Any efficiency advantage to charging slower at home?

[Ed++]

I have a Model 3 and a Tesla charger at home that delivers 48A. On a normal day, I could easily set my Model 3 to charge at 24A instead of 48A and still have time to get a full charge overnight. Is there any efficiency advantage to doing this? I am thinking if I charge slower there might be less waste heat and I might get more kilowatts actually in the battery. Has anyone ever tested this theory?

Secondly, maybe it could be better for the longevity of the battery.

 

[TexasEV]

No it’s actually less efficient because the charging circuit runs twice as long, and the battery doesn’t care that it’s getting 11.5 kW rather than 5.7 kW when it gets 110 kW routinely at superchargers now and up to 250 kW at the new v3 superchargers.

Don’t worry about the battery. You’re overthinking this and you’re not going to do a better job of it than Tesla’s engineers. Just plug in, charge the car as Tesla intended, and let the battery management system manage the battery.

 

[leporsche]

As far as efficiency is concerned, it's actually, counter-intuitively, more efficient to charge a Model 3 faster.

While the car's charging, it has to be awake and running the on board charging electronics. These are largely time-based power draws, so the shorter you run them, the less energy the car consumes. Hence, minimize the amount of time the car is awake/charging, minimize energy consumption.

As for battery longevity, I think it's negligible. Charging at 240V/48A is less than 12 kW. For a 75 kWh battery, that's a (12/75) 0.16C charge rate. I've typically come across resources that say anything under .5C is not harmful to the battery.

 

[bhzmark]

for the older dual chargers that could do 80Amp, consensus was 56A was better for battery. consensus change on that?

 

[srs5694]

While the car's charging, it has to be awake and running the on board charging electronics. These are largely time-based power draws, so the shorter you run them, the less energy the car consumes. Hence, minimize the amount of time the car is awake/charging, minimize energy consumption.

Unless the fact that I'm using TeslaFi is changing the way the car operates, it appears (based on TeslaFi data) that the car remains awake after it's finished charging, for as long as it's plugged in. I've also noticed, both in TeslaFi data and in data returned by my JuiceNet-enabled EVSE, that the car draws electricity about once every hour after it's finished charging, presumably to replenish the power lost to its idle-mode operations while the car is plugged in but finished charging. Thus, to minimize unwanted draw, I'd say to unplug the car as soon as it's done charging. (I've seen suggestions that plugging in with the car controlling when to begin charging and set to complete a charge when you intend to leave, has the same effect, but I can't confirm that from personal experience.)

Of course, I may be overlooking something, so take the above with a grain of salt. Also, the above advice is premised on your not using Sentry Mode, which also prevents sleeping, where you charge. With Sentry Mode active, the car loses about 20 miles of range per day, vs. 2-5 miles of range per day when it can sleep. (Plus extra for battery conditioning when it gets too hot or cold.)

 

[SSedan]

Fast charging a cell phone in an hour with no cooling is harder on the battery.

In a car though even at 48amps a 310mile range car getting 44mph is still taking 7 hours to charge from empty to full ignoring the taper at the very top. That doesn't sound all that stressfully quick does it?

 

[Ed++]

No it’s actually less efficient because the charging circuit runs twice as long, and the battery doesn’t care that it’s getting 11.5 kW rather than 5.7 kW when it gets 110 kW routinely at superchargers now and up to 250 kW at the new v3 superchargers.

Don’t worry about the battery. You’re overthinking this and you’re not going to do a better job of it than Tesla’s engineers. Just plug in, charge the car as Tesla intended, and let the battery management system manage the battery.

I probably am overthinking it, I'll give you that.
But we do know that it is not good to use the superchargers too much, and that it is not good for the battery. Unless they have changed that now too.

 

[Ed++]

Unless the fact that I'm using TeslaFi is changing the way the car operates, it appears (based on TeslaFi data) that the car remains awake after it's finished charging, for as long as it's plugged in. I've also noticed, both in TeslaFi data and in data returned by my JuiceNet-enabled EVSE, that the car draws electricity about once every hour after it's finished charging, presumably to replenish the power lost to its idle-mode operations while the car is plugged in but finished charging. Thus, to minimize unwanted draw, I'd say to unplug the car as soon as it's done charging. (I've seen suggestions that plugging in with the car controlling when to begin charging and set to complete a charge when you intend to leave, has the same effect, but I can't confirm that from personal experience.)

Of course, I may be overlooking something, so take the above with a grain of salt. Also, the above advice is premised on your not using Sentry Mode, which also prevents sleeping, where you charge. With Sentry Mode active, the car loses about 20 miles of range per day, vs. 2-5 miles of range per day when it can sleep. (Plus extra for battery conditioning when it gets too hot or cold.)

I have an energy monitor on my charging circuit, and I can tell you for sure that mine does not recharge every hour. I noticed once or twice it kicked in to top off the battery, but it seems very rare to me. Days I don't drive it my monitor typically says zero power output.

I have a USB drive plugged in for video capture. I have noticed even when not operating in Sentry mode the cameras are still recording, all the time. Of course there is only the last 60 minutes of video kept on the drive, but I was still surprised that when it was not in Sentry mode, it was still awake and recording. I wonder if I removed the USB drive if it would consume significantly less power during non-drive time. Maybe I need to experiment. Or just go with it and not worry! LOL

 

[Ed++]

Fast charging a cell phone in an hour with no cooling is harder on the battery.

In a car though even at 48amps a 310mile range car getting 44mph is still taking 7 hours to charge from empty to full ignoring the taper at the very top. That doesn't sound all that stressfully quick does it?

It does not sound that quick. But I am really just talking about small differences here. Just because curious minds want to know...

 

[TexasEV]

Unless the fact that I'm using TeslaFi is changing the way the car operates, it appears (based on TeslaFi data) that the car remains awake after it's finished charging, for as long as it's plugged in. I've also noticed, both in TeslaFi data and in data returned by my JuiceNet-enabled EVSE, that the car draws electricity about once every hour after it's finished charging, presumably to replenish the power lost to its idle-mode operations while the car is plugged in but finished charging. Thus, to minimize unwanted draw, I'd say to unplug the car as soon as it's done charging. (I've seen suggestions that plugging in with the car controlling when to begin charging and set to complete a charge when you intend to leave, has the same effect, but I can't confirm that from personal experience.)

Of course, I may be overlooking something, so take the above with a grain of salt. Also, the above advice is premised on your not using Sentry Mode, which also prevents sleeping, where you charge. With Sentry Mode active, the car loses about 20 miles of range per day, vs. 2-5 miles of range per day when it can sleep. (Plus extra for battery conditioning when it gets too hot or cold.)

Wow, you really do think you know more than Tesla engineers. First, the car should eventually sleep when it's not charging. Your TeslaFi setup seems to be preventing that which is causing your excessive idle drain. (In other words, yes it is changing how your car operates, as does every other third party software that people let communicate with the car.) Ordinarily the car wakes up from sleep periodically to check it's state of charge and who knows what else. Second, your suggestion to unplug the car when it's done charging is exactly what Tesla says NOT to do. You don't want to minimize "unnecessary" draw, you want to use shore power when you can rather than the battery. Tesla says to leave it plugged in. Have you RTFM?

My suggestion-- stop using TeslaFi, plug in when you get home and unplug when you leave in the morning, and stop thinking about the battery. You don't know what it is that you don't know, and the car is smarter than you think.

 

[Dre78]

Assuming you're using the same hardware, the resistance in the wire is constant so there would be no difference in efficiency. It's only software limitation. Take this with a grain of salt as I'm not an expert, so hopefully an Electric Engineer will chime in with a real answer. For efficiency I believe higher voltage makes a bigger impact.

I probably am overthinking it, I'll give you that.
But we do know that it is not good to use the superchargers too much, and that it is not good for the battery. Unless they have changed that now too.

I'm not sure how BMS was on Model S, but on Model 3 it seems to be quite aggressive in regards to battery life. So maybe it has changed.

Here is BMS keeping Urban Supercharger low based on variety of factors, mainly State of Charge but also battery and ambient temperatures.

 

[ewoodrick]

I probably am overthinking it, I'll give you that.
But we do know that it is not good to use the superchargers too much, and that it is not good for the battery. Unless they have changed that now too.

And the cars with the highest mileage (>400,000mi) on them Supercharge at least once a day.

It's definitely not "best" but it is definitely not something that you shouldn't do. With Elon's information recently that the current batteries are expected to go 300,000 miles and next years, 1,000,000. Is the battery really as fragile as everyone treats it?

 

[srs5694]

I have an energy monitor on my charging circuit, and I can tell you for sure that mine does not recharge every hour.

I've had my Tesla for less than a month, and it's mostly been on 2019.8.x, although it just upgraded to 2019.12.1 this morning. I've seen suggestions here that the charge-once-an-hour behavior is new to 2019.8.x software, so that may be what's going on; you may have observed different behavior on older software.

Of course, I may be overlooking something, so take the above with a grain of salt.

Wow, you really do think you know more than Tesla engineers.

What part of "Of course, I may be overlooking something, so take the above with a grain of salt" did you fail to understand? Sheesh.

First, the car should eventually sleep when it's not charging. Your TeslaFi setup seems to be preventing that which is causing your excessive idle drain.

Maybe, but I first noticed the charge-once-an-hour behavior before I set up TeslaFi, via my JuiceNet-enabled EVSE. Of course, that doesn't prove it wasn't sleeping, but the once-an-hour charging pattern was unchanged once I began using TeslaFi, so I expect it's the same cause -- lack of sleep when plugged in. As above, others have reported the same thing. Also, TeslaFi does report that the car sleeps when it's not plugged in, so if TeslaFi is preventing sleep mode, it's specific to when the car is charging.

Tesla says to leave it plugged in. Have you RTFM?

Yes, I have. Cover to cover. I've also written manuals, and I know full well that the advice in manuals often does not reflect what's technically optimal; it often exists as a CYA exercise. In this case, it may exist (may; not does) so Tesla can point to that if somebody were to leave a car unplugged for half a year and then complain when battery problems result. As the manual doesn't go into specifics about why it advises leaving the car plugged in, we're left guessing about the reasons, and therefore whether they apply in every case or whether some other factor should override those reasons.

More broadly, charging schedules on an EV must be flexible, and cars must be engineered for that. People who live in apartments and condos without charging infrastructure buy Teslas, and neither the manual nor Tesla salespeople warn against such purchases on the grounds that the battery might be degraded from lack of being plugged in.

stop thinking about the battery.

My concern here is not about the battery; it's about my electric bill, and the CO2 emissions that come with wasted energy use. I work from home most days, so my car is at home perhaps 20 hours a day, on average. If we suppose 2 hours a day spent actually charging (probably an over-estimate), that's 18 hours a day at home but not charging. Taking the idling loss rate of 1 mile/hour at the EPA's 260Wh/mile rating, that's 260W of power use when idling. When sleeping, range loss is usually reported as 2-5 miles per day in moderate temperatures, which works out to 21-54W. That means that idling consumes an extra ~200W of power, or 3.6kWh/day (over 18 hours of unnecessary idling), or 1314kWh/year, if I were to leave it plugged in those 18 hours a day. Electricity in Rhode Island, where I live, is rather expensive, so that costs about $262/year. That, and the associated CO2 emissions, is more than I care to burn for no reason.

 

[ucmndd]

Assuming you're using the same hardware, the resistance in the wire is constant so there would be no difference in efficiency. It's only software limitation. Take this with a grain of salt as I'm not an expert, so hopefully an Electric Engineer will chime in with a real answer. For efficiency I believe higher voltage makes a bigger impact.

Resistivity of copper rises approximately 0.4% per degree Celsius. So yes, higher current does generate heat, which creates additional resistance, which generates more heat, and so on.

A while back someone did some fairly detailed testing of the efficiency of the onboard charger in the Model S/X at various currents. As I recall from memory, the sweet spot was somewhere around 24-32A. That said, the difference was fairly negligible. Let me see if I can find that thread.

 

[ucmndd]

Here’s what I was looking for.

For the pre-refresh Model S that has either one or two 40A chargers, Jason Hughes found they were most efficient at full tilt, 40 or 80A.

Model S Gen2 Charger Efficiency Testing

Another member did similar testing with the post-refresh 48A onboard charger and found peak efficiency to be around 25A.

Ideal Charge Rate??

I don’t think anyone has performed similar testing with the Model 3 chargers. Maybe someone should.

 

[Bruinfan]

My older UMC allows 48A charging. I've always set it to 32A, because I saw some threads and pics here on the UMC breaking down or melting, or whatever... It made sense to me to not have max heat in the handle/port chronically, if I didn't need the charge speed.

My newer UMC maxes at 32A.

 

[ucmndd]

My older UMC allows 48A charging.

* 40, not 48

 

[srs5694]

I've had my Tesla for less than a month, and it's mostly been on 2019.8.x, although it just upgraded to 2019.12.1 this morning. I've seen suggestions here that the charge-once-an-hour behavior is new to 2019.8.x software, so that may be what's going on; you may have observed different behavior on older software.

I've plugged in my Model 3 now that it's updated to 2019.12.1, and it's now sleeping after having finished its charge. It's only been 1.5 hours since it finished charging, so I can't be sure that it won't try to charge, but under 2019.8.x, it didn't go to sleep after reaching its charging set point, so I'm hopeful that what I observed was a bug under 2019.8.x that's been fixed on 2019.12.1. If so, then leaving it plugged in all the time when parked at home shouldn't waste any money or electricity.

 

[Ed++]

I don’t think anyone has performed similar testing with the Model 3 chargers. Maybe someone should.

I think so!

 

[Jedi2155]

Here’s what I was looking for.

For the pre-refresh Model S that has either one or two 40A chargers, Jason Hughes found they were most efficient at full tilt, 40 or 80A.

Model S Gen2 Charger Efficiency Testing

Another member did similar testing with the post-refresh 48A onboard charger and found peak efficiency to be around 25A.

Ideal Charge Rate??

I don’t think anyone has performed similar testing with the Model 3 chargers. Maybe someone should.

This. For the 48A charger, the ideal charge rate is in the 25A+ range. I'm also pretty sure I saw a thread that someone conducted on the Model 3 charger which showed a similar efficiency band.

There is a base loads to charging (phantom load) that tremendously impacts those charging on 120v (which I call super-slow), has a big impact. The AC-DC conversion efficiency itself is not constant through the available power range, and the individual above found the overall peak to be around 25-30A. The difference isn't tremendous though (something like 96% at 26A vs. 93% at 48A).