# Is a slower charge beneficial for the long-term health of the batteries?



## Chessie (Mar 21, 2019)

I have a SR+ and the max inverter current is 32 amps, to charge at a rate of 32 miles per hour.

When charging, there is the option to decrease the max current from 32 to something less. Most nights that I charge, it is overnight, starting before 5 pm, and lasting until the morning, say 5:30.

For the maximum life of my batteries, would it behoove me to reduce the current to something like 20 amps, or 15 amps, when I'm not in a hurry?

Does anyone else do this?

thanks!


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## Zek (Oct 28, 2018)

If you want to get max battery life stay in the range between 40% and 60% and charge from 120v. But we wont know for sure for another 10 years. I decided to stick with 16amp 120v charging at home, because I supercharge on trips. I feel that super charging this summer accelerated my battery degradation. So shallow cycles and slower charging seams to be better, but people will argue about how much better. I think slower charging will get you 1-2% less degradation after 5 years.


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## garsh (Apr 4, 2016)

Chessie said:


> For the maximum life of my batteries, would it behoove me to reduce the current to something like 20 amps, or 15 amps, when I'm not in a hurry?


*No.* For any charging rate below about 180 amps (which means, AC charging, not supercharging), battery degradation is proportional to the _time_ spent charging, not the charging rate. So charging at a slower rate will actually be _worse_ for battery degradation.



JeffC said:


> Turns out charging too slowly is also bad, in general. Here is Professor Dahn's lecture on battery degradation:
> 
> 
> 
> ...


The good news is that the battery chemistry used by Tesla is very good at resisting degradation, so I don't expect the effect to be very noticable. Some people prefer to charge at a lower rate so that they can leave with a "warm" battery on a cold morning, and therefore have full regen available. So if you have some other reason for wanting to charge at a slower rate, I think it's fine to do so. Just realize that it's definitely not helping the battery.


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## garsh (Apr 4, 2016)

Zak said:


> If you want to get max battery life ... charge from 120v.


In addition to increasing charging times (and therefore degradation), charging from 120v is less efficient than charging from 240v.
Basically, the AC charger in the car requires a fixed amount of power to operate, regardless of the voltage or amps supplied.
I should dig around for some updated data for a Model 3, but here's what I was able to find back in 2017:


garsh said:


> I wasn't able to find numbers for a Tesla, but I found several reports about the charging overheads for the Nissan Leaf.
> 
> Level 1 (120v, 12 amps): 78% efficient
> Level 2 (240v, 16 amps): 91% efficient
> ...


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## Frully (Aug 30, 2018)

Chessie said:


> I have a SR+ and the max inverter current is 32 amps, to charge at a rate of 32 miles per hour.
> 
> When charging, there is the option to decrease the max current from 32 to something less. Most nights that I charge, it is overnight, starting before 5 pm, and lasting until the morning, say 5:30.
> 
> ...


Also keep in mind that 32A at 240v is more like 21A at 350v battery voltage...
SR has 54kwh pack (154 amp hour at 350 volts). 
Charging lithium batteries is commonly referred to in 'C'. Charging a 154 amp-hour battery at 154 amps would be 1C, and is generally considered safe (yes, yes, lots of nitpicking on different lithium chemistries and active cooling).
Charging at 21A is 21/154 = 0.14C, or a mere fraction of what the battery is capable of.
Going down from there is just making the charging process longer, which as @garsh says...is more damaging than the actual C rate itself.


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## tencate (Jan 11, 2018)

Just to add a long term datapoint to this discussion.

I spent my first year home charging with a 120V/20A circuit and occasionally using ChargePoints at work and Supercharging when on longer trips. 120V/20A isn't very efficient though  and it requires overnight charging to get about 60 or 70 miles of added range. 
The second year Max has been charged with an upgraded 240V/30A circuit at home and on REALLY cold nights, I _used_ to lower the charge current so the battery is charging all night, keeping warm---I had ZERO research or basis for doing that, but I never saw snowflakes or limited regeneration this way which pleased me. *But* I have since read that Lithium-based batteries actually don't MIND being cold so I've taken the advice given here and minimize my charging time overnight, setting the charging time so the car to finishes charging just before work. I now see limited regeneration and snowflakes again when it's cold (10-20F) but it's not too bad. I can live with it.
Otherwise my charging patterns are pretty random. I _do_ keep the car plugged in overnight in winter, no matter what my SOC is. Not so much in the summer. I sometimes charge up to 90% but mostly charge up to 80% overnight (for work and around town driving) and a trip to Albuquerque or beyond will run the battery down to 5% a few times on long trips, no problem. With all that, after 50k miles I show 316 miles of range when I fill it up before a long trip. That's a battery degradation of less than 5% in 50k miles. Kudos to Tesla!


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## JasonF (Oct 26, 2018)

You're all forgetting that the car slow charges the batteries _while you're driving_. Between that, nightly charging, and supercharging, you're getting a very good balance of charging methods to keep the batteries healthy. The only thing you can really do to mess that up is leave the car parked for long periods (weeks at a time) without being plugged in.


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## garsh (Apr 4, 2016)

JasonF said:


> You're all forgetting that the car slow charges the batteries _while you're driving_.


You mean, all this time, I've been driving a PERPETUAL MOTION MACHINE and never realized it?


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## Chessie (Mar 21, 2019)

I just wanted to follow up with all of the good and helpful comments and discussion and say thank you!

I will plan to charge at 32A whenever I am charging at home (which is 99.9% of my charging...) I will charge to 90% or less *most* times, and I will try to experiment with a "delayed" start, so that it is finishing about the time that I am needing to drive it in the morning.

Thanks all. The car (as we all know) is fantastic, and Tesla's updates are great, but having a forum to discuss things and ask questions is also very helpful (great).

Chessie


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## tesla m3 (Mar 28, 2019)

Funny how someone is worried about battery life enough to consider slow charging then says he plans to regularly charge to 90% or below. As if 90% is great for longevity.


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## JasonF (Oct 26, 2018)

tesla m3 said:


> Funny how someone is worried about battery life enough to consider slow charging then says he plans to regularly charge to 90% or below. As if 90% is great for longevity.


I always charge to 90%. It was recommended by the Tesla service center when I picked up my car. In their words, it's best for the battery and gives me enough for most emergencies.

I do wonder though if they recommend different charging levels for different regions. Nearly all of Florida can be covered - at least one way - by a 90% charge on a long-range model.


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## garsh (Apr 4, 2016)

tesla m3 said:


> Funny how someone is worried about battery life enough to consider slow charging then says he plans to regularly charge to 90% or below. As if 90% is great for longevity.


EVs are new for many people. You hear various things out in the wild. Luckily, we're here to learn and teach as much as we can.

That said, 90% isn't really that bad for modern Tesla battery chemistries.


__ https://twitter.com/i/web/status/1068745921079345152

__ https://twitter.com/i/web/status/1118225459416682496


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## garsh (Apr 4, 2016)

And compare that to what Elon was saying back in 2014.


__ https://twitter.com/i/web/status/448466037441179649


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## Chessie (Mar 21, 2019)

tesla m3 said:


> Funny how someone is worried about battery life enough to consider slow charging then says he plans to regularly charge to 90% or below. As if 90% is great for longevity.


I'm sure that you didn't mean this reply to sound sarcastic or smarmy..... but it implied that regular charging to 90% is harmful to battery longevity, without providing a better suggestion. Perhaps you'd like to suggest an alternative, like 85 or 80% ?

I've read where the bottom and top 10% should be avoided, in general, for the life of your batteries...

Thanks for the input...


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## AutopilotFan (Oct 6, 2018)

garsh said:


> You mean, all this time, I've been driving a PERPETUAL MOTION MACHINE and never realized it?


Don't we wish. Regen gives back like 30% of the energy it took to get the car to that speed. Much better than throwing it away as heat and rubbed-off brake pads!


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## KFORE (May 19, 2018)

There is no point in lowering the max amps. At 240v with 32amps, you're charging at 7.7 kW. Regen alone can push up to 70 kW at a time. Even at 32 amps, you're essentially slow charging. It just wastes more energy to charge at lower amps.


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## tesla m3 (Mar 28, 2019)

Chessie said:


> I'm sure that you didn't mean this reply to sound sarcastic or smarmy..... but it implied that regular charging to 90% is harmful to battery longevity, without providing a better suggestion. Perhaps you'd like to suggest an alternative, like 85 or 80% ?
> 
> I've read where the bottom and top 10% should be avoided, in general, for the life of your batteries...
> 
> Thanks for the input...


The problem is that too many people think 100% = bad, 90% = good. It's not like that at all. 100% is bad, but 90% is less bad compared to 100%, but that doesn't make it good. Over time, 90% SoC daily charging will absolutely have an impact on degradation. It won't be massive, likely not enough to push your battery under 70% for a warranty claim, but it will lose you range.

How bad it is, will be determined mostly by how much heat its exposed to, and how long the battery sits at a high SoC.

If you finish charging just before leaving, high SoC issues are mostly limited, especially if you arrive at your destination with 55% or less SoC so that heat doesn't have a massive impact on the cells.


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