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Comment by p1necone | Hacker News Reader

I'm pretty sure most (all?) manufacturers already do (the first part of) this. 100% can't be the point at which the battery literally stops taking new charge - if you did that they'd catch fire all the time.

It's just that the manufacturers idea of what "100%" should be generally skews more towards the immediate battery life end of the tradeoff, whereas some users prefer to have "100%" closer to the maximizing long term lifetime end of the scale.

Some devices do let you configure stuff like this - I know my android phone will let you cap max charge at an arbitrary level (although it won't report that level as 100%). Many laptop bioses will have a similar setting (good idea to set this to ~50% if you're using the laptop plugged in all the time, or just using it like a server with a builtin UPS).

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You are right but I disagree.

Yes, devices do not charge batteries to 100% as that is directly damaging. And they don't discharge to 0% for the same reason.

So they charge to something slightly less. But that slightly less is for all intents and purposes the only definition of 100% we, as end users, have.

It's like saying that your CPU is actually only running at 70% clock frequency but overclocking it to 100% would require massive cooling and reduce the lifetime to months.

I agree a better definition and preferably one directly comparable to all brands and battery types were available, but I'm not aware of any and I'm sure that even if we had it you wouldn't trivially be able to get that information.

Noone should ever charge to more than 80%. That is short sighted and environmentally moronic. After less than two years a device will have more capacity at 80% than the same battery will have at 100% if it was charged to 100%.

While I agree with your general take and hate that my iphone can't be capped to 80% [0], and love that my otherwise crappy hp laptop can be capped to something like 85% (and reports both cap and max capacity), I don't agree with your final take.

To take the phone example, mine usually ends the day around 85%. I just don't use it too often. But I sometimes absolutely do and would trade months of battery life for not being left without battery in the middle of nowhere. And the 20% can make all the difference.

A few years ago, I rented a car in a foreign country on a foreign continent. It didn't come with a USB port, and I didn't have a cigar adaptor. When I arrived at my destination, at night, my phone's battery was 10%. I sure as hell am happy that it was charged to a "full" 100% instead of dying on me one hour earlier.

Sure, I could have grabbed an external battery or something. Not sure how much environmentally smarter that is, since it would basically rot away in a drawer somewhere, since I would basically never use it.

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[0] Where does this 80% come from? Would it be even better if I stopped charging at 60%? I rarely use more than 20% of my phone's battery, so that would be largely enough, but I sometimes absolutely do and would hate to have to lug around external batteries "just in case".

Of course. All we're asking for is an optional "limit charge to 80%" feature. And, just like macOS has, you can have a very easily accessible "charge to full now" option for those times when you know you're going to need a full charge.

Don't forget, "100%" isn't really 100% any more after a year or two of charging to 100% all the time. You've probably lost 20% or so of your battery's capacity and it will be charging slower due to increased internal resistance.

> "Where does this 80% come from? Would it be even better if I stopped charging at 60%?"

Yes, but only marginally better since there's diminishing returns. 80% is generally considered to be the sweet spot that best balances practicality with battery health.

> just like macOS has

Can you please direct me to where this is possible in MacOS? I can't see any option to configure a charging cap except some "smart" option which has been on this whole time but always charges to 100%.

It's not possible to set a hard limit in macOS without third party utility like AlDente [1].

I was referring to the built-in "Optimised battery charging" feature. When this is active and has decided it will cap charging at 80% [2], you get a "charge to full now" option in the battery menu.

[1] https://apphousekitchen.com

[2] I do agree that this feature sucks and rarely actually works, unless you have a very predictable charging schedule.

I leave my laptop plugged in 95% of the time so in practice this caps it at 80% (helpful).

I suspect quite a few people do that so it probably is net-good overall, but agree it doesn’t work well if you’re moving around a lot.

Absolutely agree.

But my response was to the parent comment (emphasis mine):

> Noone should ever charge to more than 80%. That is short sighted and environmentally moronic. After less than two years a device will have more capacity at 80% than the same battery will have at 100% if it was charged to 100%.

My point is that I find there are legitimate uses cases for sometimes charging to 100%. And, indeed, those cases are much better served if most of the time I only charge less. In my case, this could be much less (50% of my iphone's battery is more than enough 360 days a year – and I know beforehand when it's going to be one of those 5 remaining days)

But that 20% that makes all the difference will only make a difference when the phone is new.

Otherwise the opposite is true. Refraining to charge fully makes all the difference and saves you from a tough spot.

Which in turn is why (was a bigger problem before) the sentiment that it doesn't matter how good/bad battery life I have as long as it lasts me a day. Well, if you do a complete cycle every day a year old phone will not last a day anymore...

This used to be an easy problem to fix, by replacing the battery.

Shame that seems to be harder and harder to find on new phones.

> I know my android phone will let you cap max charge at an arbitrary level

Yeah I used to have old Pixel phones as Home Assistant clients on my walls until they all started bulging one by one and I had to treat them as a fire hazard.

I now use Samsung tablets, ironically, that have a cap at 80% feature and they seem to be doing fine.

Are those cells 4.4v? Samsung makes great cells, and if you only charge them to 4.2V or less they should last quite a while. When charging to 100% means different things in different cases for the same battery technology then things get murky.

>I now use Samsung tablets, ironically, that have a cap at 80% feature and they seem to be doing fine.

Yes, this feature is one of the most underrated features on Samsung devices. Massively underrated.

I think there's some confusion on what this actually looks like on the manufacturer's end.

When charging a lithium battery, your termination point is tied to only one value: voltage. 4.2v per cell is generally considered the maximum safe termination voltage. This is the theoretical 100% charge. And yes, if you keep pushing volts in, it will keep taking them until the call fails catastrophically.

Personally, I use 4.1 as my termination voltage. That's somewhere around 90-95%. You 'lose' a little of the maximum capacity, but lifespan is increased. I find it's fairly common for battery charge controllers to use either 4.1 or 4.2 as their default termination, but it's almost always selectable. You have to make a decision somewhere.

The flaw in this is that you're trading away a banner spec for something that users won't appreciate. You have to decrease the stated capacity and runtime of your battery in exchange for an indefinable amount of extra total lifetime. That's not something that consumers can/will take into account. They just look for the device with the most mAh or runtime.

Because of the flatness of the voltage curve, limiting your charge to the middle range makes the state-of-charge estimation a much more difficult problem (less voltage signal). So there's actually a trade-off between longevity and accuracy of charge reporting.

iPhone 15 occasionally charges to 100% when 80% is set for this reason.

Many, many confused and complaining posts are to be found on Reddit and elsewhere about this even if it’s explained in the docs and I think next to the setting itself. It should show a notification when it goes to 100%.

Almost everyone caps below max voltage, but a toggle between small margin and big margin is a lot less common. And if you have to reboot into the bios, it only half counts.

As a non-battery engineer end user, I suppose all I care about is 100% usefulness. If the phone lasts more than a day under normal operation for a few years & doesn’t get too slow, I’m happy enough.

> I'm pretty sure most (all?) manufacturers already do (the first part of) this. 100% can't be the point at which the battery literally stops taking new charge - if you did that they'd catch fire all the time.

It's a really tiny amount, like 101% or 102% is the maximum. It becomes obvious on phones if you have an app that shows battery percentage over time - above 90% on a slower charger (like using USB instead of AC) it tapers off in a curve, even on a brand-new battery.

I wish my Tesla did that

The Prius Prime supposedly does this. It charges to 80% internally but displays as 100%, although usually drivers have the display set to "miles remaining" rather than the battery percentage. I vaguely remember the manual referring to this.

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