Best lithium configuration , parallel or serial first

[Samsonite801]

Remember that when you swap out a cell that is in parallel, the voltage needs to be near the same as the rest of the battery. Otherwise you are going to dump a massive amount of current from the new cell to all of the older cells. That could trip your BMS. So the idea that serial first is the best solution for fault tolerance is complicated by this fact. You can't just start flipping breakers on and off to spare cells.

Ok fair enough... So knowing that we all may have our own reasons and preferences to all things we each decide, and I respect whatever decisions anyone else makes, I will go through these items, based on my own perspective, and personal preference (what other perspective can I report on other than my own here hehe).

On this first point above... If I am ready to bring a battery module back into the cluster, then I log into each BMS, confirm the SOC and voltage are similar, if close enough (for my liking), or exactly matching (if I'm maybe feeling more OCD that day), then close breaker and bring it back in.

In addition the general rule of battery design is that all cells are similar. That is why we match cells and don't mix different brands, chemistries or ages. By implementing a hot swappable design you are violating this concept. Even if you have spares that are of the same brand, model, and age, the fact that they are spares implies that they have a different usage history. You would need to periodically rotate your spares into the battery to ensure that all cells remain equal.

I don't really care if my cells are matched or not, I never capacity tested any of mine, and didn't even top balance them first (let the BMS balancing handle aligning them), if a cell goes bad and I replace it, the bank will basically have as much capacity as the smallest Ah cell. Will they breathe at different rates, maybe. That's fine, it's been fine so far. Even if say, 280Ah cells were someday not available anymore, I wouldn't have a problem replacing some of my cells with 304Ah cell or something.

The idea of matching everything perfectly, is more for the tight optimization approach on systems where a lower margin for error is required, or space constraint issues drive designer into a push for the highest degree of precision. My battery bank is sized well enough for my uses, that the lowest DoD I've ever taken it down to is maybe 59%, and if my capacity ever suffers, I will likely just add more cells. Like maybe try to buy some extra cells every year.

Again for smaller batteries, the best plan is to keep a complete spare battery around, not just a spare cell. For larger batteries that is not possible due to expense, weight, and installation complexities, so a hot swap model is required. I don't think most people meet this criteria.

If you have a spare battery around, might as well use it. I do have 32 cells sitting in my storage unit right now earmarked for my permanent 48v system, and I cringe a bit thinking about how they've just sat there for more than a year and I'm not yet using them.

If the goal is to always have a backup cell available if one fails in a parallel configuration, then the best design is to have the backup cell already in the battery and simply run off of all but one of them. That way they can be rotated by flipping a breaker off and letting the others handle the load. But if you are going to do that, then all of the cell may as well be in use. That eliminates the need to rotate them and lets you use the money you invested in all of the cells. That is the point of parallel connections, they are all backups all the time. Its already automatically a redundant design.

Yeah, and again, I don't really care about matching cells, as long as they aren't horribly out of whack from the group, and standing out like a sore thumb. If the battery bank is sized well with extra buffer there, it should have enough extra reserve so you're not running too close to limits (unless you have space or budget constraints I suppose). I am on a tight budget myself, but I have all the space in the world. I'd rather cut a few luxuries in my life for awhile to spend a little extra and just have more KWh available...

Ideally everything would be in parallel. Serial connections are what cause the need for a BMS. Because we don't have 3.2V inverters and inexpensive lightweight large-gauge wires, we need serial connections to push the voltage up. Once we do that we need a BMS. So the whole question is when to do that.

Small = PS
Large = SP

Well, there are other reasons for running higher voltages, like lowering current, smaller gauge cables, transporting through longer cable lengths, etc.. It's also more efficient to up the voltage via serial cells, rather than try to boost convert it up once it gets to the inverter, less loss...

I honestly wish the industry would move to higher battery bank voltages than 51.2v nominal for inverters and such, like the APC 5KW UPS I put into my Prius for example, the nominal voltage on that inverter is 192v (which is 16x 12v batteries in series)... Then the inverter only has to boost 192v-230v DC to 240v AC, which results in less conversion loss.

The BMS also serve other functions, besides just to accommodate for serial connected banks. Like temperature monitoring, overcurrent, etc..

Again the voltages must be nearly the same when you introduce a new cell into the mix. This is even more problematic with larger cells since they can transfer higher currents. So to change out a bad cell requires that you wait until you can bring all cells back to full charge. If you don't want to wait, then you must have a full battery on standby and switch between them mutually exclusively.

More reason to just have a bank full of multiple battery modules, and just take problematic module out of the loop (Maintenance Mode), give it a full manicure, pedicure, massage therapy, acupuncture treatment, and when it's all nice and shiny again, leave the SoC at high-end, and pick a time in your day where your SoC matches on the rest of the bank, and bring it back into the cluster gracefully.

Again, this is just my personal kind of ideology I tend to prefer here. Others will have all the reasons for doing things some other way.