[colechristensen]

Someone please correct me if I'm wrong.

It seems like the battery packs on cars are engineered to manipulate people, and this would be an easy way of making two very different cars seem the same.

Lithium batteries degrade. With heat, with time, with charge cycles. Article after article writes about how Tesla's battery packs barely degrade! 8% capacity loss over 100,000 miles.

This has to be a lie. Unless they've come up with a new magical battery chemistry.

Lithium batteries degrade. 15-20% capacity loss per year. 2-5% per hundred cycles.

There are ways to cheat though, design your battery electronics to pretend to have a lower capacity at first and then gradually allow them to discharge more and more as the battery ages. It's true that there's a small boost to capacity retention if you don't fully discharge.

A new Tesla might well have an actual 400 mile range the day it comes off the lot, but they figure consumers would be really disappointed if that 400 mile range went to 350 after a year and so on... instead I'm guessing they pretend the battery capacity starts lower so that it's more consistent. In the end when they run out of spare room they'd initially left themselves the battery would fail fast.

What's to prevent a competitor to cut just a few corners on this strategy – don't give yourself as much future wiggle room so that you can say your capacity is higher?

I would actually love to be wrong, but what you see from the battery performance metrics just doesn't match up with any of my existing knowledge about how battery cells work.

[jblow]

Your figures are way wrong.

I have an original Tesla Roadster, bought in 2010, with a battery that is basically the first thing they figured out how to do in order to put a car together. (The Model S battery is much more advanced). I drove the Roadster daily for 6 years, and I had about 12% capacity loss after those 6 years. This was a much better situation than Tesla projected (I don't remember what they said at the time, but it was something like 30-40% loss at 7 years, and for a relatively low price they sold an optional battery replacement plan that kicks in at 7 years).

Supposedly the Model S's chemistry is much, much better. Just saying "they're lithium batteries" is kind of a red herring, because there are many many subclasses of lithium battery, and at least according to Musk the fact of lithium is not nearly the most important part, but what really matters is the composition of the cathode and anode: https://chargedevs.com/features/tesla-tweaks-its-battery-che...

[Edit: And the theory that they would have preemptively hobbled the car's maximum range by (.85^6) is just crazy, because it means they could instead have advertised a car that had THREE TIMES THE RANGE on its initial launch, and "range anxiety" was one of the biggest issues they had to overcome. They could have said OUR CAR GOES SIX HUNDRED MILES ON ONE CHARGE, which would be way more important than hiding some degradation.]

[NickM]

> Lithium batteries degrade. 15-20% capacity loss per year. 2-5% per hundred cycles.

This is a gross overgeneralization. There are many different lithium-based battery chemistries available, with wildly differing lifespans.

The voltage at which you stop charging also makes a huge difference in lifespan. Charge a li-ion battery 10% less full and it can make an order of magnitude difference in its lifespan. When you charge a li-ion battery to full, it's not really truly "full"; the charging system just stops you from charging any further past a certain point, in order to stop you from degrading the battery too much. It's up to the device manufacturer to decide exactly when the battery is "full", based on what kind of tradeoffs they're willing to make on capacity vs lifespan.

High temperatures can also really increase the degradation rate for li-ion batteries, and Tesla employs a sophisticated cooling system to prevent the batteries from getting too hot while they're charging or discharging. The original Nissan LEAF had no active cooling, and their batteries degraded quite quickly, especially in hot climates. This was partially addressed in later model years, but it's now clear that this makes a huge difference.

Like most people, you're probably used to seeing heavy degradation over time in your cell phone or laptop battery. Just bear in mind that it's much easier for companies like Apple or Samsung to advertise "our phone gets 20% longer battery life!" than "our battery capacity degrades 5% less per year than our competitor!" Add to the fact that many people replace their cell phone every few years anyway, and the incentives are clearly aligned to keep pushing higher charge voltages, leading to greater short-term capacity at the expense of longevity. It's also a hell of a lot harder to fit a good battery cooling system in a smartphone than a car.

[jartelt]

I think a variety of things could be going on. Tesla does it's own research on batteries, so it very well could be making batteries that simply last longer than standard Li-ion cells. They also likely try very hard to design the battery management system such that it does not overcharge or totally discharge the batteries, which improves longevity. They also probably try really hard to keep the batteries cool. Batteries in cell phones and laptops typically die fast because people repeatedly discharge them to 5% and charge them to 100%, which puts the cells through the maximum amount of stress due to volume change. There is also poor thermal management in these devices, so the batteries are often hot when you are using your phone/laptop a lot. Lastly, you get a new phone every two years, so phone makers optimize for the highest initial capacity rather than best capacity over time.

[jwatte]

There are many lithium chemistries, and variations within chemistries.

My Nissan has over 30% loss after four and a half years. They clearly use lithium ion/polymer, the lightest but least durable chemistry.

Meanwhile, I have a lithium ferrous phosphate battery pack for a robot that has lost very little capacity over the same time.

It's pretty clear that different manufacturers use different design margins already.

[gizmo]

How much lithium batteries degrade depends on their type. High current cells have different characteristics from high capacity cells, for instance. Tesla puts them in groups of 75 cells in parallel so they draw a minimal amount of current from each cell, thereby prolonging the life of the battery. They also monitor the health of the battery cells so they degrade as evenly as possible.

100,000 miles at 265 per charge (85kWh) is 377 charges. During typical behavior (commuting) the car will remain above 70% charged at all times. That by itself cuts the degradation roughly in 3rds. So we're looking at a 125 full charge cycle equivalent after 100,000 miles. So about 8% capacity loss sounds about right. No additional trickery required.

Apparently there is significant room for improvement here too:

> CEO Elon Musk once referred to a battery pack Tesla was testing in the lab. He said that the company had simulated over 500,000 miles on it and that it was still operating at over 80% of its original capacity.

https://electrek.co/2016/06/06/tesla-model-s-battery-pack-da...

[slededit]

SSDs do this exact same thing. The individual flash cells have much lower durability than the drive itself.

[colechristensen]

Hard disks do too but the difference is that for disks it's more or less an essential function.

If you don't leave plenty of spare sectors over the very inevitable bad sectors will cause the drive to fail outright.

Perhaps that's a decision I'd like to make myself but it's still an essential function no matter what.

Leaving spare capacity in my battery isn't an essential function. That extra hundred or two miles the first few years of ownership has value and if I use it right away I don't lose anything (except marginally faster wear on the battery pack)

The disk issue is about essential function. If I left no spare room the disk would have unrecoverable failures in days or weeks.

The battery issue is only about perception. If I left no spare battery charge the battery would function exactly the same. It's life would only be reduced in that I'd be using it more (this is assuming that my above theory is correct)

[feelix]

In practice, although HDD's are meant to be able to deal with bad blocks, and remap them, I have found the opposite to be true.

(I've been working at a low level with disks for over a decade, writing data recovery apps and such)

I have found in practice that by the time a HDD develops a single bad block, 99% of the time it's on a death spiral and it's going to completely die within a month or two.

It seems like none of this bad block remapping, spare space, or anything of the sort has any effect whatsoever. The only thing that keeps them alive is being well enough engineered not to break at all.

Please correct me if my experienced misinformed me and I'm wrong.

[patrickg_zill]

In the past, a utility did the determination and remapping via off-disk software.

My understanding was the traditional "bad block" behavior was actually hidden - there are bad blocks that occur but the firmware silently remaps everything.

When you see a bad block at the level you can see via e.g. smartctl program, that means the space set aside for bad block remapping is full - the drive has already been silently failing for a while now.

[slededit]

Only because our file systems aren't designed with shrinking media in mind. A system could be designed to mimic the behavior of UDFS on CD-R media where the size of the filesystem shrinks with usage.

[EdHominem]

> There are ways to cheat though, [...] A new Tesla might well have an actual 400 mile range the day it comes off the lot

That's not cheating, that's responsibility. That's presenting an honest product expectation. Cheating would be implying that the initial capacity would last.

It's also probably not the case, as people are pointing out. But even if it were it would actually make me think more highly of the company.

> What's to prevent a competitor to cut just a few corners on this strategy – don't give yourself as much future wiggle room so that you can say your capacity is higher?

Does it make it through the warranty period? If so, then what's wrong? For N years those customers had an X/kwh battery.

If not, lawsuit.

[mikestew]

Someone please correct me if I'm wrong.

Lithium batteries degrade. 15-20% capacity loss per year.

I'll correct you with empirical evidence: there are a ton of five year old Nissan Leafs that, by your math, shouldn't be able to pull out of their own driveway by now. My wife drives ours to work every day five years after we bought it.

There are ways to cheat though, design your battery electronics to pretend to have a lower capacity at first and then gradually allow them to discharge more and more as the battery ages.

After five years, given the geekery surrounding the Leaf and other EV, I'd be shocked that someone didn't figure out that the Leaf doesn't really have a 24Kwh battery.

[Thorncorona]

Keep in mind you wouldn't regularly fully discharge a car battery pack, however you would discharge a phone battery. Thus capacity reduction occurs at a slower rate than a laptop or phone.

[torpfactory]

Why don't electric car manufacturers build in some time-dependent margin into their range estimates?

E.g. Only display a range of 350 miles at the start of a battery pack's life even if the real range is 400 miles. To the user, the batter pack performance would appear static (to some limited lifetime, anyway). At the end of life the margin would be zero, and the actual life would be equal to the true battery capacity at that time.

I know Tesla and GM would much more happily report the new battery range than that of a tired, old pack...

[NickM]

Some do that. The Mercedes B-Class Electric is one example. Tesla is more "honest", so you actually get to see the full capacity.

This does mean that you can see the capacity very slowly decrease over months and years, but I would personally prefer to know what's actually happening and be able to get the maximum range out of my car that I can.

[jwatte]

The trade off is that the mere option to have higher range up front actually increases the rate of degradation! So, would you rather have high range first and low range after six years, or a motte even range that degrades more slowly and is better after the first few years? I prefer the latter.

[colechristensen]

The yearly losses are fixed, unrelated to usage. The per-cycle losses are indeed less when you don't fully discharge, but only by a relatively small amount.

Lets say you have a battery with a capacity of 100. Empty it completely 100 times and the capacity might be around 95.

Empty it only 20% 500 times (for the same total energy usage) and it's capacity might have only degraded to 97.

Better, but not nearly enough to erase the other effects.

[_ea1k]

That assumption is actually incorrect. Data: http://batteryuniversity.com/learn/article/how_to_prolong_li...

Table 2 shows the lifespan of a typical Lithium Ion battery based upon various discharge depths. 100% discharge rates degrade to less than 70% of original capacity after only 300-500 cycles.

25% discharges hit the same value after 2,000-2,500 cycles. That level of discharge is fairly common with cars like the Tesla.

Having said that, you do have a point about overbuilt batteries too. Some Teslas do have much bigger than rated batteries that are software limited. It is completely possible for someone else to "beat" them on range by utilizing more of the available capacity.

[colechristensen]

>25% discharges hit the same value after 2,000-2,500

This is exactly the point (and easy to miss)

The degredation per energy used is about the same.

1 cycle of 100% = 4 cycles of 25%

So the 300-500 cycle loss is equivalent to 500-625 when you divide the number of recharge cycles by 4. You _must_ do this for an apt comparison because what you should be interested in is how your capacity deacys with usage.

Artificially smaller capacities also means that using the supercharger is more common. Another feature of lithium battery chemistries is losing capacity faster at higher charge/discharge rates. Trickle charging overnight will cause significantly lower cycle decay than supercharging in an hour (or whatever period it is)

[_ea1k]

I'm not sure that I understand your point right now.

With 300-500 cycles at 100% utilization, a 300 mile car would have degraded its battery to 70% after 90,000 - 150,000 miles. With 2,000-2,500 cycles of 25%, the car would have traveled 150,000 to 187,500 cycles.

That seems like a significant improvement to me?

Your point about supercharging is similarly valid, but also a little misleading. The big factor there is temperature and the Tesla packs use active cooling to reduce the hit from more aggressive charging. I'm not sure that anyone has really seen a significant impact from this.

Basically, battery quality and management matter. Tesla does those things pretty well.

[mynameisvlad]

I mean, even with your calculated numbers, it still shows 25% discharge gives back better results than 100% dicharge, based on the ranges alone.

[slg]

Shouldn't this be an easy think to prove/disprove by just measuring the electricity used to charge the car over time? If my car required 15-20% more gas each year, it would be pretty obvious by simply keeping an eye on the amount of money I spend on gas.

[colechristensen]

No.

Imagine the batteries that are like a gas tank that shrinks over time.

My assertion is that (for example) a car starts out with a 30 gallon tank that shrinks over time. When the car is new, it's engineered to be "empty" when the tank is half full.

It'll take the same 15 gallons to refill on the first day as it does a few years down the road when the tank has actually shrunk to be 15 gallons in size.

[slg]

Yep, you are right. I was thinking of it in terms of efficiency and not capacity.

[ebbv]

You seem to have bought into the internet myths about Lithium ion batteries. What you claim just isn't true. They do degrade over time but your figures are nuts.

My four year old rMBP has barely degraded at all and my 2.5 year old Nissan LEAF has barely degraded as well.

Try getting some more real world experience with these products before quoting a bunch of internet horseshit.