Why is charging to 100% a bad thing?

[Cobra427]

The Mach E we have on order (Premium AWD SR) will be our first BEV, so we are trying to learn what it will be like to live with a beast of a different breed.
The owner's manual and many posts on this forum warn against charging to 100%. Got that, but why? Back in the Dark Ages of rechargeable batteries, warnings abounded about the Ni-Cad battery's so-called "memory", where if you didn't charge them fully, they wouldn't charge fully.

But now almost all rechargeables are Li-Ion, like in all our 'devices' and my tactical flashlights. I never have heard or received a warning not to charge my iPhone to 100%. So why all the warnings about the MME? Same technology, yes?

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[JohnnyForensic]

The Mach E we have on order (Premium AWD SR) will be our first BEV, so we are trying to learn what it will be like to live with a beast of a different breed.
The owner's manual and many posts on this forum warn against charging to 100%. Got that, but why? Back in the Dark Ages of rechargeable batteries, warnings abounded about the Ni-Cad battery's so-called "memory", where if you didn't charge them fully, they wouldn't charge fully.

But now almost all rechargeables are Li-Ion, like in all our 'devices' and my tactical flashlights. I never have heard or received a warning not to charge my iPhone to 100%. So why all the warnings about the MME? Same technology, yes?

There are a lot of threads here on this, but basically Lithium Ion batteries like to live as close to 50% charged as they can for maximum longevity. When living at the ends of their capacities (one way or the other), they will degrade faster.

With the buffer in the battery of about 10%, charging to 90% (as recommended) is basically like charging to 80%, which is the generally accepted "safe top end" for maximum battery life over time. That said, Ford has said they fully expect people to charge often to 100% and that it's fine to do so (because you're still really only charging to 90%), but you don't want to charge to 100% and keep it there for extended periods of time, just like you don't want to run it dry and keep it that way for long either. If you're storing your Mach E for an extended period, you want the battery at about 50% before you do.

Similarly, if you charged your iPhone only to 80% every time, you'd have a much longer battery lifetime than charging to 100%, which is why modern iPhones use adaptive charging and will charge to 80% and then wait until it knows you're going to get your day started, and then charges the last 20% to coincide roughly with when you're going to use it--so you're not sitting at 100% for hours and hours overnight, every night. Also, larger battery arrays degrade slower since there are more batteries to take up the brunt of it. A smaller iPhone battery doesn't have much luxury for keeping buffers at both ends, so they have to use everything they have. Also, a phone's expected lifetime is two years, whereas a car's lifetime is much longer. This also allows Apple more freedom to treat the battery a little more harshly in the name of performance than Ford can with a car.

At the end of the day, however, these cars are meant to be used, so charge to 90% daily, and feel free to charge to 100% anytime you expect to have an extended driving day or at the start of a long road trip. It's fine.

 

[ARK]

Lithium ion batteries are happiest with a midrange charge. But the most concern is when the battery is between 0-20% and 80-100%. The Mach-e recommends no more than 90% charge on a daily basis unless you need it for this reason (Ford doesn’t say 80% because Ford already put in a bit of a hidden buffer, as pointed out).

This actually is also true for your cell phone, and one reason why the battery starts to go after a couple of years (numerous charge cycles also affect this).

Fast charging also isn’t great for the battery if you do it all the time, but since L3 charging tends to cost a lot more than L2 charging, most people stick with L2 charging at home anyway instead of going to a commercial L3 charger on a weekly basis.

 

[dtbaker61]

So why all the warnings about the MME? Same technology, yes?

there are a couple considerations people haven't mentioned above.

1. it would not be good for the battery to be 100%, and then have temperature drop; cold reduces the capacity, and my understanding is that if the molecular lattice is 'full', and you shrink the lattice with cold.... it's not good. But, I wouldn't worry about this one as long as the battery pack enviromental controls work right and keep the pack above freezing

2. The major reason EVs are different that anything else with a battery is that you want the 'headroom' available for regenerative braking to put some energy IN as you brake or go down a long hill. If the battery is already full, the regen can't kick in. I wouldn't worry about this unless you live a the top of a big hill and want to use 'L' or 1-p driving to slow your decent.

 

[Cobra427]

there are a couple considerations people haven't mentioned above.

1. it would not be good for the battery to be 100%, and then have temperature drop; cold reduces the capacity, and my understanding is that if the molecular lattice is 'full', and you shrink the lattice with cold.... it's not good. But, I wouldn't worry about this one as long as the battery pack enviromental controls work right and keep the pack above freezing

2. The major reason EVs are different that anything else with a battery is that you want the 'headroom' available for regenerative braking to put some energy IN as you brake or go down a long hill. If the battery is already full, the regen can't kick in. I wouldn't worry about this unless you live a the top of a big hill and want to use 'L' or 1-p driving to slow your decent.

OK, now this is clear and understandable, especially point #2. Headroom, never considered that.

Still, wondering about what "like to live as close to 50% charged" and "are happiest with a midrange charge" actually mean. Not disputing this but what about the battery construction or chemistry makes this true? Thanks in advance.

 

[RockwallRick]

My Wife always wants it charged to 100% but we don’t do it on a daily basis, more like once per week.

 

[dtbaker61]

OK, now this is clear and understandable, especially point #2. Headroom, never considered that.

Still, wondering about what "like to live as close to 50% charged" and "are happiest with a midrange charge" actually mean. Not disputing this but what about the battery construction or chemistry makes this true? Thanks in advance.

Lithium cells are more likely to be damaged by high rate of charge/discharge than state of charge in the range of 20%-80%. As the amps going in or out climb above 'C' rate determined by capacity of a given cell or parallel set of cells, you run the risk of growing what is called dendrites withint the cells which carry the anode material to the cathode as the electrons move through the lithium sort of like stalagmites. This is why High amp DC DC charging is not necessarily a good thing for the battery pack, and level 2 at something less than 60amps is preferable for longer life.

Below 20% or above 80% you run the risk of one or more cells being slightly out of balance with the rest and going into an over voltage for under voltage condition if they are not well-balanced. It t is best to operate within that range for the safety of all the cell's unless you have total trust in the battery management system keep things balanced at the bottom and the top at the end of charge to avoid irrecoverable damage if a cell hits 0 under load, or overcharges into thermal runaway and bursts or vents.

 

[ARK]

OK, now this is clear and understandable, especially point #2. Headroom, never considered that.

Still, wondering about what "like to live as close to 50% charged" and "are happiest with a midrange charge" actually mean. Not disputing this but what about the battery construction or chemistry makes this true? Thanks in advance.

This article jumps into the technical side of lithium ion battery degradation. Figure 6 is relevant to this discussion, where they tested lithium ion battery capacity after using the same type of battery repeatedly with a 100-25% charge cycle, 100-40% charge cycle, 85-25% charge cycle, etc.

You can see how the battery will degrade with use no matter what, but that certain charge cycles are more damaging, and how it makes much less of a difference once you are avoiding the extremes (i.e. by not regularly charging to 100% if you don’t need it).

 

[timbop]

There are a lot of threads here on this, but basically Lithium Ion batteries like to live as close to 50% charged as they can for maximum longevity. When living at the ends of their capacities (one way or the other), they will degrade faster.

With the buffer in the battery of about 10%, charging to 90% (as recommended) is basically like charging to 80%, which is the generally accepted "safe top end" for maximum battery life over time. That said, Ford has said they fully expect people to charge often to 100% and that it's fine to do so (because you're still really only charging to 90%), but you don't want to charge to 100% and keep it there for extended periods of time, just like you don't want to run it dry and keep it that way for long either. If you're storing your Mach E for an extended period, you want the battery at about 50% before you do.

Similarly, if you charged your iPhone only to 80% every time, you'd have a much longer battery lifetime than charging to 100%, which is why modern iPhones use adaptive charging and will charge to 80% and then wait until it knows you're going to get your day started, and then charges the last 20% to coincide roughly with when you're going to use it--so you're not sitting at 100% for hours and hours overnight, every night. Also, larger battery arrays degrade slower since there are more batteries to take up the brunt of it. A smaller iPhone battery doesn't have much luxury for keeping buffers at both ends, so they have to use everything they have. Also, a phone's expected lifetime is two years, whereas a car's lifetime is much longer. This also allows Apple more freedom to treat the battery a little more harshly in the name of performance than Ford can with a car.

At the end of the day, however, these cars are meant to be used, so charge to 90% daily, and feel free to charge to 100% anytime you expect to have an extended driving day or at the start of a long road trip. It's fine.

Couldn't say it better myself

OK, now this is clear and understandable, especially point #2. Headroom, never considered that.

Still, wondering about what "like to live as close to 50% charged" and "are happiest with a midrange charge" actually mean. Not disputing this but what about the battery construction or chemistry makes this true? Thanks in advance.

When leaving the car idle for long periods (greater than a week) it is best to leave the car at 50% state of charge - as mentioned leaving it charged too low or too high is bad. So, exactly in the middle is farthest from the worst states of charge at either end

 

[Cobra427]

Thanks to all who replied, especially ARK, for providing the link to Battery University. Very good reference.
One thing that was enlightening was that the Li-ion battery could not function long without Battery Management System software. Whoever authors this software gets to decide the balance between safety, max charge, rate of charge, and battery life.
Once again, it comes down to software. Good software equals good user experience, bad software...

 

[benk016]

Just as a data point, I charge my car to 90% every day.

Anytime I plan on going out of town, about 2 hours before I plan on leaving, I set the car to then charge to 100% so that its either almost at full, or just reached it by the time I am leaving.

I just don't want to charge it to 100% and then let it sit. Occasional 100% charges are fine though.

 

[AZBill]

Couldn't say it better myself


When leaving the car idle for long periods (greater than a week) it is best to leave the car at 50% state of charge - as mentioned leaving it charged too low or too high is bad. So, exactly in the middle is farthest from the worst states of charge at either end

This is the technique I use when going to the airport on a trip, I want to arrive at the airport with 50-60% charge.

I fly RC planes and can tell you that if you charge a lithium-polymer battery to 100% and leave it in that state for 1 week, it is very likely to puff up and damage the battery. That is slightly different chemistry and there is not normally a built-in BMS, but I have seen it happen. I have some newer Spektrum "Smart" RC batteries that actually have a built-in BMS and will automatically discharge to 50% after X hours of non-use. I set the timer (X) to 48 hours. I charge the battery to 100% the night before flying, and if for some reason I do not fly the next day, the battery discharges to 50% on the second day.

 

[agoldman]

I always charge at home to 100. No ill effects yet. I read somehwre that Ford designed it to go to 100 safely. May be long term issues, but I don't keep cars very long.

 

[RockwallRick]

I always charge at home to 100. No ill effects yet. I read somehwre that Ford designed it to go to 100 safely. May be long term issues, but I don't keep cars very long.

They built in a buffer because they know that most people will charge to 100%.

 

[Timelessblur]

I have my car set to Fords recommended 90% max charge for daily use. I tend to let the battery drain until like 65-70% then charge it back up to 90%. I will charge it to 100% the night before I plan on doing a long distance drive so it is fully charged in the morning ready to go.
For example Friday I am driving from Austin to Fort Worth. I will be telling the car when I go to bed on Thursday to charge to 100%. Any hotel I stay out while in DFW I will let it charge to 100% as it should be again a 1-2 night only thing in preparing for my drive back home.
I did the same thing when I drove to Houston. 100% the battery the night before got to the hotel at a lower state of charge and 100% it there. I got back to my house in Austin with a 10% charge.

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