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Comment by dmitrygr | Hacker News Reader

This is fundamentally flawed because lithium batteries mostly degrade not because of amount of use, but because of time (and their charge state and temperature during this time). No matter if used or not, they will lose capacity. The rate of capacity loss is mainly dependent of average charge (best is 40%, worst is 100%) and temperature (best is 5 degrees C, the higher the worse).

(This is also why phone batteries fare so badly: they are often near 100% charge at 36.6 C in your pocket)

http://www.sciencedirect.com/science/article/pii/S0378775316...

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I do not believe that the passage of time is the main cause of lithium ion battery degradation. The source that you linked, http://www.sciencedirect.com/science/article/pii/S0378775316..., does not seem to support that idea either.

Figure 3 from the paper, "Cause and effect of degradation mechanisms and associated degradation modes," shows time as just one of 8 degradation mechanisms affecting lithium ion batteries. Other figures/tables from the paper do not emphasize time as the primary degradation mechanism, nor does the body text. Nor is there any explicit passage-of-time term in the final diagnostic model that the authors develop.

Phone batteries do poorly because the SoC generates a ton of heat when used which gets dumped right on the battery pouch.

It's the same reason that you see early Leaf cells die out in Arizona while Tesla's have held up to 100k+. Thermal management makes or breaks longevity.

yes... I said that...high temp is bad

Sure, but you lead with:

> not because of amount of use, but because of time

Which makes it seem like time is more important that heat.

heat != use

think of it more as

BADNESS = integral(abs(temp - 5), over time) * integral(abs(charge_amt - 40%), over time)

heat = (discharge + charge)

use = (discharge + charge)

hence: heat = use ;)

To be fair for phones you focused on: 100% charge and pocket (body) heat.

You did mention 100% and temperature being the problem generally though...

It seems like the opposite might be true, and that batteries degrade from cycle count and not because of time. This is why they are rated in cycle counts (i.e. cycles until 80% capacity = life time of battery). Also important is the charge/discharge rate, but because the batteries are so oversized in Teslas they can discharge as low as .25C even at highway speeds which helps increase their lifetime.

source: http://batteryuniversity.com/learn/article/what_causes_lithi...

They are rated in cycles so consumers do not need a degree in physics to understand the reality. Feel free to inspect a REAL spec sheet for a LiPo battery from a manufacturer (I think ATL provides some online)

Can you provide links? You clearly feel we have never seen a "real" sheet, perhaps more hand holding is order. I would appreciate the experience and would not do it without the link.

Here's a "real" manufacturer datasheet. The spec the battery life in cycles. Op may be correct, I don't know battery chemistry, but I've never seen batteries spec'ed the way the op describes in any "real" datasheets I've read.

http://www.omnitron.cz/download/datasheet/NCR-18650PF.pdf

I am trying to find some non-NDA specs online for you. I saw this when I was working with these for a job and the specsheet was NDA (and I no longer work at the place this was)

Lithium batteries lose charge with use as well. Also rate of charge/discharge. It's more complicated than just one thing.

For about 16 hours a day, my phone is somewhere between 100% and 0% charge, probably around body temperature or a little higher.

For the remaining 8 hours, while I'm sleeping, it's at close to 100%; which is not ideal. I'd be curious how effective an overnight charging scheme would be that charges up to 40%, then holds it there until say 4 a.m. (or some other time based on the phone consumption, capacity and charger output) then charges up to 100% so that when you wake up it's fully charged.

So if you were optimizing your phone for low battery degradation, when would you charge it, and to what %? If you were trying to keep it around 40%, would a good strategy be to let drop to 20%, and then only charge to 60%, or would you bias it even lower to keep it away from 100%, like a working range of 10-50%?

The ideal state of charge for a lithium battery is 50%. So charging between 20% and 80% would significantly lengthen battery life. If you wanted to completely optimize for low battery degradation you'd charge between like 49% and 51% and only have a few minutes of usage, but that kind of defeats the point of keeping a lot of battery capacity.

Maybe schedule it like so:

Between 6AM and 8AM: Charge to 100% All other times: Charge to 75%

So the battery is topped off in the morning right before you put it in your pocket for the day, but otherwise doesn't fully charge to save the cell life.

They have a chart for calendar degradation. It's not weighted by mileage, but there are none of the datapoints you'd expect for that proposition (low miles, high age, high degradation).

Wow, I've never heard anybody say that before or model it that way before.

Though these tests are evidence of that, it's definitely not a common belief.

Before the Tesla data, what informed you?

In the RC world, the '50/50' rule of thumb is common for lithium battery storage. 50% charge at 50˚F. It's close enough to the 'ideal' values, and easy to remember (for those of us used to a human-centric temperature scale)

> human-centric temperature scale

One point (100 °F ~ 37 °C) doesn't make a scale.

Heh, sorry, that was just a tongue-in-cheek joke, not a central point of my comment.

0 to 100 on the Fahrenheit scale is "Really Cold" to "Really Hot".

0 to 100 on the Celsius scale is "Kinda Cold" to "Unsurvivable Hot".

When I say it's "human-centric" I mean that it maps well to the range of human comfort (Celsius is obviously vastly superior in just about every other use).

I'm not so sure I would call 100° C "unsurvivable". People go in saunas with 100° C all the time for enjoyment. If the air is dry enough, it's not that hot.

Maybe it's unsustainable, you can't survive that indefinitely. But you can't survive 0° F indefinitely either, unless we bring protective clothing into the game.

100C would send you to the burn unit. I think you mean 100F.

Edit: 100C is boiling point for water - is the ambient temp of the Sauna 100C?

I'm thinking of finnish-style saunas, which operate at 60 to 100 °C [1] (with 100°C being a frequent option). You usually stay in them 10-15 min at a time, but I guess if you keep hydrated you could stay a lot longer.

Finnish saunas are kept very dry. Sufficiently dry air makes very hot temperatures bearable because evaporation cooling from sweat gets more effective the dryer the surrounding air is. Water on the other hand makes sweating useless, which is why you can't tolerate water temperatures above body temperature (depending on how much of you is submerged).

1: https://en.wikipedia.org/wiki/Finnish_sauna#Types_of_sauna

"Air temperatures averaged around 75–100 °C (167–212 °F) but sometimes exceeded 110 °C (230 °F) in a traditional Finnish sauna." [0]

Apparently so! TIL.

[0] https://en.wikipedia.org/wiki/Sauna

I'm not sure that the 0-100 thing matters that much.

I'm in the UK we switched to C for temperatures back when I was a kid (I still vaguely remember both been used on weather charts back when they stuck the things on by hand..), 0 is cold, 10C is brisk, 16C is perfect, 20C is warm, 30C is hot, 35C is "kill me now".

I provided a citation. Feel free to google around to find more info. This is basically how everyone models LiPo and Li-Ion cells. Tesla only publishes marketingspeak (which makes sense - that makes them money). Look for actual science and you'll see

Weird, was that there when I replied? I definitely don't remember seeing it there... Perhaps I was just rushed.

In any case thanks for the source!

I've recently had to become pretty familiar with this subject for work, and your comment is only partially correct.

Li-Ion battery degradation is generally modeled as two (roughly additive) components, called calendar and cycle aging. Calendar aging is what you're talking about in your comment - it's basically determined by the temperature and state of charge. Both of these impact the rate of chemical processes that lead to loss of lithium and active material. This is what happens when the battery is sitting on the shelf.

Cycle aging, on the other hand, happens when you charge or discharge the battery and is driven mostly by the actual volume change in the anode and cathode when they get lithiated (it's quite significant, up to 15% or so in some materials). This introduces mechanical stress, which can break the protective film that forms between the electrodes and electrolyte and allow chemical degradation to proceed at a faster rate. The degree of mechanical stress is mostly determined by the depth of discharge, although rate of charge and discharge is also believed to be important. Cycle degradation is also impacted by the temperature at which the battery is cycled, in a similar way to calendar aging.

With high temperature variance (low temperatures will wreck your batteries too, and I can't recommend keeping them at 5C), calendar aging is the dominant mode. This is absolutely the case with cell phone batteries. However, electric vehicles, especially higher end ones like the Tesla, have an active cooling system that keeps the battery at a constant temperature, even when the car is "off". This is why many EVB warranties will be voided if you let them run out of charge for more than 14 days - at that point, the cooling system isn't working and your poor battery is at the mercy of the elements.

Under closely controlled temperatures, cycle aging is the most important factor, and it's dominated by depth of discharge. If you drain the battery as far as it will allow you every day, it's going to be hosed. If you only go down to about 80% of the allowed charge every other day, it'll last for quite a while. These numbers are roughly correlated, obviously, with the mileage on the car, but the relationship is not simple, and I'd caution against considering mileage to be a good determinant of battery degradation.

Some good sources if you want to learn more:

A fairly simple model of degradation (ignores charge/discharge rate): https://www.researchgate.net/publication/303890624_Modeling_...

A thesis on the subject, goes into great detail and has an excellent bibliography: http://webfiles.portal.chalmers.se/et/Lic/JensGroot.pdf

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