Better to charger higher or run battery lower?

[Allistah]

Hey all —

M3 owner for a little over a month. We live in the Central Valley in California and commute to Pleasanton every day. We have a M3 AWD LR and we use about 50% of the battery every week day.

Since it takes longer to get up to 90% which costs more money in power, and it charges faster from lower SoC, would it be better to only charge to 70% and run it down to ~20% every day or charge it up to 80, 85, or 90% every day and only run it down to 15-25%

I would think it would cost less to go with a lower amount as getting to 70% versus higher levels would take longer and thus cost more.

Any thoughts on this?

 

[AlanSubie4Life]

Since it takes longer to get up to 90% which costs more money in power, and it charges faster from lower SoC

It does not take longer (per mile added) at higher SoC when using L2 home charging. Only above 96-97% will it slow down and potentially hurt charging efficiency, which is not relevant here.

Just charge to 90% and don’t worry about it. Sounds like you definitely need 90% for margin and flexibility.

 

[pdx_m3s]

In your case, I would certainly charge to 90%. Although it may appear to be taking longer near 90% SOC, it’s just the system trying to approximate the SOC (i.e. it’s just a presentation issue). The power going into (and “filling up”) the battery is constant from 40% to 90%.

 

[Allistah]

Ah ok. Yeah, I did the math and have been averaging about 51% usage a day during the week. I've tried to lower it a little bit so I've been setting it to about 85% and that has worked pretty well.

On a side note, I'm not stressing about it at all. Learning this stuff is fun for me, little tweak here, little tweak there. Part of the fun of owning these cars. I think I'll just continue to run at 85%. The average low point at the end of the day during the week has been 34% left when charging to ~86%. Sounds like I'm pretty much right on the money where I should be considering the amount of usage we go through.

Can't wait to gather all the charge rate information tonight when I get home. In another thread, I promised to gather efficiency data in 2A increments. I'm going to measure current at the breaker panel, and what the car reports. Will capture all that data in a spreadsheet to see exactly what point is the most efficient. People are calling out that 32A is going to be the sweet spot to minimize loss so we will see what my setup is. I have about 45' of 6/3 cable on a 50A circuit running through a high quality 14-50 outlet that goes into a Tesla WC. The outlet gets warm so I know that's contributing to some loss. Will be really interesting to see how much loss I have and what current charging rate will be the most efficient for my configuration.

 

[AlanSubie4Life]

People are calling out that 32A is going to be the sweet spot to minimize loss so we will see what my setup is

Might have got the wrong impression what what I said there. I think it is possible that 33/34A are not as efficient relative to what you expect compared to 32A. But I would expect 40A to be the best for you. 48A would be better most likely but you can’t check it.

 

[Allistah]

Might have got the wrong impression what what I said there. I think it is possible that 33/34A are not as efficient relative to what you expect compared to 32A. But I would expect 40A to be the best for you. 48A would be better most likely but you can’t check it.

Ah, no worries! Can't wait to see! lol - like a little kid waiting for a package at home or something. At least I'll know which may hopefully help others.

Now this begs another question in regard to the health of the battery pack. Since I know it can charge at a really high rate at SuperChargers, I'm guessing that charging with 40A at home won't be a problem for it over the long run. Have not been to many SuperChargers yet but I thought it was pretty crazy that it was charging at 529/miles an hour. LOL. I think I get under 50 at home I think?

I am also thinking of raising the temperature inside the car during the day. It's set to default right now. I know that just raising it by 5-10 degrees can make a big difference in power usage while my wife is at work during the hot days and it will still keep the car to a reasonable temp when she gets into it.

 

[darth_vad3r]

I am also thinking of raising the temperature inside the car during the day. It's set to default right now. I know that just raising it by 5-10 degrees can make a big difference in power usage while my wife is at work during the hot days and it will still keep the car to a reasonable temp when she gets into it.

“While your wife is at work” ? ... are you using “Keep climate on” to maintain temp in the cabin all day?

 

[Allistah]

“While your wife is at work” ? ... are you using “Keep climate on” to maintain temp in the cabin all day?

Yeah my wife drops me off at a Park & Ride where I catch a shuttle the rest of the way to where I work. The option to keep the cabin cool is indeed on. The loss doesn't seem to be too much so I haven't focused on it too much. Teslafi is what I'm using to keep an eye on that and it's a few miles loss, but not a ton. Happy wife, happy life. lol

Update: Went and looked at last Thursday.. Nearly five hours, the car lost 5.54 miles in range which was 1.23 kWh. I've gone through all the math and it's about 19 cents per kWh for us with summer rates (don't have time of day rates) so for 24 cents, it's worth it to keep my wife happy every day when she gets back into it.

 

[darth_vad3r]

Yeah my wife drops me off at a Park & Ride where I catch a shuttle the rest of the way to where I work. The option to keep the cabin cool is indeed on. The loss doesn't seem to be too much so I haven't focused on it too much. Teslafi is what I'm using to keep an eye on that and it's a few miles loss, but not a ton. Happy wife, happy life. lol

Update: Went and looked at last Thursday.. Nearly five hours, the car lost 5.54 miles in range which was 1.23 kWh. I've gone through all the math and it's about 19 cents per kWh for us with summer rates (don't have time of day rates) so for 24 cents, it's worth it to keep my wife happy every day when she gets back into it.

That’s “Cabin Overheat Protection” that keeps the car from killing pets/babies accidentally left in the car, but you can’t adjust that temperature. It’s around 40C/105F hard-coded.

You mentioned changing the set temp by 5-10 degrees so I thought you were using “Climate Keeper”. That is when you hit the Fan icon after putting it in park and setting it to “on” (or “dog mode”) and it keeps the climate system running at your chosen set temp.

If you aren’t using that, then adjusting the set temp doesn’t affect how much energy is used while you are parked.

Or are you actually using Climate Keeper? I can’t see it only using 1.23 kWh for Climate Keeper in California over 5 hours.

You can just pre-cool the car to your preferred temp 5-10 minutes before arriving at the car and save on 4h50m of AC use.

 

[Allistah]

Ah wow ok, thanks for the info on that. For some reason, I thought there was a set point that I could adjust but I guess I'm mistaken. I had not dug in to the settings to look for it so I just assumed that I could change that. The temp doesn't seem to go much about 100F in there so I guess that feature is on and I'll just roll with it since my wife likes it. Thanks again for the info.

 

[darth_vad3r]

Ah wow ok, thanks for the info on that. For some reason, I thought there was a set point that I could adjust but I guess I'm mistaken. I had not dug in to the settings to look for it so I just assumed that I could change that. The temp doesn't seem to go much about 100F in there so I guess that feature is on and I'll just roll with it since my wife likes it. Thanks again for the info.

Was just editing while you replied, see note I added about pre-cooling ... the wife will be never more happy! 5-10 minutes is plenty to drop from 100F to 70F

 

[Allistah]

I've got my spreadsheet ready to go for tonight with a corresponding chart for the net loss. I can't think of anything else that would be good to know when I gather this data. If anyone has any suggestions, please feel free to chime in.

If anyone is curious, here is some additional data about the columns:

 

[Tam]

...it takes longer to get up to 90% which costs more money in power...

In some states that prohibit electric reselling, Tesla would charge you my minutes and not kWh. In this scenario, quicker make sense.

However, I assume you are talking about home charging. Utilities charge you by kWh and not by the time duration.

So, the last 1 kWh may take 10 hours or 1 minute at home, I still only have to pay for 1kWh.

 

[AlanSubie4Life]

So, the last 1 kWh may take 10 hours or 1 minute at home, I still only have to pay for 1kWh.

This incorrect, due to overhead of about 200W (this is approximate, don't assume it is exactly correct) when actively charging.

For your 10 hours (extremely unlikely) example, that would be 3kWh to add 1kWh, vs. ~1kWh for your (impossible since it would require 60kW AC) 1-minute example.

 

[Allistah]

So, the last 1 kWh may take 10 hours or 1 minute at home, I still only have to pay for 1kWh.

I'm having a difficult time understanding the last line there. kWh is a measure of watts over one hour of time. The part that is confusing me is in the scenario of the car. If I set the car to a 30A draw and going from 90-100% takes 2 hours, but going from 50-60% only takes 1 hour, the amount of power consumed (and thus the amount I would pay for) would be double on the 90-100 portion. Thats right, right?

 

[AlanSubie4Life]

I can't think of anything else that would be good to know when I gather this data. If anyone has any suggestions, please feel free to chime in.

Sounds like you have TeslaFi. I guess for each charging level, you will need to leave it charging for some amount of time. I guess I would get TeslaFi's data on how many kWh you add for each charging interval (you'll presumably need to interrupt the charge). But you do need to understand exactly what TeslaFi is reporting from the API (there may be multiple fields for the amount of energy - one for energy added to the battery, another for the power out of the AC-DC chargers - they won't be the same!). It's important to understand what the numbers TeslaFi reports mean.

I expect that the Breaker amps and the amps reported by the car should basically be the same. But you'll have better resolution on your breaker amps probably.

You should record the voltage that the car sees in a column. This is going to change depending on your current setting. Voltage matters as well as current to the on-board chargers because they are converting AC input power to a DC output power (with some loss). The input current is only half the info needed to know the input power.

It would be interesting to see the exact overhead amps from your current clamp when the car is sitting there not charging (since it will have better resolution than the in-car display), but that 2A (that's what I observe it to be when the door is open and the screen is on - so 500W) does not last long before the Wall Connector opens its relay. You also have to record the voltage at that 2A to figure out the static power. It is good info to have, but I guess you just need to record it once.

I don't think you need to subtract that 2A or whatever from your numbers, though - it's part of the overhead you're trying to determine. And the number you see on initially plugging in probably is not the same when the car is just sitting there in idle mode with the display off and just charging (that seems to be closer to 200W, not 500W, just based on the energy consumption of idle mode from other data).

There are a lot of other details I'm not covering here. You'll probably have to gather data and then iterate on the method as you discover the shortcomings of the approach.

Ideally you have measurement of the voltage at the current clamp location and at the Wall Connector input, too, but probably not realistic.

 

[AlanSubie4Life]

I'm having a difficult time understanding the last line there. kWh is a measure of watts over one hour of time. The part that is confusing me is in the scenario of the car. If I set the car to a 30A draw and going from 90-100% takes 2 hours, but going from 50-60% only takes 1 hour, the amount of power consumed (and thus the amount I would pay for) would be double on the 90-100 portion. Thats right, right?

No. Because it will not be drawing 30A at the end. It'll be tapering way below that set charge level at some point (and you'll see it on the screen). That being said, the original statement was also not true. See above.

 

[mswlogo]

It will most definitely take longer to get to 90%, that is partly why some SC now limit to 80%. If we are talking SC you are definitely better off going lower. Some SC are time based and it’s more costly. It doesn’t take more energy though. SC backs off on the watts long before it reaches 90%.

That all said, charging at home to 90 daily would be fine. But if it’s all super charger, I think 80% would be more time efficient and possibly cheaper in some situations (maybe). Assuming you have enough room on the bottom end. Down to 30% daily would fine. I wouldn’t want to go to 20% daily though. You want more buffer in case of traffic or weather event.

 

[Allistah]

No. Because it will not be drawing 30A at the end. It'll be tapering way below that set charge level at some point (and you'll see it on the screen). That being said, the original statement was also not true. See above.

Ah ok, I've never watched the actual draw when it nears the end and just assumed that it was 30A constant the entire time. Thanks for the info on that.

 

[AlanSubie4Life]

It will most definitely take longer to get to 90%,

From context, I assumed the OP was talking about home charging (presumably no one would use a Supercharger every day if they only used 50% of their battery each day - generally good to avoid a Supercharger at all costs!). But yes, the Supercharger situation is different than that discussed above.