Low Amp Charging?

[jmichaels]

Given my daily trips are only around 25 miles and only using about 10% of range, any reason not to do my daily charge at 240V/ but lower amps to make it easier on battery?
I have read a post from an owner who did a study that showed that charging efficiency is the same at 240V/24Amps as 240/12Amps. Given I am typically charging from 55% to 65% or so on a daily basis, and time to charge is no concern, why not charge the few hours my refill takes at 12-16Amps/240V (less than my house capacity of 24Amps) to be gentle on battery for my typical daily cycle? Lower amps are less heat, right, and heat is one of the enemies of battery charging, correct? Thoughts?

 

[Zoomit]

Anything less than about 0.25C would be "gentle" on the battery.

For an LR battery, 24A is ~0.08C and 12A is ~0.04C.
For an SR battery, 24A is ~0.12C and 12A is ~0.06C

Those are rough numbers but the bottomline is that it makes no difference.

 

[jrweiss98020]

Don't overthink it. If you're only cycling between 55 and 65%, "it just doesn't matter"! The electronics are fully capable of optimizing the charging.

 

[Gasaraki]

Given my daily trips are only around 25 miles and only using about 10% of range, any reason not to do my daily charge at 240V/ but lower amps to make it easier on battery?
I have read a post from an owner who did a study that showed that charging efficiency is the same at 240V/24Amps as 240/12Amps. Given I am typically charging from 55% to 65% or so on a daily basis, and time to charge is no concern, why not charge the few hours my refill takes at 12-16Amps/240V (less than my house capacity of 24Amps) to be gentle on battery for my typical daily cycle? Lower amps are less heat, right, and heat is one of the enemies of battery charging, correct? Thoughts?

As the others said, if you don't care how fast it charges, do the lower amps, doesn't matter, do what you want. However it makes no difference to the car. The batteries are not even remotely stressed at 24amps. That's only half of what the battery can take.

 

[brkaus]

You're not building up much heat in the battery during the short charging, so I would say no difference.

Also, when you charge longer the pumps and electronics need to stay on longer.

 

[gavine]

I charge at higher amps in the winter to warm up the battery before leaving. I charge at lower amps in the summer because I don't need to warm the battery and it's less-taxing on the summer grid.

 

[camalaio]

In addition to the above good points, I should point out the efficiency statement you quoted is probably not entirely correct.

• 240V * 24A = 5.76kW available
• 240V * 12A = 2.88kW available

If I assume these are "equivalently efficient" in terms of power conversion (let's take 100%), I can then consider other losses. The car uses about 250-300W while charging for running pumps, computers, etc. for the duration of the charge. These loads do not change depending how much power is available, so the more power you provide, a higher ratio of it will be going to the battery.

• @ 24A: 5760W available - 300W overhead = 5460W to battery (94.8% going to battery)
• @ 12A: 2880W available - 300W overhead = 2580W to battery (89.6% going to battery)

You gain over 5% more energy going to the battery instead of being wasted running pumps & computers if you charge faster.

Anecdotally, at 32A filling about 70%, my battery raised about 5°C (9°F) in a closed garage. That's fairly insignificant and for a much larger percentage than you would be charging for.

 

[BEPA400]

In addition to the above good points, I should point out the efficiency statement you quoted is probably not entirely correct.

• 240V * 24A = 5.76kW available
• 240V * 12A = 2.88kW available

If I assume these are "equivalently efficient" in terms of power conversion (let's take 100%), I can then consider other losses. The car uses about 250-300W while charging for running pumps, computers, etc. for the duration of the charge. These loads do not change depending how much power is available, so the more power you provide, a higher ratio of it will be going to the battery.

• @ 24A: 5760W available - 300W overhead = 5460W to battery (94.8% going to battery)
• @ 12A: 2880W available - 300W overhead = 2580W to battery (89.6% going to battery)

You gain over 5% more energy going to the battery instead of being wasted running pumps & computers if you charge faster.

Anecdotally, at 32A filling about 70%, my battery raised about 5°C (9°F) in a closed garage. That's fairly insignificant and for a much larger percentage than you would be charging for.

That is not entirely correct. You forgot to account for higher voltage losses between the panels and the car. It could be only 1-2% difference but it will come off the 5% you mention.

on top of that if you are like me and use sentry mode always, then while charging it is actually more efficient to charge at the lower amps since the computer overhead is essentially an ongoing function regardless of charging status.

so for me overall I save a few percent by charging at a lower amp rate.

 

[camalaio]

That is not entirely correct. You forgot to account for higher voltage losses between the panels and the car. It could be only 1-2% difference but it will come off the 5% you mention.

on top of that if you are like me and use sentry mode always, then while charging it is actually more efficient to charge at the lower amps since the computer overhead is essentially an ongoing function regardless of charging status.

so for me overall I save a few percent by charging at a lower amp rate.

Fair, but more situational. You need about 100ft before you get 1% @ 12A vs. 2% @ 24A. For some people this world be accurate, but many the run is actually quite short (garage panel to garage outlet).

Using Sentry 24/7 is equivalent to about 11,000 rated miles per year on your battery. If you use it that much I'd really suggest a dedicated dashcam setup with internal battery that uses much, much less power. A year of using that instead of Sentry would pay for the dashcam setup via saved electricity! According to info people have found here, using Sentry 24/7 may also prevent the BMS from calibrating properly since it needs a rest period.

 

[gavine]

Using Sentry 24/7 is equivalent to about 11,000 rated miles per year on your battery.

If my math is correct (using Model 3 @ 4mi/kWh), that would mean that Sentry Mode uses about 300 watts? That seems like a lot. Either my math is off, or Sentry uses a lot of juice.

 

[camalaio]

If my math is correct (using Model 3 @ 4mi/kWh), that would mean that Sentry Mode uses about 300 watts? That seems like a lot. Either my math is off, or Sentry uses a lot of juice.

Correct. I'm glad my math is reversible haha. Coolant pumps need to be on (at minimum, the AutoPilot computer is liquid cooled) and the computers themselves draw a lot of power (they don't need to, but they're not optimised to be glorified dashcams as that's far from their designed purpose).

Any situation that keeps the car awake will use about this much power. Sentry Mode. Summon Standby. Waking the car to check it via the app. The car waking itself to do whatever it does regarding updates, sending data, etc. (which it does frequently for many people).

When asleep, the car is actually fairly well designed to reduce power drain from what I can tell. There's just so many ways to keep it awake. It's a shame really. One department making the car really reduce power usage, another making it use a lot to do things it wasn't designed for (whether that's for flashy features like Sentry or data collection for AutoPilot/service/fleet data).

 

[BEPA400]

If my math is correct (using Model 3 @ 4mi/kWh), that would mean that Sentry Mode uses about 300 watts? That seems like a lot. Either my math is off, or Sentry uses a lot of juice.

I don’t know.. it is a lot. Those numbers suggest 7.5kwh per day, but I only measure draw on average day of 5kwh, suggesting 2/3 of the energy camalaio suggests? Though I think maybe Tesla is using my computer to mine bitcoin while it’s awake..

 

[camalaio]

I don’t know.. it is a lot. Those numbers suggest 7.5kwh per day, but I only measure draw on average day of 5kwh, suggesting 2/3 of the energy camalaio suggests? Though I think maybe Tesla is using my computer to mine bitcoin while it’s awake..

7.2kWh, not 7.5kWh. But anyhow. How are you measuring? There might be other differences too, like HW3 vs. HW2.5 (I'm on HW3). Most people say 250-300W, I see anywhere from 250-350W averaging about 300W. Notably if you drive in the 24h period, you will of course have a lower amount for "Sentry-only" usage since some time in the day was indeed not only Sentry. Same goes for charging like you pointed out. 11,000mi/year is indeed a worst case scenario thought exercise as no one is truly using it 24/7.

Even if it's less at 5kWh/day, that's still a very significant amount of energy IMO, equivalent to about 7,600mi/year (rated miles).

Sort of back to topic, these are good points for edge cases to low-current charging. I expect this applies to very few people, and where it does, I will always suggest a dedicated dashcam setup. At 5kWh/day like @BEPA400, that's 1825kWh/year for Sentry. After factoring in charging losses (highly variable depending on your setup), let's say you pull 2000kWh from the wall for that amount. In money terms, assuming $0.12/kWh, that's about $240. You can take that $240 and get a battery-backed dashcam setup which records audio (unlike Sentry), has much clearer video, and will save you money in future years by using it instead of Sentry. At this point, I'm not sure there's much left for benefits to charging at low amperage at home?

 

[jmichaels]

7.2kWh, not 7.5kWh. But anyhow. How are you measuring? There might be other differences too, like HW3 vs. HW2.5 (I'm on HW3). Most people say 250-300W, I see anywhere from 250-350W averaging about 300W. Notably if you drive in the 24h period, you will of course have a lower amount for "Sentry-only" usage since some time in the day was indeed not only Sentry. Same goes for charging like you pointed out. 11,000mi/year is indeed a worst case scenario thought exercise as no one is truly using it 24/7.

Even if it's less at 5kWh/day, that's still a very significant amount of energy IMO, equivalent to about 7,600mi/year (rated miles).

Sort of back to topic, these are good points for edge cases to low-current charging. I expect this applies to very few people, and where it does, I will always suggest a dedicated dashcam setup. At 5kWh/day like @BEPA400, that's 1825kWh/year for Sentry. After factoring in charging losses (highly variable depending on your setup), let's say you pull 2000kWh from the wall for that amount. In money terms, assuming $0.12/kWh, that's about $240. You can take that $240 and get a battery-backed dashcam setup which records audio (unlike Sentry), has much clearer video, and will save you money in future years by using it instead of Sentry. At this point, I'm not sure there's much left for benefits to charging at low amperage at home?

Thank you, Camalaio...I agree, back to topic, now thinking that really not much, if any, benefit to lower amp charging. Based on several posts and input from many, seems 24A is more efficient (vs 12A) and the heat/stress (or wear/tear) on battery on 24A vs 12A is negligible. Anyone disagree?

 

[gavine]

A little off-topic, but when I got my first Tesla back in 2014, I started looking-into house appliance vampire drain. I had a plasma TV which apparently has a lot of vampire drain. I don't recall the math, but I think it equaled about a 1,000 miles of driving. I happened to have an outlet behind the TV on a switch and decided to plug the TV into the switch. Voila....1,000 miles of free range.

 

[acarney]

Given my daily trips are only around 25 miles and only using about 10% of range, any reason not to do my daily charge at 240V/ but lower amps to make it easier on battery?
I have read a post from an owner who did a study that showed that charging efficiency is the same at 240V/24Amps as 240/12Amps. Given I am typically charging from 55% to 65% or so on a daily basis, and time to charge is no concern, why not charge the few hours my refill takes at 12-16Amps/240V (less than my house capacity of 24Amps) to be gentle on battery for my typical daily cycle? Lower amps are less heat, right, and heat is one of the enemies of battery charging, correct? Thoughts?

You don't have to worry about the battery at level 2 speeds. These batteries can hit 170kW (SR+) or 250kW (LR) when supercharging. Less than 5% of that from a level 2 (7.6 to 11.5kW) isn't going to do anything to the battery, zero, nothing. In fact, batteries like to be warm when charging... again this very very low rate probably doesn't need to be warm but in deep winter it might actually be better on the battery to be warm while charging at L2 speeds.

You're keeping it in a great range (50 to 70%), you're charging mainly at home on L2 power, and you're hardly discharging very much, that battery is going to last. Don't worry one bit. I would have it charge as quick as possible so it's always "full" (based on however much charge you want), so it gets some heat into the battery in the winter before leaving, and so the car can go to sleep as soon as possible (turn off computer, fans, etc and rest in a very low power state).

(I only have a little under 7000 miles on my SR+ right now so not a huge history, but I charge daily at 32 amps [max for an SR+] and have supercharged a handful of times, maybe 15, and with a usual charge of 75% and maybe 20 miles or so a day, I'm currently seeing less than 1% battery degradation which means about 2 miles less max range... and honestly that might come back if I charge to 100% a couple times. I also occasionally will charge a couple times a day... come home from work, plug in and I'm done charging in 15 minutes but then I run out to the store. Come home and plug back in for another 4 kWh or something and then maybe run to a movie that night, etc. The battery health seems rock solid... though I probably should go for a long drive fairly soon to discharge down to 20% or so and then charge all the way back up just to keep the computers happy with their algorithms.)

 

[Rocky_H]

why not charge the few hours my refill takes at 12-16Amps/240V (less than my house capacity of 24Amps) to be gentle on battery for my typical daily cycle? Lower amps are less heat, right, and heat is one of the enemies of battery charging, correct?

This question is a bit ridiculous if you think about it. You're looking to nitpick about the difference between 6kW or 3kW power levels when the battery can take over 200kW. Dinking around with your home charging amps is insignificant and not noticeable to the battery. But it's a valid point someone made about how there is some fixed overhead energy consumption during the charging process, so you lose some efficiency if you charge slower and extend that time.

The only thing that could be a valid consideration of turning down amps is not related to the battery. Tesla has been kind of known for building their charging equipment at just barely handling the maximum current they are rated for and can get kind of hot running at max level. And a lot of frequent daily cycles getting hot/cold/hot/cold is kind of stressful on materials, solder joints, electronics, etc. So with the old wall connectors running at max 80A or the old mobile connectors at max 40A or the newer mobile connectors running at max 32A can get pretty warm. So not for the sake of the battery, but for probably extending the life of your charging equipment, it may be good to turn it down a few amps from maximum. I still have my original 1st generation mobile connector I have been using daily for over 6 years, but I keep the amps set at 31 instead of 40 just to keep it cooler and not wearing it down sooner.