Note from Bill Gates on Lithium-ion Batteries

[ABCCBA]

I happened to read an article written by Bill Gates. The context was for Home Energy Storage solutions. He was trying to calculate the cost of the Storage Solution. In the article, he wrote:

"..This figure is based on the capital cost of a lithium-ion battery amortized over the useful life of the battery. For example, a battery that costs $150 per kilowatt-hour of capacity with a life cycle of 500 charges would, over its lifetime, cost $150 / 500, or $0.30 per kilowatt-hour..."

So, he states the life-cycle of a Lithium-ion battery is 500 charges. I immediately started to think if this held true for the Model S battery. If it does, and the life-cycle is 500 charges, then that could affect whether to keep your vehicle plugged in each and everyday, of let the battery drain down to a 10% to 20% SoC before recharging, to increase the number of miles driven over the 500 charges.

I am not a battery guy, so I don't know if the 500 cycle is full discharge to fully charged cycles, or if it is any time that the battery cycles from a lower SoC to a higher SoC.

More thoughts if the 500 charge life-cycle is close to true: I have a 85KWh battery that at 100% charge has a range of 270 miles. Based on those numbers, I tried to calculate the number of miles drive in 500 charges based on the delta in the SoC each cycle.

270 miles per 85KWh = 3.17 miles per KWh
270 miles per 100% SoC = 2.7 miles per %SoC

Theoretically, if I could drive and charge 100% every charge cycle, the maximum miles on the battery would be 270 miles * 500 charges = 135,000 miles over the life of the battery. However, that is not possible or practical if it were. So, if we come up with some average delta SoC cycles, here are miles:

Average Charge Cycle delta SoC%

10% - 2.7 miles * 10 * 500 = 13,500 ( 27 miles driven between charges avg.)
20% - 2.7 miles * 20 * 500 = 27,000 ( 54 miles driven between charges avg.)
40% - 2.7 miles * 40 * 500 = 54,000 (108 miles driven between charges avg.)
60% - 2.7 miles * 60 * 500 = 81,000 (162 miles driven between charges avg.)
80% - 2.7 miles * 80 * 500 =108,000 (216 miles driven between charges avg.)

I would love to hear from someone who know how the Tesla battery pack really works, what is considered a life-cycle charge, and if the 500 life-cycle charges from Bill Gates makes sense.

TIA

 

[jcaspar]

I am curious why you would give credence to anything Bill Gates said about Li-ion batteries. Can't imagine he has any more expertise than the typical Tesla owner and certainly far less than several contributors on this site.

 

[andrewket]

There have been many white papers and discussion here on this. The cycle count is full discharge and full recharge. You're much better off doing many shallow cycles (staying in the 20-80% SOC range) than fewer deep cycles.

 

[Saghost]

First of all, as Andrewket said, a cycle isn't defined as whenever you start/stop charging/discharging - it's putting the equivalent of the full capacity into/out of the battery. The thing is, though, that the battery wears a lot more near the top and bottom end - it'll last a lot longer if it never goes above 80% or below 20%, even if it sees a lot more cycles.

Second, I think you're reading far too much into the quote. Bill is trying to get the idea of amortizing the battery initial cost as an expense per kWh of energy moved through the battery across. He isn't saying that the average battery lasts 500 cycles, or that the best does - just that as an example, a $150/kWh battery with a 500 cycle life ends up costing about $.30 per kWh to the end user.

Presumably the intent of this is to show that a more expensive chemistry might actually end up being cheaper if it lasted longer...

 

[ecarfan]

I agree with @Saghost. Gates is doing a theoretical calculation based on a specific set of assumptions. That's a reasonable approach. We already know that for a Tesla pack you can do a partial charge over a thousand times and the battery capacity degrades by just a few percent, based on the documented experience of multiple owners of early production Model S. See the Plugin America study.

 

[sorka]

Can't recall where I saw the lifecycle graph, but it's not linear. The extreme ends kill battery life far faster. The chart I saw showed equivalent full cycles if you only charge between certain ranges. 20% to 80% was something like 3000 cycles of equivalent 0 to 100% if you stayed in the narrower range. Some even debate if there's much of difference between 80% and 90% yet nobody debates that it's not good to go over 90% regularly.

 

[wdolson]

There are people out there who bought a Model S in 2012-2013 who have charged the battery to 90% almost every night and the battery is still at 90-95% of its original capacity. There are Roadster owners who have even more charge cycles on their batteries.

Gates may be basing his calculations on his experiences with laptop batteries which are usually charged to 100% on every charge. A BEV with a good battery management system like Tesla can set the charge level to something that doesn't stress the battery much and they last longer.

We will eventually get to a point where the batteries start to fail in older Teslas, but very few owners have seen any dramatic decline yet.

There is a video out there of Gates doing an interview in a Model X:
Bill Gates drives a Tesla Model X and talks end of the world with 'Seveneves' author

It's unclear if it's his own car, but it might be. If he just got an X, he is probably thinking about all the long term details about owning a BEV which may have led to the battery speculation.

 

[techmaven]

Here is a research paper on NCA chemistry published by Panasonic employees which is the closest analogy to Tesla's batteries:

Development of High Power and Long Life Lithium Secondary Batteries

Note the 2nd graph where they charge/discharge at 2C from 3.6 volts to 4.05 volts and get maybe just over 10% degradation out to 3,000 charge cycles. It's not directly applicable to Tesla vehicle cell longevity because of several changes... notably the cell discharge rate is as much as double, and the cell charge rate is less than 2C and the cells are subjected to a lot more cold than nearly so much hot (50 degrees C/122 degrees F). But there is a wide temperature variance while the lab testing is at one temperature. Also, lab testing tends to have charge/discharge quickly while real world has cycling at far fewer times/day.

In any case, it shows that the cell life is dramatically increased when limiting the charge and discharge extremes, enough so that even half that number of charge cycles, 1,500 at 180 miles a charge = 270,000 miles just over 10% degradation.

 

[sorka]

For some perspective, an S 85 or P85 at 500 full cycles is 132,500 miles. There are a dozens of 125K mile Model S's that are still at 90 to 95%.

 

[daxz]

There is a video out there of Gates doing an interview in a Model X:
Bill Gates drives a Tesla Model X and talks end of the world with 'Seveneves' author

It's unclear if it's his own car, but it might be. If he just got an X, he is probably thinking about all the long term details about owning a BEV which may have led to the battery speculation.

I would say it is Neal Stephenson’s Model X , the plate in front says 7 EVES

 

[AWDtsla]

"cycles" are just a really terrible way to measure li-ion lifetime.