one on all oddities is to check that all connections are properly torqued, crimps are of high quality, etc.

You have to eliminate any potential for bad contact. If you're stacking ring terminals on the same post, you have to make sure they are perfectly flat.

Did you confirm that the BMS for each battery was true and accurate via fully charging and synchronizing the individual batteries?

You likely need to top balance the bank at your peak absorption voltage and linger there for a couple hours. This is needed to ensure that the BMS are individually synchronized to 100%.

Those were the first things that I did. I do not have any ring terminals stacked. When charging, the top battery on the chart charges at about half the rate of the other two, and discharges at about half the rate as well. What makes the most sense would be connections, since all cables are the same, but they are all tightened the same. Is there another way to check with a multimeter? I have considered moving the BMS from the one battery to another just to see if the problem moved to a different battery, but I thought I would beg for advice here first.

 

[sunshine_eggo]

Those were the first things that I did.

Bus bars torqued to spec?

No washers in the current path?

I do not have any ring terminals stacked. When charging, the top battery on the chart charges at about half the rate of the other two, and discharges at about half the rate as well. What makes the most sense would be connections, since all cables are the same, but they are all tightened the same. Is there another way to check with a multimeter? I have considered moving the BMS from the one battery to another just to see if the problem moved to a different battery, but I thought I would beg for advice here first.

SoC variations can substantially influence current imbalance.

Have you fully charged the entire bank to absorption voltage (56.8V min) and held it there for a couple hours? If communication is present, you need to disable any communication and set the voltages manually.

 

[piper18o]

I did some experimenting yesterday after the sun went down and found that when I isolate the batteries that seem to charge and discharge slower, the wattage is wrong. For example, with only the one battery online, Solar Assistant said that it was pulling about 500 watts but my inverter was using about 1000. When I isolated one of the other batteries, Solar Assistant said that the battery pull was matching the inverter load of 1000 watts. What does this mean?