As long as you wire entire packs in parallel, there should never be an issue with charging and discharging. I don't see how it's in any way safe to do hot-swapping (or hot-switching, I guess). What if your code glitches? What if your arduino's voltage sensing drifts? What if a cosmic ray hits your microcontroller? These kinds of things are done in automotive hardware and battery switching, but they require intrinsically safe coding practices and hardware redundancy. If you do it wrong, what you get is something close to short circuit currents running between packs.
Just put packs in parallel. It's the only safe way to go about adding capacity, outside of replacing the main battery pack.
Also, out of curiosity, IsaacZachary, why would you want to use LiFePO4s in automotive? To me that seems really strange, but I might be missing something. Here's what I know about this tech - and I consider myself fairly knowledgeable considering batteries are core to my line of work:
- LFP used to have a flatter discharge curve, but NCA pretty much has the same characteristics, yet at a higher voltage
- LFP used to have better power density, but that's... just not true anymore, it's a lot worse than any modern chemistry now.
- LFP used to have higher cycle life, but that's totally not true anymore
- LFP used to be considered more environmentally safe, arguably the cobalt in NCA still makes it a bit more toxic but with 811 NMC coming up this is moot as well
- LFP cannot thermally runaway. It technically does have this advantage, as it makes it a somewhat inherently safe battery, but this aspect is so incredibly moot these days. NCA is so close to inherently safe that you have to massively abuse it to get it to do anything strange. The higher internal resistance and lower energy density (thus higher system complexity) of LFP makes it, IMO, more dangerous than more modern chemistries.
The low energy density (~2.5x difference with NMC) also makes that you really do need a trailer to pack 60kWh of LFP (=1150 lbs!!), whereas you can probably fit 60kWh of NMC (=450lbs) in the stock battery shell and a few briefcase extenders in your trunk, likely without even increasing total vehicle weight if you replace the original pack. Much more convenient, and always available, not just when you're pulling a trailer. Not to mention the cost difference and the sheer amount of extra work. 60kWh of NMC is still somewhat affordable, LFP is... just crazy. You can buy a new 2018 Leaf for that kind of money.
LFP really only makes sense as a lead acid replacement in front, not as a main traction pack.
I don't believe that one crazy video where the guy, out of frame, pretends to switch battery packs on a Leaf. Sure it could work, but I don't consider it demonstrated or particularly useful. Also, I don't consider the Leaf pack to be particularly desirable as a range extender pack. It's really heavy for its capacity and the chemistry they use inside is really inferior to even stuff like Yuasa LEV50s. As stated before, they somehow managed to get only 24kWh into a volume that can comfortably hold 45-50kWh, and still get significant voltage sag on just 80kW of load and very high wear even in mild climates. The Leaf battery is just not very good overall, I wouldn't use it for anything tbh.