This is a study of LFP aging and capacity recovery that might interest you.
TLDR:
- Shallow cycles in the middle of the SOC range (~20-80%) cause higher capacity loss in LFP cells than deep cycles or cycles that cross certain SOC ranges around 20% and 80%.
- Here's a graph showing capacity loss percentages after 350 Full Equivalent Charges at different cycle ranges. The study says that crossing between voltage plateaus 2 and 3 (around 18%) and between 4 and 5 (around 82%) are what keeps capacity loss low. As you can see, cycling between 15-35% had the least capacity loss, followed by 0-100% and 10-90%, whereas cycling between 40-60% had the most capacity loss.
- View attachment 928021
- The capacity loss is not due to BMS calibration drift. It's because the voltage curve is so flat in the middle of the SOC range that it causes non-uniform lithium distributions in the electrodes. Higher voltage gradients in the lower and upper SOC ranges help to redistribute lithium more evenly.
- Most (up to 90%) of the capacity loss due to this effect is recoverable with the right procedure.
- The most effective recovery procedure is holding SOC at 0% for 1-3 weeks depending on temperature (1 week at 45 C to 3 weeks at 25 C). Holding SOC at 100% also worked somewhat, but it was much slower and much less effective. Note: When your Tesla gets to 0%, it's really around 4.5% due to the buffer. I'm not sure how that affects the recovery and whether you need to drain it all the way out or not. I also imagine it won't be great for the low voltage battery, but maybe if you keep the car plugged in and not charging at very low SOC, that would work?
Even though most of the capacity loss from this effect is theoretically recoverable, this combined with the fact that deep cycles barely have any affect on LFP batteries (unlike NCA and NCM batteries), led me to avoid the desire to do 40-60% or 30-50% cycles, which would be great for NCA or NCM batteries. Instead, I charge to 100% just before I know I'll be going somewhere, then let the SOC get all the way down to 15-20% before I charge it back to 100% again, kind of like you would do with a gas car. Letting the battery get down low lowers the average SOC, which is good for calendar aging, but this method also lets the SOC cross those areas with higher voltage gradients to keep lithium distributed evenly. I do still plug in at home every time (so I can get preconditioning from wall power, etc), but I stop the charge manually when I'm not ready to charge yet.
If you're set on not charging to 100%, you might at least want to do something like 15-55% cycles instead of like 30-55% or 40-55%, to make sure you're crossing that area with a slightly higher voltage gradient around 18%.