Thanks! That was what I was looking for. Everything seemed to reference the C-rate for discharge, and I couldn't tell if they were equal.
Interesting how the article states (for Li-Cobalt-Oxide, 18650 cell):
Charge (C-rate) 0.7–1C, charges to 4.20V (most cells); 3h charge typical. Charge current above 1C shortens battery life.
Discharge (C-rate) 1C; 2.50V cut off. Discharge current above 1C shortens battery life.
So this is for a cell. Mr. Graves' blog looked at the whole unit.
If we assume a linear relationship between the cell and the pack, then the P85D low end discharge C-rate of 5.25 (see post above) could translate into a charge C-rate of 5.25 x 0.7* = 3.675.
Is this a correct assumption?
*I chose the lowest charge C-rate for the 18650 from the article.
I would say there are way too many assumptions in this.
But to start off, the current Teslas uses the NCA chemistry (the Roadster did use LCO), so the relevant stats are these:
Charge (C-rate) 0.7C, charges to 4.20V (most cells), 3h charge typical, fast charge possible with some cells
Discharge (C-rate) 1C typical; 3.00V cut-off; high discharge rate shortens battery life
Second, these are *very* general stats. It all depends on the specific architecture of the battery cell, with the specific cathode material, anode material, seperator and electrolyte used. Plus, as you say, the cooling of the cell and the way it's integrated into the battery pack. There's no way of knowing the relationship between max charge rate and max discharge rate without assessing the actual properties of the cell and pack.
Third, timing is important. It might be possible to charge/discharge a cell at max 1C for 30 minutes without issues, but also be able to charge/discharge that same cell at max 4C for 30 seconds without issues. When you look at the 5.25C discharge of a P85D, that's not over any significant amount of time. Once you approach top speed the power drops off.
Furthermore, there's a big difference between what a cell can be charged at without bursting into flames, and what a cell can be charged at without unacceptable capacity loss over time. We know for instance that the current cells use a partial silicon anode. Not much is known about the specific anode material Tesla is using, but it is generally known that adding silicon is good for energy density, while not as good for durability.
Now, maybe Tesla has limited the charge rate of the partial silicon anode cells to err on the side of caution, but has over time seen that the durability will be acceptable even with higher power charging. If so, I would expect a *modest* improvement in charging power.
