newbe question on soc solved

I have an 18KP invertor with 16kw solar and 12 48v lifepro batteries. Originally installed with only 6 battery pack and every thing worked great. We would run the batteries down to 30% and they would all charge to 100% with no problems. Added the second bank of 6 batteries to the system as we were going to add extra load and were worried about being short of power overnight. Everything was fine the first day of operation. Bank1 was charged to 95% and Bank2 was only at new battery charge of 50%. Ran the banks down to 15% and let system charge but could only get 83% charge. Thought that was because of the extra load to get bank2 up to bank1. The next day we were at 40% to start charge and could only get to 90% charge. Then again from 40% to 89%. That is when I started to look at the battery communication information and all batteries in bank2 were balanced at 99-100% but bank1 was all over the place from 70-83%. Now the question I have is that if the battery that shows 100% has all the cells within 3.337 to 3.338 at 53.6 volts then how can a battery at 70% show cells at 3.334 to 3.337 at 53.6 volts. Am I missing something here? Does this just mean that the bms has drifted and the batteries are all ok or do I have to worry that I am having battery failures?

I am going to take a blind stab at this. You need to get the new batteries fully charged first before connecting them to your existing battery bank. Then maybe your bms will do its job and keep all of them balanced.

Cabin Rising said:

I am going to take a blind stab at this. You need to get the new batteries fully charged first before connecting them to your existing battery bank. Then maybe your bms will do its job and keep all of them balanced.

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Thanks for your response. You are correct that I should have only hooked up bank2 and charged them to 100% but as mentioned above bank2 (the new batteries) are well balanced at 99 to 100%. The problem is that bank1 now has 3 batteries only registering 60 to 80% with one registering 100% and the other 2 between 70 to 85%. Bank1 was always balanced before adding bank2 now bank2 is well balanced and bank1 is out of wack. But when I look at all the batteries I see that all cells are within 2mohms of each other and all are at the same voltage as measured. The question is if a batteries bms is reporting 60% with the identical cell voltages and battery voltage how can one be at 100% and one be at 60%?

Thanks Cabin Rising for your suggestion. I disconnected bank2 from the system and let bank1 charge and behold all came back to 100%. Disconnected bank1 and connected bank2 and let them charge to 100%. After that I connected both banks and everything is back to normal.
I guess that I answered my own question that if the cell voltage and the main voltage is the same as a higher % battery then the bms for the lower % battery is out of wack and not a bad battery. Always learning from my mistakes. Thanks again.