Or if someone already has a formula for me to use, let me know
I have a thought, rather than a formula.
I think it will be hard to have "paired charge" detection, per se. If I am paired the other car might be at 10% SoC ... a long way from tapering ... or at 90% SoC in which case I may be getting as much as possible, or something very close to it. So don't bother to attempt to detect that.
I think my wife's expectation would be a good target to aim for!: Plug in, walk away, come back after 10 ... 30 minutes, be surprised/angry if the car is not charged as expected!
How about compare current charging rate to "best ever" (i.e. for this car). Over time the "Best ever" might change - especially for a new car. Record / Remember the best ever charging rate from 10% to 20% SoC, 20% to 30% SoC and so on.
In case helpful for me (S90D) it is approximately this:
0% - 10% 5m - 282 mph
10% - 20% 5m - 282 mph
20% - 30% 5m - 282 mph
30% - 40% 5m - 282 mph
40% - 50% 5m - 282 mph
50% - 60% 5m - 282 mph
60% - 70% 7m - 198 mph
70% - 80% 9m - 156 mph
80% - 85% 6m - 120 mph
85% - 90% 7m - 102 mph
90% - 95% 12m - 60 mph
95% - 100% 28m - 24 mph
100% - 100% 24m additional time to Balance cells (this is probably very variable)
Then display the variation between current charging session and the Best-Ever. A bit like getting "5 bars" on a mobile phone when the signal strength is good.
I'm not very good at Visuals, so don't have a suggestion for one, but I would like to have a display indicator that would indicate that:
a) charging rate is poor - this would show as charging rate being, say, only 30% of normal. (Not sure how easy it would be to differentiate "paring" from "Stall not working properly". My thought it that Pairing will improve over time, whereas a stall that is only delivering 30% of maximum won't improve AND might taper less). This would indicate that it might be worth moving to a different stall, and is the main thing I would like an alert for.
b) I was Paired at the start, and that has finished - i.e. charging started at, say, 49% of "best-ever" whilst SoC went from 10% to 30%, then after that charging has been consistent at 99% (compared to best-ever).
c) Charging started off fine, but has tapered off far
more than normal. This would be "something abnormal" (I have read threads where people are suggesting that some Superchargers in SoCal are throttling back). That would be useful to get an alert for - I might delay finishing my meal
Perhaps the baseline for "ideal charging rates" could in some way "guess" if the charger was a Supercharger, L2 or something else? Does Volts or somesuch give a clue? I think the API has a Supercharger attribute, I've never charged on L2 but maybe that has a different attribute value?
Lastly, not sure if "Expected Charge Complete Time" is available via the API? If so then, additional to whether the current charge rate is normal for SoC perhaps also "alert" if that time changes - that might detect the SoCal scenario where charging throttles back much more than normal. Perhaps it would also indicate when Pairing had finished - a sudden drop in "Expected Charge Complete Time"
I'd be very happy to send you all my Supercharger logs (recorded using TeslaFi, which I could export to XLS or similar). I don't think I've ever been paired ... but I have charged at a stall that was not working properly, not sure if I using TeslaFi back then.