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Charge Efficiency Chart From Our Model 3

Allistah

Member
Jul 6, 2019
214
165
Central Valley, CA
I wonder if I could somehow validate the data myself. I think we already know the car uses about 250w of power in overhead, and I know that it's really pulling right at 40A from the breaker panel. I would think I could take the amount of time, the current out of the wall and the beginning/ending energy in the battery to come up with a number... Sounds like that's enough data to figure it out right?
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
9,411
11,298
San Diego
I wonder if I could somehow validate the data myself. I think we already know the car uses about 250w of power in overhead, and I know that it's really pulling right at 40A from the breaker panel. I would think I could take the amount of time, the current out of the wall and the beginning/ending energy in the battery to come up with a number... Sounds like that's enough data to figure it out right?

There is probably a way, yes. Depending on what you want to capture, you could use the voltage reported by the car, the voltage you measure at the input to the Wall Connector, or the voltage at the breaker.

Getting the time period exactly correct is important too of course.

You could also manually integrate the TeslaFi data points to see whether they are self consistent.

Somewhere along the way you’d probably discover the exact discrepancy.
 

Chisale

Member
Sep 28, 2019
217
195
Ohio
If you're worried about this sort of charging rate, I would be really worried about using any regen in the car, too. (Which I'm not.) It very, very easily exceeds 11.5kW, unless regen is severely limited.[/QUOTE]

I agree with you and I'm absolutely certain that the regen charging rate is well in excess of the maximum AC charge the car is capable of. My previous car was a HEV with a 1.3 kWhr battery in it and on a long downhill highway grade and not hitting the full regen capacity it would take the battery from completely depleted to completely full in about 2-3 minutes. On a severe downhill highway exit ramp with braking where you can get the full regen potential it would charge it to full in way less time than that. Maybe 1 minute. So giving those approximate values the charge rate would be somewhere in the neighborhood of 40-60 kW. Maybe more. And the Tesla has way more regen capacity than that because it is designed to and has so much more storage for it.
Sometimes people chime in and wonder why the regen capability is limited during cold weather. It's powerful. That's why.
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
9,411
11,298
San Diego
I wonder if I could somehow validate the data myself. I think we already know the car uses about 250w of power in overhead, and I know that it's really pulling right at 40A from the breaker panel. I would think I could take the amount of time, the current out of the wall and the beginning/ending energy in the battery to come up with a number... Sounds like that's enough data to figure it out right?

Example from my session today:

Added 106 rated miles (245Wh/rmi * 106 rated miles = 26kWh)

On a Chargepoint session of 4:37 minutes, at 30A, with voltage between 198 and 201V (as indicated in the car).

Chargepoint says the session was 27.324kWh

Hand calculations using approximate car numbers says 30A*199V*4.62hr = 27.6kWh

So efficiency was about 26kWh/27.3kWh = 95.2% - which is slightly better than I've measured in the past for 6kW charging (I expected 93-94%). Obviously there is large (245Wh) potential rounding error on the charging kWh (about 1% error). Also using the car numbers (26kWh/27.6kWh) gives 93.4% efficiency - so it could be the Chargepoint is reporting a low number due to miscalibration. Obviously there is error on that in-car value as well, since I don't know exactly where the voltage was the whole time.

Still, I wonder whether they have optimized the charging overhead with recent updates - would allow them to claim better MPGe without any increase in range. Probably not. Enough error sources here that the efficiency may well be close to 93-94% as expected.

But way worse than the TeslaFi numbers. This is why I don't trust them (plus the huge amount of variation you see). You can obviously do a similar test yourself.

I've screwed this math up in the past (this post below is incorrect - I failed to multiply through the hours correctly...):

Model 3 Range Constant Check

But if the overhead is actually 250W from the wall, then this implies that AC-DC conversion efficiency is:

Efficiency * (27.6kWh - 4.62hr*250W) = 26kWh

=> AC-DC conversion efficiency is 98.3%. (this is not the overall efficiency, which is worse, of course)

That's probably too good, so either it's not really 250W from the wall of overhead (less), or some rounding error has created error. But it could be as high as 97% I would think. Only way to easily check the overhead is to fit it to other charging rate datapoints.

In any case, I guess it's possible that TeslaFi "corrects" for this overhead, and is presenting that result. But that would not produce data that would be what you would want for your original purpose. And I'd expect less variation on this efficiency over the range of currents you presented in your original plot, if they are removing the charging overhead (it dominates the lack of efficiency at low charging powers).

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