Well, I just don't want to advocate as if this is some sort of 'best practice' to mix battery chemistries or anything, that's my disclaimer. A lot of people disagree with the idea of running different chemistries on same bank, and even I have occasionally posted against it in past posts.
On paper it isn't truly optimal, if you compare the discharge curve charts between LFP and AGM specs sheets examples, they do not discharge at same rate on the charts. You're also not supposed to discharge AGM (depth-of-discharge) below 50% or they puke out, where LFP you can take right down to 10-14% and get long life out of them.
I was just saying, in my case, since I really don't care about the fate of my old AGMs as much, but they still have some useful cycles left in them, I simply chose to be a simpleton and hard-bridge them together (with a manual disconnect switch in between). I favor my charge profiles on my charge controllers towards long life on my LFP battery bank, since I paid a lot of money for those cells and for sure want that array to last as long as realistically possible.
And, the charge also being on my old AGM bank keeps them alive for longer (so they are not just sitting in the shop self-discharging), but if I lost them, I would just take them out and dispose of them at a recycler. With that said, my normal stance from a professional point of view is to not mix battery bank chemistries, and not try to integrate them together, for best life on the non-favored bank.
But from my previous experience in the farming industry, sometimes you just work with what you got and not worry about things so much and there is the kind of real world, boots-to-the-ground mentality, I decided to run them together, paralleled, and two years later (they were old batteries before), they still work fine and have not had any issues. I never take my bank down lower the 55-60% state of charge anyways, so I've never hammered on the AGMs too hard. I don't even know how much extra life that bank realistically adds to my system. It does kind of seem like the SoC goes down slower since I connected the AGMs back in.
Since the two banks have a bit different discharge curves, there may be areas in that SoC curve where one bank is a bit of an asset or a liability on the adjacent bank (one bank takes more charge from the other, it's not like a 1:1 ratio throughout the discharge or charge curve).
I just look at it as a way to get a little extra watts on the system (since you already have them), but when they die, just take them out and replace them with LFP.
I don't have any schematics to create here since I'm not good with those kinds of tools. There are lots of other threads (controversial) about mixing chemistries in parallel, so I would encourage you to search here using the search page and read more about it.
I will provide an example here showing some of the varying opinions about it:
A friend has an AGM battery (has been sitting disconnected for a few years) that he has offered to give me but I already have a battery bank with two Battleborn lithium batteries. Is it OK to have a battery bank with different battery types? I'm thinking not. If it is OK, will the AGM battery...
diysolarforum.com
I guess all I am saying is that I don't want to say that I am really for or against it, I can understand why some say to never do it, but we don't live in a perfect world afterall, so we sometimes do what seems to work for us at any given time, and in this case it looks to work ok for me on this run, and doesn't seem to have any negative side effects for my system. Individual results may vary though.
