Thanks for that. But the LFP battery cars don’t seem to charge above 70 kw. For me this is an issue if my Made in China LR AWD car will only supercharge that fast.
There is normally a fairly wide range of rapid charge speeds, caused by several different factors, some relating to the car, some relating to the charger, so trying to pin down an exact reason for a particularly slow rapid charge rate may not be easy, and, as always, correlation does not prove causation.
The biggest single factors affecting maximum DC charge rate will be state of charge and temperature, with temperature being a really major factor for all lithium ion cells. When cold they simply will not charge at a high rate. I've built packs using LiFePO4 cells and their cold temperature performance doesn't seem to be much different to that for other chemistries.
They do tend to have a much flatter open circuit voltage versus state of charge characteristic though, and this makes it significantly more difficult for anything that's trying to estimate state of charge in the mid range (i.e. not when nearly charged, or nearly discharged). It's possible that the battery management system may be erring on the side of caution and limiting the charge rate if it believes that it may be higher than it really is. Given that there can be no cell group balancing process running during a rapid DC charge, there needs to be an abundance of caution within the battery management system algorithm to ensure that no cell group gets pushed to an excessively high terminal voltage during the charge, and requesting the charger to reduce the current may be a good way of ensuring this in some circumstances.
If I had to guess, given the short time it took Tesla to test and qualify the LiFePO4 cells, I'd suggest that the BMS may need some fine tuning, once enough data has been collected, in order to get the best balance between battery pack health and maximum rate of charge for any given condition. Tesla have over a decade of experience with what are essentially variations of the basic LiCoO2 chemistry cells (currently LiNiMnCoO2), that have a typical fully charged open circuit terminal voltage of ~4.2 V per cell. They have barely more than a year's experience with LiFePO4 cells, that have a typically fully charged terminal voltage of ~3.65 V.
I don't think that Tesla are likely to stick with using LiFePo4 cells long term, TBH. All their in-house cell development seems focussed on a reduced cobalt LiNiMnCoO2 type of cell, with a higher energy density than the 2170 cells that are in production at the moment. The Chinese LiFePO4 cells are way down on energy density, around 13% to 15% lower than current LiNiMnCoO2 cells, so the battery pack needs to be bigger/heavier in order to have the same capacity, and that probably has some impact on vehicle efficiency. I'm sure the switch to LiFePO4 cells in the Shanghai made cars is most probably because cells of that type are more readily available in China. Because LiFePO4 cells are also a great deal safer, there may even be regulatory pressure there to use them.
