Why shouldn't I charge my 64 kWh Kona EV to 100% every day?

[electriceddy]

Charging to 100% on occasion(say once a month) will give the cells a top end balancing which is good but I wouldn't do it all the time. I think it is more important for long life duration of the pack not to leave it at a high SOC for long (days) and the more importantly for low SOC .

 

[brulaz]

...
I think it is more important for long life duration of the pack not to leave it at a high SOC for long (days) and the more importantly for low SOC .

I too have heard that its best for Li batteries to be maintained at mid-level (20-80%) SOC.
But Hyundai says otherwise as quoted earlier:

Now in the Kona Electric owners manual, I do see the following paragraph in the "Electric Vehicle System Overview" section under "High Voltage Battery (lithium-ion polymer)":

"If the vehicle will not be used for an extended amount of time, it is recommended to fully charge the vehicle to 100% before storing, and then charge the vehicle periodically (approximately every 3 months) to prevent the EV battery from dis- charging completely."

So I guess that what Hyundai reports to the user as 100% SOC is quite a bit less than what is actually the true battery 100% SOC. And their BMS (battery management system) prolly takes care of a lot of issues internally that home-brew Li battery pack builders have to do manually (like cell balancing and over-charging).

 

[Pushmi-Pullyu]

Where does this "best practice" come from?

My understanding is it originated with the home energy storage market. That's been the rule of thumb for quite a few years now. An 80%/ 20% charging strategy (charge to 80% of full capacity, discharge to 20% before recharging) has been found to produce the best return on investment, long-term. Use more of the full capacity and the batteries wear out too fast; use less capacity and you have to spend more money than necessary buying more battery cells. 80%/20% has been found to be the "sweet spot".

I would have thought EV manufacturers would build that into their charging schemes.

Yeah, I would have too. Here's one example of why that's not the industry standard it ought to be: The Leaf originally had an option to charge to 80%, but that was eliminated in later years because elimination of that option gave it a higher EPA range rating, without Nissan having to increase the capacity. Crazy, innit?

To say that an EV has a certain max range (but you shoudn't use it every day by charging to 100%) is rather deceptive.

Well, sadly we don't live in a perfect world. As it is, I recommend that anyone buying a BEV should buy one with 40% more range than they think they'll need for their daily commute. That is a concept that I think all EV advocates should be spreading, but unfortunately aren't. A 40% margin in range/battery capacity should allow sufficient margin for the range reduction in bitterly cold weather, as well as a bit of loss of capacity over time as the years pass. That 40% extra range will also be more than enough margin for everyday charging to only 80%.

Those who live in Mediterranean climates, such as coastal California or Florida, may be able to get away with, let's say, only 30% more range than they think they'll need. They have little need to worry about significant loss of range due to sub-freezing weather.

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[Pushmi-Pullyu]

Ya, this battery stuff reeks of wive's tales and urban legends. Just charge it and forget about it.

Some people have to learn things the hard way.

Just like your teeth. No need to brush or floss them every day. Just ignore them, and they'll go away!

 

[Pushmi-Pullyu]

I read the manual and to me it seems to suggest charging past 80% is worse when charging via DC Fast than AC. I charge level 1 AC most of the time and plan to charge to 100%. But if I use DC Fast on a longer trip, I plan to only charge to 80% (and I set the limit at that in the car settings).

Experienced BEV drivers only charge to 80% during a DC fast charging session, but more because it takes an excessive time to charge that last 20% (from 80% to 100%), not because of the additional strain on the battery pack. Battery packs charge fastest when they're nearly empty of charge and the charge rate tapers off to a slower and slower rate as the pack approaches 100% charge.

You're better off leaving the DCFC session with an 80% charge, and if you need more, making another charging stop further down the road. You'll get to your destination faster that way.

It's true that DC fast charging does put more strain on the pack than slow charging, but unless you're using DC fast charging quite frequently, that shouldn't have a significant impact. Besides, it can reasonably be argued that the additional strain isn't that great, because the Tesloop company charges their Tesla rental cars at DC fast chargers every day, usually (altho not always) charging to 100%, and their battery packs haven't degraded all that much worse than the average Tesla car.

(Caveat: If you own a Leaf, then using DC fast charging more than once during a day probably does have a significant impact on battery life. The lesson there is if you get a Leaf, just lease it; or if you decide to actually buy a Leaf, against good advice, then never use a DC fast charger more than once before parking the car and letting the battery pack cool down for several hours.)

Besides, the battery pack has a lifetime warranty, right?

Unlikely. It's generally just an 8 year warranty, and it's also limited to a maximum mileage. I don't think any auto maker offers a "lifetime" or unlimited mileage warranty for a BEV battery pack.

I mean "worst case" scenario you get earlier degradation of the charging capacity .. say at 8 years instead of 10 or 12 or 15. Well... then the warranty kicks in.

No, the worst case scenario would be the pack degrading to 71% of capacity in 8 years, just barely above what most BEV makers use as the cutoff point (70%) below which they'll replace the battery pack. Fortunately, unless you drive a Nissan Leaf and you keep it for 8 years or longer, that's very unlikely to happen to you.

EV-haters used to claim that EV owners would have to replace their battery packs every few years. But in practice, we see that this simply isn't true. Unless you buy a Leaf, your battery pack should last the lifetime of the car.

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[Pushmi-Pullyu]

"If the vehicle [Kona] will not be used for an extended amount of time, it is recommended to fully charge the vehicle to 100% before storing, and then charge the vehicle periodically (approximately every 3 months) to prevent the EV battery from dis- charging completely."

This is the first time I've heard of actually recommending to charge to 100%. Even though its only talking about storage of the car and not the normal driving of it.

Indeed. Letting a battery pack sit at 100% charge for an extended time -- longer than 24 hours -- is one of the worst things you can do for battery life in a li-ion battery pack, so I'm amazed that any BEV manual would recommend doing that.

What is reported elsewhere is that for long-term storage, you should charge to 60% and disconnect the starter battery, and charge periodically as necessary (perhaps every 3 months?) back to 60%, to keep the battery from draining too close to 0%.

I don't know why the Kona would be any different, altho I would hesitate to ignore what the manual recommends. If my understanding is correct, most BEVs don't use li-ion polymer batteries, so perhaps those are more tolerant of sitting for a long period at 100% charge?

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[Pushmi-Pullyu]

Wonder if that "extra" range is beyond what they advertise. But I doubt it. The model 3's range per battery kWh are higher than anybody else's and maybe pushing the batteries limits is one way they do it (in addition to their fine engineering of course).

Tesla now advertises the official EPA range, which according to most reports is a pretty good average of what you'd get in daily driving in mixed conditions; some highway, some city driving.

However, in practice, generally the only time you'll be concerned with range if you're driving a Tesla car, or any other 200+ mile BEV, is when you're taking a long trip, which means almost all highway driving. So don't expect your BEV to get its full EPA rated range when you're driving on the freeway at 70-80 MPH. The EPA's highway tests average only 55-60 MPH, so you'll almost certainly get less range at the speed most people drive on the freeway.

For example, the Tesla Model S85 had an EPA range of 265 miles on a 100% charge, but in normal freeway driving, an average driver would get only about 220 miles, and less than that on a cold day when running the cabin heater.

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[Pushmi-Pullyu]

The model S and X definitely have some extra capacity. Remember when the hurricanes stuck Florida and Tesla unlocked extra range in the cars for people to get out?

That was only for cars sold as the "Model S60" or "Model X60" which actually had (nominally) 75 kWh battery packs, but electronically limited to using 60 kWh. That was a limited time marketing promotion used by Tesla to boost sales. Unlocking the extra capacity in the aftermath of a hurricane, for that limited number of cars, was great publicity for Tesla, but it won't apply to the Model 3.

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[Pushmi-Pullyu]

So the Kona reduces the DC charging rate at high SOC, presumably to preserve the battery.

All BEVs taper off the charging rate as the battery pack approaches full charge. Charging efficiency is lost as the cells approach full charge; that's just a basic limitation of electrochemical batteries. Losing efficiency means the amount of waste heat generated by charging increases, which requires the charging to be further slowed to avoid overheating the battery. It's a vicious cycle.

The only way to avoid that is simply to stop charging before you reach 100% charge. That's why experienced BEV drivers usually stop at 80% charge when using a DC fast charger; it takes too long for that last 20%, and it's better to stop for another charge further down the road. You lose less time that way.

BTW this is yet another reason why BEVs benefit from bigger battery packs, and why a smart car buyer will buy a BEV with a substantially longer range than he needs for his daily commute. (I personally recommend 40% over that minimum.) Those who say it would be better for BEVs to have just the bare minimum capacity they need for daily charging... well, they're wrong. Period.

 

[Pushmi-Pullyu]

Don't some DC fast chargers cut you off at some point if you're charging too slow?

Not according to anything I've ever read, no. But some DC fast chargers are programmed to end a session after a certain amount of time, even if the car isn't fully charged; perhaps that's what you're thinking of?

I recall a comment from someone who said that at one particular DC fast charger he had developed a practice of returning to his car before it was fully charged, unplugging it, and plugging it back in to start a 2nd charging session, all to avoid having the charger cut off before fully charging his car. But that wasn't a Tesla Supercharger, it was some other brand of DC fast charger.

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[Pushmi-Pullyu]

Charging to 100% on occasion(say once a month) will give the cells a top end balancing which is good but I wouldn't do it all the time. I think it is more important for long life duration of the pack not to leave it at a high SOC for long (days) and the more importantly for low SOC .

I've seen a lot of argument over whether or not that's actually beneficial to battery life. But even if you do that, it shouldn't be done more than once every 3 months. No need to do that every month just for a supposed benefit of "balancing the cells" or "recalibrating the BMS".

And if you do charge to 100%, make sure you drive the car soon after. You really don't want to leave your BEV sitting at 100% state of charge for 24 hours or more; that's very bad for battery life.

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[Pushmi-Pullyu]

I too have heard that its best for Li batteries to be maintained at mid-level (20-80%) SOC.
But Hyundai says otherwise as quoted earlier:

So I guess that what Hyundai reports to the user as 100% SOC is quite a bit less than what is actually the true battery 100% SOC. And their BMS (battery management system) prolly takes care of a lot of issues internally that home-brew Li battery pack builders have to do manually (like cell balancing and over-charging).

I suppose that's possible, but generally speaking, only PHEVs, with their significantly smaller capacity battery packs, reserve a significant portion of the pack's full capacity to prevent charging to nearly full. If my understanding is correct, BEVs typically use somewhere around 92-95% of full battery cell capacity.

It's true that if you charge what the car reports as "100% capacity" it's not really 100% of what the battery cell manufacturer lists as full name-plate capacity, but that doesn't mean it's "okay" to charge the typical BEV to 100% of usable capacity on a daily basis. It's not a binary, either/or situation. It's not the case that you put strain on the battery only if charged to 100%, and don't strain it if charged only to 90-95%. The reality is that li-ion batteries "like" to be kept at 50% state of charge, and the further away from that happy medium, the more strain is put on them.

Even if the Kona is different for some reason, then that applies to only the Kona; it doesn't apply to BEVs in general.

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[Pushmi-Pullyu]

Now in the Kona Electric owners manual, I do see the following paragraph in the "Electric Vehicle System Overview" section under "High Voltage Battery (lithium-ion polymer)":

"If the vehicle will not be used for an extended amount of time, it is recommended to fully charge the vehicle to 100% before storing, and then charge the vehicle periodically (approximately every 3 months) to prevent the EV battery from dis- charging completely."

Now, wait: Are you sure you are quoting from the Kona Electric (BEV) manual, and not the Kona PHEV manual?

What you quoted would make more sense, at least to me, if it was for a PHEV rather than a BEV.

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[ajstjacques]

Now, wait: Are you sure you are quoting from the Kona Electric (BEV) manual, and not the Kona PHEV manual?

What you quoted would make more sense, at least to me, if it was for a PHEV rather than a BEV.

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I can find this in both my French and English owners Manuel for my Kona EV.

 

[brulaz]

So P-P, even if you're charging a Kona with a level 1 (or 2) charger, you wouldn't let it charge up to "100%" as shown on the dash?

 

[Pushmi-Pullyu]

So P-P, even if you're charging a Kona with a level 1 (or 2) charger, you wouldn't let it charge up to "100%" as shown on the dash?

We're talking about the Kona Electric, the BEV, and not the Kona PHEV, right?

Not unless I was planning on an extended trip that day, no. There's no good reason to stress the batteries by charging to 100% (of usable capacity) on a daily basis. Why would you want to do that? That would be as foolish as red-lining your sports car's engine every time you took it out for a drive. Do that very often, and it won't last long!

Now, if you're planning on an extended trip that day, or you're not sure if you'll have sufficient range because you're taking a side trip or otherwise exceeding your normal daily range, then sure, go ahead and charge to 100%. But if you're doing that on a daily basis, then you're abusing the battery pack. BEVs are not built for that.

Caveat: As I said in an earlier post, it's possible that for some odd reason, the Kona doesn't follow the normal rules for BEVs. It's possible I'm committing the fallacy of applying a general rule to a specific case. But without knowing a lot more about how the Kona is engineered and built, I wouldn't assume that somehow its battery cells are immune to the same limits of physics and chemistry which affect every other BEV's battery cells.

 

[brulaz]

Yes the BEV.
How do we know the Kona's reported 100% SOC is not actually 80% of the real usable capacity?

 

[Wildeyed]

I've seen a lot of argument over whether or not that's actually beneficial to battery life. But even if you do that, it shouldn't be done more than once every 3 months. No need to do that every month just for a supposed benefit of "balancing the cells" or "recalibrating the BMS".

And if you do charge to 100%, make sure you drive the car soon after. You really don't want to leave your BEV sitting at 100% state of charge for 24 hours or more; that's very bad for battery life.

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I've really got to ask for citations at this point. Reapeating the same advice as though it was gospel does not make it true. Why would a manufacturer with warranties and reputation on the line not recommend the 80/20 gospel if it were true?

 

[Wildeyed]

Yes the BEV.
How do we know the Kona's reported 100% SOC is not actually 80% of the real usable capacity?

We're talking about the Kona Electric, the BEV, and not the Kona PHEV, right?

Not unless I was planning on an extended trip that day, no. There's no good reason to stress the batteries by charging to 100% (of usable capacity) on a daily basis. Why would you want to do that? That would be as foolish as red-lining your sports car's engine every time you took it out for a drive. Do that very often, and it won't last long!

Now, if you're planning on an extended trip that day, or you're not sure if you'll have sufficient range because you're taking a side trip or otherwise exceeding your normal daily range, then sure, go ahead and charge to 100%. But if you're doing that on a daily basis, then you're abusing the battery pack. BEVs are not built for that.

Caveat: As I said in an earlier post, it's possible that for some odd reason, the Kona doesn't follow the normal rules for BEVs. It's possible I'm committing the fallacy of applying a general rule to a specific case. But without knowing a lot more about how the Kona is engineered and built, I wouldn't assume that somehow its battery cells are immune to the same limits of physics and chemistry which affect every other BEV's battery cells.

There is no Kona PHEV.

 

[Esprit1st]

I recall a comment from someone who said that at one particular DC fast charger he had developed a practice of returning to his car before it was fully charged, unplugging it, and plugging it back in to start a 2nd charging session, all to avoid having the charger cut off before fully charging his car. But that wasn't a Tesla Supercharger, it was some other brand of DC fast charger.

I stopped at a EVGO DCFS that cut me off after 45 minutes. I restarted another charging session because I needed the extra range. (However I'm not the person with that habit, just had that experience)

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