OK, so with that said, what does "same age" mean? Same year? Same month?
What does same capacity mean? What's the allowable variation?
I mean theoretically, based on that statement, I may not be able to go with the same brand. I mean I may buy more cells in 6+ months to make a 200 AH setup. Then maybe a year down the road I will buy more to expand it further.
A big question based on the eddy current issue, what damage is done if it is in the "bad" range? Are there methods to reduce or eliminate the issue if it occurs? Does having the same brand/model of BMS help? Like if I put the Overkill Solar BMS on the SOK?
Sorry for the questions, I need detail, facts, data.
Same rating, purchased at the same time, same brand. That's the recommendation.
There's what's recommended and then there's what's done. It's a choice. If you search the forum, you may find examples of where people have tested their bank to confirm operating currents are acceptable. I personally can't think of an example where I read there was a problem, but I haven't read the whole forum.
I have 8X (4S2P) T-1275 150Ah 12V Trojan FLA with TESTED capacities ranging from 75-95% of rated. They also range about 2 years apart in age. I pay attention to them and treat them as though they are all 75% even though one whole string is in really good shape. I also have active balancing on them. Their current flows were within about 10% running about 50A and were well within limits. I also limit charging of the entire bank based on the weakest battery to make sure that I'm not straining the weakest one.
Note the above is a worst case type of scenario. I have 4 batteries in a string that all have different capacity - never recommended. Ever. It works fine because I pay attention to it. If one of the strings was 150Ah and the other 100Ah, but all string components were well-matched, I wouldn't pay much attention at all.
If you find the batteries are out of limits, you reconfigure to try and shift current, or you split them up and come up with a plan B.
This is a DIY forum. If you need rigid rules, indisputable facts to justify
all actions/decisions, you're probably going to be disappointed.
Consider that you're accepting the "same brand, same capacity, same age" as gospel because that's the recommendation. None of them have hard data to back it up, it just passes the logic test - they
should perform the same. When you can take DISsimilar batteries and verify they are performing within limits, that's hard data.
Hopefully
@SteveS will chime in because I have a faint recollection he said something about his results.