SOK 48V battery guidance needed

I have eight SOK 48V batteries (two racks of four batteries 1-4 and 5-8 with one set of cables per bank to the inverter. All cables are identicalin length) connected with communications to Sol-ark 15K. The batteries have been in service six months at this point. Starting 02/07 the report SOC on the Sol-Ark has stopped reaching 99-100% and then sending power back to the grid. Each day the SOC has stopped at a lower percentage and stayed there for the day. As of yesterday it stopped at 95%. I have looked at the batteries and four of them showing a lower achieved SOC per day (#4, #5, #6, #8), number four for example stopped at 91.67% yesterday. The individual pack voltages are very close as are the cell mV readings. The max spread on pack cycles is 70-80. I have logged quite a good deal of data from the batteries if it would help.

I would appreciate any advice on how to interpret the data and how to correct this situation.

Thanks
Ryan

I have no idea. What I would do:
1) Power cycle the inverter and all the batteries.
2) Use voltage mode to charge up the batteries to full. See if any cut out in the process.

I have now tried your suggestions. So far no changes have occurred. Anybody got anything else?

Mercracing said:

I have now tried your suggestions. So far no changes have occurred. Anybody got anything else?

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What voltage did you charge to? Did the low batteries actually get to that voltage?

I had the BMS calling the shots until this afternoon when I switched the system to voltage. At that time the batteries had stopped taking a bulk charge. I have the Sol-ark set to charge at 57.1V (the log in the inverter shows the system touched 57.1V and quickly dropped to float voltage) and float at 55.2V. In the SOK manual it talks about a 15 minute absorption period. Do you (or anyone else) know if that is something the batteries are doing internally or is it a setting in the Sol-ark (if it is I don't know where it is located).

Mercracing said:

Float at 55.2V. In the SOK manual it talks about a 15 minute absorption period. Do you (or anyone else) know if that is something the batteries are doing internally or is it a setting in the Sol-ark (if it is I don't know where it is located).

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There doesn't seem to be a sol-ark setting for how long to float. You can set equalization to the same 55.2v and do that for 8 hours. If the battery is way out of balance, it may need time.

After fully charging, as soon as the inverter drops from charge voltage to float voltage, turn off all your batteries. Then turn on one battery at a time and check their voltages. See if there are any outliers.

You can also try charging one battery at a time.

DIYrich said:

There doesn't seem to be a sol-ark setting for how long to float. You can set equalization to the same 55.2v and do that for 8 hours. If the battery is way out of balance, it may need time.

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What constitutes being "way out of balance " and when is the correct time to look at cells to determine this? I have been taking readings of all the different battery info, but I don't really know what they mean.

To that point. Last night my #3 was showing a high cell reading at 3320mV and lowest cell at 3312 mV (@ 84.13% SOC at reading and ended the charging cycle @ 98.55%) while #4 (the pack reaching the lowest SOC @ 79.65% at reading and ended the charge cycle 90.76 % was showing high @ 3317 mV and low at #3308 mV.

My cells are balanced to within 5mv. Yours are not too far off. But 3.32v per cell is 53.1v, which seems a little low for fully charged resting. I'm guessing one of the batteries is ending things early for everyone. I would charge each individually to see if one is having problems.

DIYrich said:

My cells are balanced to within 5mv. Yours are not too far off. But 3.32v per cell is 53.1v, which seems a little low for fully charged resting. I'm guessing one of the batteries is ending things early for everyone. I would charge each individually to see if one is having problems.

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Sorry. I should have been more clear. The cell voltages I listed were during discharge. The cells on #4 were highest 3558 mV and 3523 mV lowest. While #1's (highest SOC) highest cell @ 3640 mV and lowest @ 3377 mV.

Mercracing said:

Sorry. I should have been more clear. The cell voltages I listed were during discharge. The cells on #4 were highest 3558 mV and 3523 mV lowest.

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Measure right after charging to full, and no charging or draw.

Set absorb and float to 57.6v and let it sit there. No need to float so low.

@HighTechLab dont you recommend to Absorb/Float at the same voltage? Or am I mis-remembering?

Lt.Dan said:

Set absorb and float to 57.6v and let it sit there. No need to float so low.

@HighTechLab dont you recommend to Absorb/Float at the same voltage? Or am I mis-remembering?

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Does holding the batteries that high pose any danger to the cells (I am asking out of ignorance)? When the communications were on the bank would come up to 57.1-57.2V and then ramp down to 56.4V and then back up to 56.9V throughout the afternoon. I'm still kind of at a loss as to how the batteries got out of sync with one another SOC wise (I probably should have mentioned this earlier but the batteries are charged and discharged daily and had been reaching a full charge daily (albeit at a lower SOC each day).

DIYrich said:

My cells are balanced to within 5mv. Yours are not too far off. But 3.32v per cell is 53.1v, which seems a little low for fully charged resting. I'm guessing one of the batteries is ending things early for everyone. I would charge each individually to see if one is having problems.

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DIYrich

Catching the batteries during a time frame like you described the cells are not as well balanced. These numbers are from 2/14 @ 5:09 p.m.
Pack 1- high- C5- 3558 mV
Low- C7- 3348 mV
Pack 2- high- C5- 3668 mV
Low- C10- 3501 mV
Pack 3- high- C13- 3656 mV
Low- C15- 3509 mV
Pack 4- high- C13- 3623 mV
Low- C16- 3504 mV
Pack 5- high- C5/C9- 3585 mV
Low- C14- 3516 mV
Pack 6- high- C5- 3654 mV
Low- C7- 3504 mV
Pack 7- high- C5- 3638 mV
Low- C4- 3350 mV
Pack 8- high- C13- 3620 mV
Low- C11- 3514 mV

Also, not sure if this would matter but when the batteries went into service the #4 and #7 packs had incorrect/ different programming from the other six batteries. This caused #4 and #7 to shut down as fully charged earlier than the other batteries. Initially this caused the batteries SOC to go pretty far out of balance, with 4 and 7 showing 100% charged and the other packs only getting to 75-80%. The programming issue was resolved and the bank came back into balance (BMS reported SOC) and this was several months ago. However after the correction #4 and #7 do not charge and discharge at the same rate as the other packs, an example from 2/14 @ 8:57 p.m.
#4 and #7 were putting out 1.52A and 1.71A respectively while the other six batteries were outputting 2.12 to 2.33.

Mercracing said:

Does holding the batteries that high pose any danger to the cells (I am asking out of ignorance)? When the communications were on the bank would come up to 57.1-57.2V and then ramp down to 56.4V and then back up to 56.9V throughout the afternoon. I'm still kind of at a loss as to how the batteries got out of sync with one another SOC wise (I probably should have mentioned this earlier but the batteries are charged and discharged daily and had been reaching a full charge daily (albeit at a lower SOC each day).

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Charging and holding at 57.6v is no problem for a few hours or even a day or two. Keeping them at that high of a voltage for weeks or months (like in storage), will degrade them slightly over time. Don't forget they are spec'd to charge to 58.4v (3.65v per cell).

I personally charge to 56.8v and have it float for a few hours everyday to give the passive balancers time to balance out. This keeps my batteries within a perfect 10-15mV delta.

Lt.Dan said:

Charging and holding at 57.6v is no problem for a few hours or even a day or two. Keeping them at that high of a voltage for weeks or months (like in storage), will degrade them slightly over time. Don't forget they are spec'd to charge to 58.4v (3.65v per cell).

I personally charge to 56.8v and have it float for a few hours everyday to give the passive balancers time to balance out. This keeps my batteries within a perfect 10-15mV delta.

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Lt.Dan,

What type of inverter do you have? My 15K does not appear to have the ability to set different stages to the charging process (I have float, absorption, and equalization- which Sol-ark said not to use on LFP). Ultimately what I would like is to figure out how to get the inverter and the batteries to function correctly in closed loop, but for now I will settle for charging correctly. Thank you to everyone for trying to help. I appreciate it.

Mercracing said:

Lt.Dan,

What type of inverter do you have? My 15K does not appear to have the ability to set different stages to the charging process (I have float, absorption, and equalization- which Sol-ark said not to use on LFP). Ultimately what I would like is to figure out how to get the inverter and the batteries to function correctly in closed loop, but for now I will settle for charging correctly. Thank you to everyone for trying to help. I appreciate it.

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I have a Sol-Ark 15k as well. I just set my Absorb, Float, and Equalize all to 56.8v. My use my batteries to arbitrage sell, so they are all charged by 1-2pm, and then hold 56.8v until 4pm, where it dumps everything back to the grid.

I personally use open loop communication.

Lt.Dan said:

I have a Sol-Ark 15k as well. I just set my Absorb, Float, and Equalize all to 56.8v. My use my batteries to arbitrage sell, so they are all charged by 1-2pm, and then hold 56.8v until 4pm, where it dumps everything back to the grid.

I personally use open loop communication.

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During the non air conditioning seasons (provided we get decent sun) my batteries are discharged (per the closed loop BMS) to about 71-73% when the sun starts charging us up. On a normal day the batteries are charged by noon or shortly after and we start selling to the grid. In your opinion how long is to long to let the batteries sit at 56.8 or 57.6? I don't think I had asked, but what brand of batteries do you have? I am looking at my SOK's currently and they had reached the set voltage and are now showing voltages ranging from 54.95V (1- was one of the batteries that was showing a full charge) to 56.75V (5- one of the batteries that was low) amd do not indicate they are taking in any power. #1 has cell voltages from 3439-3509mV and #5 is 3543-3556mV. The line voltage to the batteries is still at 57.6V. Should I turn it down?

Mercracing said:

During the non air conditioning seasons (provided we get decent sun) my batteries are discharged (per the closed loop BMS) to about 71-73% when the sun starts charging us up. On a normal day the batteries are charged by noon or shortly after and we start selling to the grid. In your opinion how long is to long to let the batteries sit at 56.8 or 57.6? I don't think I had asked, but what brand of batteries do you have? I am looking at my SOK's currently and they had reached the set voltage and are now showing voltages ranging from 54.95V (1- was one of the batteries that was showing a full charge) to 56.75V (5- one of the batteries that was low) amd do not indicate they are taking in any power. #1 has cell voltages from 3439-3509mV and #5 is 3543-3556mV. The line voltage to the batteries is still at 57.6V. Should I turn it down?

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Even 24hrs is totally fine imo.