Battery Cycles

[TruckDaddy]

When they say a battery pack will last 2k, 3k or 4,000 cycles, does that mean each time you plug in and charge for even 10 miles counts as a cycle?

What about a solar panel on the roof? Does each day count as a cycle and contribute to degradation more than it would help?

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[ajdelange]

I am sure there are standardized definitions but I have not seen them. The capacity of a battery is rated in terms of the number of coulomb or ampere hours (1 ampere hour is 3600 coulombs) it takes to charge it from empty to full and is symbolized by C. As you are probably well aware the definitions of "empty" and "full" are two somewhat arbitrarily chosen open circuit voltages which are temperature and chemistry dependent. To me a "cycle" is 2C round trip coulombs i.e. enough to charge from empty to full and then discharge back to empty. Thus I would consider charging from 20% to 70% and then discharging back to 20% to be half a cycle because it is 1C round trip charge. A battery scientist writing a paper about his work will have a more detailed and precise definition of what he means by a "cycle". So no, I don't think replacing phantom loss of a few percent each day costs you a whole cycle.

With solar cells I believe it is just chronological age that determines the degradation.

 

[Crissa]

Well, for solar cells it's hours exposed to sunlight.

hours divided by intensity would be a decent measure for solar cells.

-Crissa

 

[Newton]

Most cycle charts show something like this... this isn't tesla chart or even lithium

The c rating stuff AJ was talking about confuses me, but alot of charts use that instead. But as u can see life is based on how far you discharge usually.

Example as I know it.
Remember this isn't a tesla chart, I have no idea about their battery stufff.
But using this chart if you use 30% of you battery a day(which would be 90 miles for a 300mile range car) you can recharge 6000 times or about 16 years going 90 miles every single day...
But if you use 50% of the battery, 150 miles a day, you can only recharge about 3000 times or about 8 years.

 

[TruckDaddy]

So, it doesn't really matter if you keep it daily "topped" up at ideally 80% like the Prius does with its ICE. It's all about maintaining the middle range.

 

[TruckDaddy]

Cell phone batteries would last much longer if they limited charging to 90%?

 

[Jhodgesatmb]

Most cycle charts show something like this... this isn't tesla chart or even lithium

The c rating stuff AJ was talking about confuses me, but alot of charts use that instead. But as u can see life is based on how far you discharge usually.

Example as I know it.
Remember this isn't a tesla chart, I have no idea about their battery stufff.
But using this chart if you use 30% of you battery a day(which would be 90 miles for a 300mile range car) you can recharge 6000 times or about 16 years going 90 miles every single day...
But if you use 50% of the battery, 150 miles a day, you can only recharge about 3000 times or about 8 years.

How does this work with Tesla wanting us to keep the car plugged in? We go from 250 down to 150 in our Model 3 pretty frequently and I worry all the time about the lifetime of our batteries.

 

[ajdelange]

Most cycle charts show something like this... this isn't tesla chart or even lithium

The c rating stuff AJ was talking about confuses me, but alot of charts use that instead. But as u can see life is based on how far you discharge usually.

You aren't alone in that. Let's look at your chart a little further. It says you can charge and discharge it 1000 times from full to empty. C is the capacity. The chart says you can start with it empty, add C ampere hours and then take them all back out. That's 2C round trip ampere hours per charge and thus the life expectancy of the battery is 2000C. Now suppose you charge it to 100 percent and then only discharge it to 80% before recharging it to 100% and so on. In each such cycle you put in 0.2C and take out 0.2C for a total of 0.4C round trip ampere hours. The chart says you can do that 8500 times for a total round trip lifetime of 0.4*8500 = 3400C. Thus you can substantially increase the total amount of current you can run through this battery by more shallow discharge. But suppose you ran 0.4C cycles between say 45% and 65%. What would the lifetime capacity be then? This curve does not tell us that. Since we know we ought to stay away from 100% SoC in the current Lithium batteries it is possible that we might get even more round trip capacity. So unless we have a much more extensive set of curves we can't really do better than to say that this battery has a lifetime capacity of about 2700C.

 

[ajdelange]

How does this work with Tesla wanting us to keep the car plugged in?

As I said in my first post the daily replacement of phantom drain represents very shallow discharge so, in the first place, each 0.06C cycle takes very little from the total round trip capacity and, in the second, as the cycles are shallow the round trip capacity is high.

We go from 250 down to 150 in our Model 3 pretty frequently and I worry all the time about the lifetime of our batteries.

People do obsess over this. Get Tesla Stats or TeslaFi and it will keep track of the degradation for you. The best chance of achieving long life/minimum degradation given the available information to us is to follow the advice Tesla, who has tons of data, gives in its manuals:

1)Keep it plugged in when at home
2)Don't charge above 90% except as necessary
3)Avoid SuperChargers to the extent possible
4)Approach super chargers by navigating to them with the car's navigation system rather than just driving to them. The car will precondition the battery's temperature for optimum charging if you do that.

I interpret 2 and 3 to mean that one should stay away from the high SoC charge region and so set my charge threshold to 70%. This is more than enough for most of my daily driving and, of course, if I need more for a road trip I simply put it in. I wish I could tell you that this is a great success and the way to go but the car hasn't got that much mileage on it yet. Seven months in its capacity is what it was the day I picked it up. It is also a random thing.

 

[ajdelange]

It's probably worth mentioning that we don't really care about the lifetime of the battery as it will, for 99.99% of users, exceed the time the owner has the car. What we really obsess over is the rate of degradation. During discharge of the battery lithium ions are released by the anode and migrate towards the cathode. But not 100% reach the cathode. A tiny (fortunately) percentage react irreversibly with the SEI and are lost to the lithium pool which can store charge. During charge lithium ions migrate from the cathode to the anode where, again, 100% do not make it. One of the goals of battery design is to make sure that the percentage of lithium ions reaching the electrodes has lots of 9's in it to the right of the decimal point. For example, if 99.99% of lithium stays in the pool after a cycle then the pool contains 90.4% of the original amount after 1000 cycles. If retention is 99.999% then 99.9% remains after 1000 cycles and 90.4% after 10,000. Battery "life" is defined in terms of some level of remaining charge capacity. Clearly that depends on the number of 9's as does the rate of decline to the end of life level. So as the example shows adding a 9 (order of magnitude improvement) increases battery life by a factor of 10 but also decreases the degradation at a particular time or mileage by a factor of 10. That's what drivers care about.

 

[ajdelange]

Cell phone batteries would last much longer if they limited charging to 90%?

Don't know about "much" longer but longer, yes, and in fact Apple made mention of this in a note that was included with my last cell phone purchase. They claim that this version has new software which learns the user's use pattern and limits charging to less than 90% to the extent possible. Must say, however, that I have not noted any visible indication of this but then I don't study my cellphone battery the way I study my Tesla's.

 

[Newton]

You aren't alone in that. Let's look at your chart a little further.

I see, that helps a bit. C is the batteries capacity.

Question...a 20c battery mean you can use or charge the battery at 20 times its capacity? So like if you have a 100amp hour battery, you can use or charge it at 20x100= 2000 amp hours?

 

[Jhodgesatmb]

It's probably worth mentioning that we don't really care about the lifetime of the battery as it will, for 99.99% of users, exceed the time the owner has the car. What we really obsess over is the rate of degradation. During discharge of the battery lithium ions are released by the anode and migrate towards the cathode. But not 100% reach the cathode. A tiny (fortunately) percentage react irreversibly with the SEI and are lost to the lithium pool which can store charge. During charge lithium ions migrate from the cathode to the anode where, again, 100% do not make it. One of the goals of battery design is to make sure that the percentage of lithium ions reaching the electrodes has lots of 9's in it to the right of the decimal point. For example, if 99.99% of lithium stays in the pool after a cycle then the pool contains 90.4% of the original amount after 1000 cycles. If retention is 99.999% then 99.9% remains after 1000 cycles and 90.4% after 10,000. Battery "life" is defined in terms of some level of remaining charge capacity. Clearly that depends on the number of 9's as does the rate of decline to the end of life level. So as the example shows adding a 9 (order of magnitude improvement) increases battery life by a factor of 10 but also decreases the degradation at a particular time or mileage by a factor of 10. That's what drivers care about.

It is true that most people don’t own cars so very long, but everyone wants their car to have trade in value if/when they move on. I think one of the things that people like me think is of value in the CATL battery announced in March/April is the claim that it is a 1.25 million mile battery. If that were true (by most people’s standards) the battery would last through several owners.

 

[ajdelange]

I see, that helps a bit. C is the batteries capacity.

Yes.

Question...a 20c battery mean you can use or charge the battery at 20 times its capacity? So like if you have a 100amp hour battery, you can use or charge it at 20x100= 2000 amp hours?

Not quite. A 32 A-hr battery (the 12V battery in an X) is said to have capacity C = 32 A-h. A charge discharge cycle would involve putting 32 A-h in and taking 32 A-h out for a round trip charge transfer of 64 A-h. If the battery has a useful life of 1000 cycles that's 2000C equal to 32000 A-h of charge and 32000 A-h of discharge. If charged at 96 A we would say it is being charged at 3C or C/.333.

 

[Crissa]

Cell phone batteries would last much longer if they limited charging to 90%?

Yes. But then they'd weigh more and need to be recharged more often.

-Crissa