LFP4 internal resistance test vid, cell/battery match, parallel/series, could use some help

[Stokeme]

Early on with LFP4 I was told (along w/other chemistries) that you wanted to use the same Generation for parallel or series configuration. I had a vague idea, but not really “why”. Buying my latest battery, cell matching was discussed, internal resistance (IR) seemed to be the compatibility issue. All my batteries are prismatic cells, even my first one. (Apologies to Rando for suggesting cylindrical superiority a while back)
The latest battery dealer thought with BMS (unless IR was notably different), doing its duty, you would stand to lose only 5% battery life over 10 years, (w/diff Gen) matching up in parallel. He thought, ballpark, 6 months. It was a quick calculation by him. I do not intend using a parallel setup more than a day(s) at a time.
I could really use some opinions from knowledgeable WTW electrical guys. I have recipe knowledge & a bit more, but lack the understanding of many contributors on this Forum. I’d like to understand the ramifications with running 2 different Gen, & different company’s, prismatic cell batteries, in parallel. He recommended using same Ah to same Ah only. Help?
Looks like a good detailed vid below ...

 

[rando]

You are on the right track with internal resistance. With two batteries in parallel, the battery with lower internal resistance will receive slightly more of the charge current than the battery with higher internal resistance. Similarly during discharge, the battery with lower internal resistance will provide slightly more of the discharge current than the one with higher internal resistance. Typically internal resistance increases slightly as a battery ages, thus your older battery probably has higher IR, but it is also a function of the BMS, that also adds resistance to the circuit.

However, there is kind of a fortuitous balancing effect. The IR changes as a function of SOC. At both high and low SOC, the IR increases (see the figure below). So even if your newer battery starts off with a lower SOC and is taking slightly more charge or discharge current, if it gets ahead of the older battery, the IR will increase, giving the other battery some time to catch up.

These are pretty small effects, practically speaking, put them in parallel and don't stress about it, they will be fine.

 

[Stokeme]

I appreciate the post Rando. It provides needed info & some understanding of IR dynamics. I was especially looking for “practically speaking” and hoping for “pretty small effects”. The reference to “fortuitous” also sounds good. This takes a load off my mind. Now looking at two different prismatic 50Ah batteries (with very similar build) running solo or parallel.
This gives me a lot of modular flexibility. Stoked!