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Old 01-12-2020, 10:49 PM
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Optimal EV charging


I am hoping this GQ rather than IMHO answerable-

Assume a car with 200 to 250 miles of range and daily usage of 40 to 50 miles.

The easy is what NOT to do for battery longevity. Donít use rapid DC charge unless you have to, certainly not often. Limit the amount of time near max state of charge (SOC). But is it better to charge a lesser amount more frequently, maxing at 60% SOC using about 20% of the capacity and replacing that 20% each day, or charging more fully less often, once every three or four days? Not sure if frequent small recharges has its own stress.

If one is better is it meaningfully so?
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Old 01-12-2020, 11:07 PM
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A lithium-ion battery is happiest at around 50% state of charge. If the range of the EV is 200 miles, and you are expecting to drive 50 miles, you should (ideally) charge it to 125, and at the end of the day you will be at 75.

Do that 4 times, and that is one battery discharge cycle (you’ve charged it to the capacity, ignoring any buffer at the top or bottom). Charging it from 0 to 200 would also count as one cycle, but battery degradation quickens when a battery is left at high or low states of charge for extended periods of time. This is exacerbated by heat.
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Old 01-12-2020, 11:09 PM
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Besides the electrical aspects (which you are apparently looking for), you also need to consider the behavioral aspects--in other words for me it would be better to get into the habit of plugging it in every night. While I will occasional forget, that won't be a problem because I still have plenty for the next day. But if I have to remember to plug it in when it gets down to a minimum amount, then it will certainly happen that I will forget to sometimes--resulting in problems the next day.
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Old 01-12-2020, 11:51 PM
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Originally Posted by aaelghat View Post
A lithium-ion battery is happiest at around 50% state of charge. If the range of the EV is 200 miles, and you are expecting to drive 50 miles, you should (ideally) charge it to 125, and at the end of the day you will be at 75.

Do that 4 times, and that is one battery discharge cycle (youíve charged it to the capacity, ignoring any buffer at the top or bottom). Charging it from 0 to 200 would also count as one cycle, but battery degradation quickens when a battery is left at high or low states of charge for extended periods of time. This is exacerbated by heat.
Interesting. So doing this will make the battery last longer than if you charge it each night to 200 miles, then get home at night and it is at 150 miles?

Any reason this is the way it is?
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Old 01-13-2020, 08:58 AM
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As far as I can tell, the recommendations for best battery life vary slightly for each car, so following the manufacturer's recommendations remain the best strategy. Some cars have a software limit on charging the battery, specifically in order to extend battery life. IIRC, the Volt did this, and I think the BMW i3 does this too. For these types of cars, there would be no benefit to charging to less than 100%, because the software already takes into account the desire to extend battery life.

But as a general rule of thumb, I would say that there's a consensus in EV-related media that charging to somewhere between 75-90 percent -- again subject to manufacturer recommendations -- and charging more frequently for less charge each time is the best way to go.

I am not a scientist or engineer, so I can't challenge aaelghat on the merits of his recommendation, but I have not seen any article in years in the EV-related media that recommended charging to 50%. As a frequent reader of that type of news, I am quite confident that the vast majority of articles and papers that I have seen recommend charging to 75-90%.

The couple of articles I have seen that have used a 50% charge recommendation are from those that I would consider EV critics from many years ago, and those articles go like this: "Your brand new LEAF has a range of 75 miles -- or does it? Well, if you charge more than 50%, you're going to break it. And then in winter the range goes down by 30%. And if your LEAF is more than a few years old, range goes down by another 20%. So in the real world, your LEAF can't even make it to work and back!!! Oh noes!!!"
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Old 01-13-2020, 09:52 AM
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Originally Posted by DSeid View Post
But is it better to charge a lesser amount more frequently, maxing at 60% SOC using about 20% of the capacity and replacing that 20% each day, or charging more fully less often, once every three or four days? Not sure if frequent small recharges has its own stress.

If one is better is it meaningfully so?
The answer is YES.

There's so much urban legends going around about lithium ion batteries but this question is one where the info is quite consistent and reliable.

That is:

If you charge and use 20% each day, charging between 40% and 60%, then your battery will last twice as long compared to charging 80% every four days (e.g. between 10% and 90%), everything else being equal.

Even though you charge four times as often, the battery will give you eight times as many cycles of 20% depth compared to 80% depth.

Cite: e.g. Battery University, How to Prolong Lithium-based Batteries (Battery University should absolutely not be taken as gospel but this particular info is consistent with all proper research I have seen).
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Old 01-13-2020, 10:08 AM
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Originally Posted by aaelghat View Post
A lithium-ion battery is happiest at around 50% state of charge. If the range of the EV is 200 miles, and you are expecting to drive 50 miles, you should (ideally) charge it to 125, and at the end of the day you will be at 75.
In something like a Tesla, there's not just one lithium-ion battery but hundreds. My guess is that if there is some optimal charging level for such a battery, the onboard software will manage the charge levels of each so it's optimized.
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Old 01-13-2020, 10:32 AM
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I don't think there is a GQ answer to this. It's not like an editor war, or something, where it is completely opinion based. There is science behind it, but even the experts in the field have different opinions, and not all lithium batteries are the same. Different battery chemistries, different controller software, etc. all might have different optimal charging and usage patterns. The instructions that were gospel 10 years ago may not apply today, what's best for a phone may not be best for a car, etc.

One thing I read, which unfortunately I can't find now, from I believe a Tesla engineer, said that the difference in battery degradation between charging to 75% daily and 90% daily was so small as to amount to 1% of charge loss in 300,000 miles, or something on that order. The bottom line being that there is no reason to charge to less than 90%, because it doesn't matter very much at all. Tesla's official recommendation is that a plugged in car is a happy car---charge as frequently as is convenient, but not to higher than 90%.

Anyway, none of that is terribly satisfying as a definitive statement, but every single definitive statement that somebody drops in a thread like this is contradicted by another definitive statement from somebody else. About the only thing that I see universally agreed upon is to not charge to 100% and hold the battery there for too long. But how long is "too long?" One hour, four hours, a day?
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Old 01-13-2020, 10:50 AM
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What is the shape of the curve of SOC to impact on degradation? Is it linear or does it more rapidly increase as one gets over some threshold? IOW is the difference between 70 and 80 less than the difference between 80 and 90? Is the negative impact the same sitting at 10% as charged to 90%?

If so does the fixed buffer that prevents overcharging keep out of that steeper portion?

Our EV can time the charging but ISTM that being able to specify to what level to charge would also be useful: e.g. be at 65% capacity at 8 am and charge no further.
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Old 01-13-2020, 12:12 PM
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It is true, that the particular battery chemistry changes the degradation curve of a lithium-ion battery, but degradation is highest at the extremes (100% and 0%).

Much of the degradation happens because of anode interactoin. If we take Tesla batteries as an example, Roadster and Model S batteries had a pure graphite anode in the 18650 cells, and model 3 batteries have a silicone/graphite anode.

During charge, lithium gravitates to the graphite anode (negative electrode) and the voltage potential changes. Removing the lithium again during discharge does not reset the battery fully. A film called solid electrolyte interface (SEI) consisting of lithium atoms forms on the surface of the anode. The SEI layer grows as the battery cycles. The film gets thicker and eventually forms a barrier that obstructs interaction with graphite.

This is basically one of the main causes of battery degradation.

The degradation curve is not linear. Degradation increase is higher going from 90% to 100%, than say from 80% to 90%. EV manufacturers put in a top buffer so that charging to 100% isn't really charging to 100%, as well as a bottom buffer so that you can't completely discharge the battery.

The reality though is that you have the EV for a reason, and the difference between charging between 70% and 90% may be just a few percent over several years.

Many manufacturers are starting to design their products to not fully charge. As an example, iPhones with IOS 13, will only charge your battery to 80%, until they think you are ready to leave, so the battery isn't sitting at 100% all night long.

https://www.howtogeek.com/423451/how...harging-to-80/

Last edited by aaelghat; 01-13-2020 at 12:13 PM. Reason: typo
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Old 01-13-2020, 12:25 PM
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Quote:
Originally Posted by aaelghat View Post
EV manufacturers put in a top buffer so that charging to 100% isn't really charging to 100%, as well as a bottom buffer so that you can't completely discharge the battery.
And this "buffer" can be quite large. The 1st gen Chevy Volt had a 16 kWh battery but only used 10.4 kWh of it. Tesla had (has?) some versions of the Model-S that had a software-limited battery capacity (you can pay Tesla for an unlock code to use full capacity).
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Old 01-13-2020, 03:14 PM
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In something like a Tesla, there's not just one lithium-ion battery but hundreds. My guess is that if there is some optimal charging level for such a battery, the onboard software will manage the charge levels of each so it's optimized.
This is important. Electric vehicles have many cells. When the car is charging, it doesn't send the power to every cell equally. When the car is consuming power from the cells, it doesn't discharge power from every cell equally. The charging software makes its own decisions about what is best. It's likely balancing cell lifetime vs instantaneous power.
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Old 01-13-2020, 03:26 PM
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Originally Posted by aaelghat View Post
...
Do that 4 times, and that is one battery discharge cycle (youíve charged it to the capacity, ignoring any buffer at the top or bottom). Charging it from 0 to 200 would also count as one cycle, but battery degradation quickens when a battery is left at high or low states of charge for extended periods of time. This is exacerbated by heat.
Battery charge cycles is more of a industry standard holdover to the former battery chemistries like NiCd and Nimh and apply a lot less then Li-ion types. Things like time at depth of discharge and temperature are much bigger factors.
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Old 01-13-2020, 03:31 PM
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For a Tesla model 3, we were told to set the charge limit to 80% to 90% (We use 80%) unless it's a rare occasion where you are going on a long trip. Also recommended to avoid discharge below 20% as a habit. Keep in mind, they Tesla that had its battery pack replaced at 300,000 miles was used as a taxi between LA and Vegas and supercharged often, daily, several times a day. Some people are pushing 100,000 miles plus (or a lot more) on Model 3's with no reports of significant range loss.

One issue with supercharging (or high power charging) is heat generated. Tesla uses metal-clad batteries like AA batteries immersed in coolant to limit heat/expansion damage, while apparently early Leafs did not use hard casings or coolant. Some Leafs allegedly lost up to 50% of range due to damage from thermal expansion. You can also see YouTube videos of people driving Tesla on the Autobahns at over 220km/h - funny, the computer senses battery overheating and slows the car to 90kph (55mph) to let the batteries cool for a while - then off they go again. The computer supposedly does a good job of limiting problems with batteries.

I suppose if you left your EV charged at 50% and never used it, it would be better off. But that's not the point of a battery.

Last edited by md2000; 01-13-2020 at 03:31 PM.
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Old 01-14-2020, 01:29 PM
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This is important. Electric vehicles have many cells. When the car is charging, it doesn't send the power to every cell equally. When the car is consuming power from the cells, it doesn't discharge power from every cell equally. The charging software makes its own decisions about what is best. It's likely balancing cell lifetime vs instantaneous power.
I just skimmed through this and I don't see any mention of a system that manages each cell and selectively charge or discharge them. There are balancing circuits to equalize cells with different voltages, but that seems to be about it.
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Old 01-15-2020, 09:54 AM
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I just skimmed through this and I don't see any mention of a system that manages each cell and selectively charge or discharge them. There are balancing circuits to equalize cells with different voltages, but that seems to be about it.
Sorry, I guess I wasn't clear enough. I don't think any system manages each cell separately. But large batteries do often manage subsets of cells separately.
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