SR (limited?)battery degradation

[Rocky_H]

So then it's not really 'hiding' the top end, is it, as someone else mentioned?

Yes it is.

That's why I'm confused.

You're confused because you are frequently going back and forth, interchangeably referring to two completely different things as if they are one thing.

even if one charges a 60kWh battery to 100% daily, I'd expect to see no real degradation.

This is not true. Regardless of how you use a battery of what level you charge it up to, it will have some degradation over the years. All batteries do. So the total capacity of the battery will be reduced some as it ages.

The charging limit, as controlled in software charges from the bottom up, and does not charge all the way to the top of the real capacity. But you are mistaken in thinking the reduced overall capacity is subtracted ONLY from the top end that is locked out. It is really that the degraded part is subtracted from the whole, so that is split across the physical total, and the proportion you are allowed to access.

To maybe put it in simpler terms, let's say you get three fourths of whatever is there. If you start with 100, you get 75. If it degrades so there is now 80, you get 60. If it degrades to 60, you now get 45. So it keeps the same ratio of whatever the total capacity is. It is still charging up the bottom portion of the battery and leaving the top uncharged, but because it keeps scaling like that, it effectively does look like the degradation is coming out of the usable portion.

So you were thinking of it in terms of:
1) Fills up to XX amount first
2) Then any reduced capacity would be out of the top, which wasn't being charged anyway.

But it's really:
1) Total reduced capacity is calculated
2) It fills to XX% of that.

 

[MP3Mike]

So given that, even if one charges a 60kWh battery to 100% daily, I'd expect to see no real degradation. Any degradation that happens will be in the top end of the battery, so should be invisible to the end user, yes?

No, degradation happens evenly in the entire battery.

 

[dmurphy]

To maybe put it in simpler terms, let's say you get three fourths of whatever is there. If you start with 100, you get 75. If it degrades so there is now 80, you get 60.

... that sort of makes sense. I get how it works, technically, but that's not really congruent to what one purchases.

My point being... What you purchase is 240 miles' worth of range. Not 80% of 310 miles' worth of range.

Similar, but different.

To my feeble mind, the behavior I would expect would be a 75kWh battery that software-limits the top-end charge to 60kWh. Then empty is empty, and we never worry about being too high of a charge for regenerative braking or speeding up battery degradation by filling to 100% repeatedly. Seems there's a couple of advantages that way.

Instead, what we get is a 75kWh battery that fully charges but then uses a multiplication factor (.775) for range. Still charges all the way up etc - but only shows valid range as a percentage of the battery's overall capacity.

I'm absolutely sure there's an advantage to doing it via the percentage calculation vs. a charge limit, just don't know what that is.

Again, moot point in my situation since I have an LR model, but just trying to understand the advantage of the percentage method vs. a charging cap.

 

[dmurphy]

No, degradation happens evenly in the entire battery.

Of course, given proper cell balancing and such, I'd expect each cell to degrade evenly. It's not like a gas tank where the top 2" is empty or something.

But my point is if we're using a charging-cap method where I can only charge each cell to, say, 75% of initial capacity anyway, that gives me a 25% "buffer" before I'd even notice the inability of the aged cells to fully charge. But it appears that's not how it works - it's using a percentage of current charge range calculation instead.

 

[MP3Mike]

But my point is if we're using a charging-cap method where I can only charge each cell to, say, 75% of initial capacity anyway, that gives me a 25% "buffer" before I'd even notice the inability of the aged cells to fully charge. But it appears that's not how it works - it's using a percentage of current charge range calculation instead.

They are using a charging-cap method. And no it doesn't hide degradation. In your example they charge to 75% of physical capacity. But as degradation occurs that 75% holds less energy. (Think of it like filling a bucket 75% full, but occasionally someone comes along and shaves the top of your bucket down, i.e. degradation, but you calculate the 75% fill rate based on the current bucket size each time you fill it. So you put less in it each time it gets shaved down, but it is still 75%.)

 

[dmurphy]

They are using a charging-cap method. And no it doesn't hide degradation. In your example they charge to 75% of physical capacity. But as degradation occurs that 75% holds less energy. (Think of it like filling a bucket 75% full, but occasionally someone comes along and shaves the top of your bucket down, i.e. degradation, but you calculate the 75% fill rate based on the current bucket size each time you fill it. So you put less in it each time it gets shaved down, but it is still 75%.)

Gotcha. So if new, each cell holds 5000mAh, it’d charge to, say, 3750mAh to achieve 75%. But after, say, 10 years, the top end of the battery is 4000mAh, the car would charge it to 3000mAh (75%) instead of 3750mAh — even though it’s still capable of the 3750.

I got it. Still think it’d be more customer friendly to charge to the 3750mAh per cell instead of 3000, but I get how the math works.

Thank you!

 

[Rocky_H]

Instead, what we get is a 75kWh battery that fully charges but then uses a multiplication factor (.775) for range. Still charges all the way up

No. I already addressed this in my comment that you were responding to.

The charging limit, as controlled in software charges from the bottom up, and does not charge all the way to the top of the real capacity.

So like I just said here, it is not filling the real battery all the way full but only showing you part of that energy. It has an estimate of how much energy the full battery could hold and only filling up to a portion of that.
All of the cells are being used, and they are being used from the bottom, but they do not all charge to the highest total voltage level they can store. And that partial actual fill is then referred to as being "full" for the software limited lower named battery.

 

[gary3411]

Ok so evidence seems clear the battery will degrade proportionately over time. However, I now have a somewhat new worry I hadn't considered before. In the past model S software locked versions were able to charge faster than an unlocked version at a supercharger near the end of s charge, indicating a mostly top-locked battery. This has been well established. If they do the same for the SR like they should and I'm sure intended to, awesome.

However, until Tesla actually releases the software lock, we will not know if they plan to do this. My worry is that with all the complaining coming from SR+ folks about having 'almost' the same car as a SR and demanding a refund for the difference, Tesla is now trying to find ways to differentiate the cars. One way may very well be to bottom-lock the battery instead of mostly top-lock like done previously on the S's. This would result in much slower charge for the SR, and really suck.

I honestly think this could be the hold up for the software lock and why it hasn't occured yet. They are engineering a way to make it worse (bottom-locked) solely as a response to the complaining of SR+ owners. Hopefully, making Tesla waste time and money trying to engineer ways to make a car worse will make these folks happy.

 

[Rocky_H]

I honestly think this could be the hold up for the software lock and why it hasn't occured yet.

What do you mean by "hasn't occurred yet"? They have already delivered some standard short range versions, so it has occurred and isn't being "held up". I think they're doing it just like they have always done before. The rest of this just seems like paranoia.

 

[gary3411]

What do you mean by "hasn't occurred yet"? They have already delivered some standard short range versions, so it has occurred and isn't being "held up". I think they're doing it just like they have always done before. The rest of this just seems like paranoia.

What do I mean by "hasn't occured yet?" Um as far as I can tell, nobody's SR has been software locked to 220 miles. So no, the software lock has not occured yet. Are you suggesting it will never occur? That'd be great. I'm giving that about a 1% chance. I think the situtation I painted is much more likely, not 50% likely, but maybe 10. And if it happens, it will be a huge bummer for SR owners. 20 additional miles knocked off 'real-world' supercharging range just like that. Again, I hope not and don't expect this.

 

[gary3411]

It's just one more penny in the potential 'cons' bin when weighing possibly the biggest purchase/investment I will ever make in my life.

 

[MP3Mike]

Gotcha. So if new, each cell holds 5000mAh, it’d charge to, say, 3750mAh to achieve 75%. But after, say, 10 years, the top end of the battery is 4000mAh, the car would charge it to 3000mAh (75%) instead of 3750mAh — even though it’s still capable of the 3750.

I got it. Still think it’d be more customer friendly to charge to the 3750mAh per cell instead of 3000, but I get how the math works.

It might be more customer friendly but it would be giving away that battery capacity for free. (The customer didn't pay for it.) And then after degradation they couldn't unlock the remaining 25% because there would only be 5% left to unlock, having given the other 20% to the customer for free. It wouldn't be fair to the people that paid for the full capacity.
​

However, until Tesla actually releases the software lock, we will not know if they plan to do this. My worry is that with all the complaining coming from SR+ folks about having 'almost' the same car as a SR and demanding a refund for the difference, Tesla is now trying to find ways to differentiate the cars. One way may very well be to bottom-lock the battery instead of mostly top-lock like done previously on the S's. This would result in much slower charge for the SR, and really suck.

They won't do that, as it would cause more battery degradation, and as you said charging would be slower so people would be at the Superchargers longer, and that isn't what Tesla wants. They want people to be able to charge as fast as possible and move on so they don't need to install more Superchargers.

 

[jjrandorin]

It's just one more penny in the potential 'cons' bin when weighing possibly the biggest purchase/investment I will ever make in my life.

You must never plan on buying a house / condo etc? Also, almost no car is an "investment". Most depreciate, pretty much to zero eventually.

 

[gary3411]

You must never plan on buying a house / condo etc? Also, almost no car is an "investment". Most depreciate, pretty much to zero eventually.

I may or may not buy a house, which is why I said "possibly." Renting is a pretty sweet deal. Depends on your definition of investment. What it most certainly is, is an allocation of resources/assets that could otherwise go towards a real yielding investment. So even if it's not an investment, in my case, it's taking the place of an investment so I am comparing it to one. Also why I specifically wrote "purchase/investment."