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Charge Efficiency Chart From Our Model 3

Allistah

Member
Jul 6, 2019
214
165
Central Valley, CA
Hey all --

I was thinking about what the most efficient charge rate was for our car taking into account that slower rates are better for the battery, but higher rates are more efficient. I thought there was a happy area in the middle that I could choose if I could see the data. I subscribe to TeslaFi.com. Our 3 DM AWD was bought on June 30th this year. We have a Tesla Wall Connector that is connected to the panel with a NEMA 14-50 outlet using 6 gauge wire to a 220v 50A breaker across about 50' distance. All I did was export the data from TeslaFi.com on our charges and remove any charges that were under an hour in time. Pretty interesting, I think I'm going to roll with 32A. Seems to be the most efficient while being the slowest. I would think that I could probably get away with something in the 26-28A range too, but I haven't charged in that area to get any data on it. Maybe I'll try a couple and see how those show up on the chart. Seems like charging at 32A gives me somewhere between 96-98% efficiency - almost identical to charging at 40A. When I charge around 21A, you can see a clear loss of around 2% in the range (~94-96%). Interesting data so I thought I'd share. Let me know if anyone has any questions.

Screen Shot 2019-11-17 at 4.56.32 PM.png
 
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AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
9,393
11,272
San Diego
How does TeslaFi define & calculate charge efficiency? Does it continuously monitor the AC voltage & current, integrate over time, and take the ratio with the added kWh?

Efficiency = kWh added / Integral over time of (I*V)

Or what?

This data seems 1) pretty noisy and 2) the peak efficiencies are higher than I have observed when taking the ratio of energy added to battery vs. energy from Chargepoint, so I wonder about the accuracy of the source data. If it didn't track your voltage over time or current over time, I could see errors being introduced.

I believe best case efficiency when charging at 240V/48A (11.5kW) is something like 94-95% - but that would include the losses in the Chargepoint cable.
 

Allistah

Member
Jul 6, 2019
214
165
Central Valley, CA
Thanks for the post. I'm not sure how it figures that out.. Maybe someone can chime in. The data comes directly from the car so I'm guessing that the efficiency varies based on temperature of the pack and temp in the garage, etc. and all that when it actually charged. Seems like about a 2% spread at a given charge rate. I set it to 28A so I'll let it go at that rate for a few weeks to see how it does. If anyone knows more about the charging efficiency from TeslaFi, would you mind sharing what you know?
 

sa012

Member
Nov 7, 2019
196
46
Central CT
Hey all --

I was thinking about what the most efficient charge rate was for our car taking into account that slower rates are better for the battery, but higher rates are more efficient. I thought there was a happy area in the middle that I could choose if I could see the data. I subscribe to TeslaFi.com. Our 3 DM AWD was bought on June 30th this year. We have a Tesla Wall Connector that is connected to the panel with a NEMA 14-50 outlet using 6 gauge wire to a 220v 50A breaker across about 50' distance. All I did was export the data from TeslaFi.com on our charges and remove any charges that were under an hour in time. Pretty interesting, I think I'm going to roll with 32A. Seems to be the most efficient while being the slowest. I would think that I could probably get away with something in the 26-28A range too, but I haven't charged in that area to get any data on it. Maybe I'll try a couple and see how those show up on the chart. Seems like charging at 32A gives me somewhere between 96-98% efficiency - almost identical to charging at 40A. When I charge around 21A, you can see a clear loss of around 2% in the range (~94-96%). Interesting data so I thought I'd share. Let me know if anyone has any questions.

View attachment 478139

Interesting......my rated range has taken a drop from 240 to 231 (SR+) after only 2200 miles. I have been using a NEMA 14-50 but have been using a slow amperage (10-12 amps) over a longer time frame rather then the 32 amps for a short period and charging it up to 90% SOC. Maybe that could be a reason???
 

Allistah

Member
Jul 6, 2019
214
165
Central Valley, CA
Eh, I wouldn't think so.. What I found is that the slower the charge rate, the more it costs you because there is some amount of overhead with charging. The shorter time that you charge with, the more efficient it appears to be - at least from my data. If I slow down - which is better for the batteries, the more inefficient it is and costs me more money. So I'm trying to find out where the good middle ground is for the battery and where it's still somewhat efficient.

I did a very slow charge at 5A over the weekend able 90% and whoa, that was really inefficient around 65% or something. Which means that it really wasn't worth charging at a rate that slow.
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
9,393
11,272
San Diego
where the good middle ground is for the battery

I am sure this has been discussed elsewhere, but is there any consensus on whether there is any problem at 11.5kW? Personally I think it cannot possibly make any significant difference if you are charging at 6kW vs. 11.5kW. Both are fantastically low rates of charge.

Maybe you could summarize what you read from elsewhere in discussions of this topic?

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.
 

Allistah

Member
Jul 6, 2019
214
165
Central Valley, CA
Let me say that digging into details like this is fun for me because I learn from it. You could easily charge at any rate. I'm probably splitting hairs at this point, but I'm a tech nerd and I like this sort of thing.

There is an overall assumption though that charging slower is better for the battery. It heats it up less, gives it time to rebalance itself it at the right SOC, etc. But there is an "overhead" cost of power when charging so the less time you charge, the less you have to pay in "overhead costs". When you charge at 10A, not exactly 10A is going into the battery, the car uses some of that power and some is lost through the inverters, etc. I think this is where they get the efficiency rating from. What goes into the battery, versus what comes out of the power panel. The difference is the overhead. That overhead doesn't seem to increase with higher charge rates, so you benefit by charging higher/faster which results in less overhead time since it finishes quicker. The down side is that it could heat up the battery a bit. I'm not sure if 40A could heat the battery pack at all since it seems so slow in regard to watching some of these SuperChargers run with 150kw. My little 9kw (40A) seems like nothing in comparison but who knows.

This is what I've learned from these forums and from my own data:
1) Slower charges are better for the battery, in general (less heat, gives them time to rebalance if they need to)
2) Slower charges cost you more money in "overhead" because those charges take longer (it's a give/take thing)
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
9,393
11,272
San Diego
That overhead doesn't seem to increase with higher charge rates, so you benefit by charging higher/faster which results in less overhead time since it finishes quicker.

Yeah, for sure the fixed overhead is about 250W, I understand that...

My little 9kw (40A) seems like nothing in comparison but who knows.

Yeah. I doubt charging at 32A vs. 40A vs. 48A will make any appreciable difference in terms of longevity. Tesla already throttles those rates if the battery is detected as cold, etc., to prevent any plating risk. If it actually makes a difference for balancing that's more of a software issue that Tesla needs to deal with - the car should take extra time to balance the battery, no matter what it is plugged into, if it needs to do that.

Personally I'd just charge at the maximum possible rate and not worry about it. But that's just me. Anyway, back to the topic - hopefully someone can help with the TeslaFi raw data source - as I said I don't trust it. I think it's very unlikely efficiency using AC charging is ever better than 95-96%, even at 48A.
 

Allistah

Member
Jul 6, 2019
214
165
Central Valley, CA
Yeah, for sure the fixed overhead is about 250W, I understand that...

Yeah. I doubt charging at 32A vs. 40A vs. 48A will make any appreciable difference in terms of longevity. Tesla already throttles those rates if the battery is detected as cold, etc., to prevent any plating risk. If it actually makes a difference for balancing that's more of a software issue that Tesla needs to deal with - the car should take extra time to balance the battery, no matter what it is plugged into, if it needs to do that.

Personally I'd just charge at the maximum possible rate and not worry about it. But that's just me. Anyway, back to the topic - hopefully someone can help with the TeslaFi raw data source - as I said I don't trust it. I think it's very unlikely efficiency using AC charging is ever better than 95-96%, even at 48A.

I think the data comes from the car itself as it would really be the only one to know since it has to report the data.

Do you have TeslaFi? I was just looking at the charge data from last Friday and it has some interesting info to look at. If you don't have it, I can post a file with the data in it if you're interested in taking a peek.
 

Allistah

Member
Jul 6, 2019
214
165
Central Valley, CA
Here is a screenshot of a charge summary from last Friday from TeslaFi. It gets the data directly from car from the information that the car reports.

upload_2019-11-18_12-4-10.png
 

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EVDSCar

Member
Oct 21, 2019
19
3
California
I think for everyday charging, if you can charge between 20mi - 30mi/hr is the most efficient since you can probably plug in for 2-3hrs Max. If you have to continuously charge for 8 or 9 hrs everyday then that’s too much
 

AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
9,393
11,272
San Diego
I think the data comes from the car itself as it would really be the only one to know since it has to report the data.

Do you have TeslaFi? I was just looking at the charge data from last Friday and it has some interesting info to look at. If you don't have it, I can post a file with the data in it if you're interested in taking a peek.

I don't have TeslaFi. I use Stats, which does not report charging efficiency (just reports kWh & miles added, and driving efficiency).

The API has access to the voltage and current readings on the AC side, as I understand it. So it would be possible to report charging efficiency, but the details are important.
 

Allistah

Member
Jul 6, 2019
214
165
Central Valley, CA
Here are a couple screenshots from a charge that gets recorded in TeslaFi. The first one is the top, and the second is the first ~15 mins of data below it. I think the data at the bottom is the data that the car makes available to it. Would be cool if Stats did something like that as well. I use both TeslaFi and Stats.

Screen Shot 2019-11-18 at 12.12.37 PM.png


Screen Shot 2019-11-18 at 12.13.09 PM.png
 

hugh_jassol

Member
Jan 26, 2019
715
767
Los Angeles
I don't have TeslaFi. I use Stats, which does not report charging efficiency (just reports kWh & miles added, and driving efficiency).

The API has access to the voltage and current readings on the AC side, as I understand it. So it would be possible to report charging efficiency, but the details are important.
Tesla Fi Looks at the Voltage and current from the AC side to determine kWh "used" and then looks at what the car reports as the kWh added to the battery and then the ratio is the effeiciency
 

ran349

Member
Jun 28, 2016
431
279
SoCal
I don't have TeslaFi. I use Stats, which does not report charging efficiency (just reports kWh & miles added, and driving efficiency).

The API has access to the voltage and current readings on the AC side, as I understand it. So it would be possible to report charging efficiency, but the details are important.
I used TeslaFi for a while and found it to be inaccurate on several things, including charging efficiency. I think it takes voltage and current data points periodically, and sometimes misses data points, which will bias the efficiency number high.
I wouldn't use it to make any conclusions about charging efficiency.
 
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AlanSubie4Life

Efficiency Obsessed Member
Oct 22, 2018
9,393
11,272
San Diego
Here is a screenshot of a charge summary from last Friday from TeslaFi. It gets the data directly from car from the information that the car reports.

View attachment 478390


The added kWh is equal to 245Wh/rmi * (Added rated miles).

So charging efficiency is: 49.13kWh (which is 200.09rmi*245Wh/rmi) divided by 50.2kWh.

49.13/50.2 = 97.86%


To calculate charging efficiency, you should probably do something like:

Avg A * Avg V * Charge Time (but this is not an integral so it is not guaranteed to be correct if both amps and volts are varying).

In your case: 40A * 232.6V * 5.383hr = 50.1kWh

Efficiency: 49.13kWh (Added kWh) / 50.1kWh = 98%

This number is too high based on my ChargePoint observations but I can't explain why. It may be what @ran349 described above.

How TeslaFi got 50.2kWh I also don’t know.

I would just use a model for the charging efficiency for your purposes (250W overhead and 95% AC-DC efficiency for a quick guess) and leave it at that. It is true that this efficiency number accounts for some cabling losses so the actual conversion efficiency may be slightly higher.
 
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Allistah

Member
Jul 6, 2019
214
165
Central Valley, CA
Like I said, splitting hairs for sure, just fun to look at the data and learn from it. Would be interesting to know if TeslaFi is wrong or if Stats could add something like this to their app.
 

justsomeguy

Member
Jul 4, 2019
295
288
Vancouver, Bc
Yeah, for sure the fixed overhead is about 250W, I understand that...



Yeah. I doubt charging at 32A vs. 40A vs. 48A will make any appreciable difference in terms of longevity. Tesla already throttles those rates if the battery is detected as cold, etc., to prevent any plating risk. If it actually makes a difference for balancing that's more of a software issue that Tesla needs to deal with - the car should take extra time to balance the battery, no matter what it is plugged into, if it needs to do that.

Personally I'd just charge at the maximum possible rate and not worry about it. But that's just me. Anyway, back to the topic - hopefully someone can help with the TeslaFi raw data source - as I said I don't trust it. I think it's very unlikely efficiency using AC charging is ever better than 95-96%, even at 48A.
I've heard many people repeat the old "low and slow" methodology when it comes to charging their EVs, but in my opinion a charge rate of < 1/6C is very slow as it is. I believe you're absolutely right that the BMS already eliminates the ability for most forms of poor battery care from occuring, especially when it relates to stuff people usually have no visibility over like cell temperature and balancing.
 

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