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I'm not sure that I accept your premise. If it were really better for the battery, wouldn't the engineers designing the charging circuitry simply force it to stop charging at 80%

Right?? My phone does actually have a setting under battery maintenance to have it only charge to 85% and the people at the Verizon store also told me I'd be better off charging it to 80% and that it wouldn't really affect how long the battery lasted (which makes no sense to me) because that first 15-20% drains so fast? This is why I'm confused.

A lot already do, they just say it's 100% because alot of people would be upset to see that their whole battery isn't being used all the time.

And they do. Mostly for marketing reasons because to charge vattery to 80% takes almost same time as charging from 80 to 100. Thus pr team can claim super fast charging

Two main things impact phone battery life. First is what charge level it is sitting at, it lasts longest hanging out around 50% state of charge(actual charge level not displayed) and wears out a bit quicker the further from that you get. Second and wayyy more important is charge cycles, your battery has a fixed number of times it can get charged up and down. Using 10% and then charging it back up counts for 1/10th of a charge cycle. Using 40% is 4/10ths of a charge cycle.

Your phone shows you the usable range and not the actual charge state of the battery

0% is closer to 20% charge state and 100% is closer to 80% charge state to leave room for battery degradation, prevent over charging, and extend the lifespan a little. Batteries also charge significantly faster in the middle of their range than at the high end so there's a bit of marketing there.

If your phone has a setting to only charge until 80% then you can use that and it'll charge up and hold it there. It will mildly extend your battery life as charge cycles have a significantly greater impact

If your phone doesn't have this feature then manually removing it from the charger at 80% will just increase your charge cycles causing it to wear out faster. When your phone hits full charge it just runs itself off the charger and the battery sits there full, if you're constantly unplugging it when it hits 80% and doing 3% extra of charge cycles every night because of this then after a year you'll have added another 11 full charge cycles and made things significantly more complicated in your life

The sub would have not let me answer your question with just a link, but since this is not a top-level answer, I can.

https://www.nytimes.com/2020/09/04/smarter-living/phone-charging-advice.html

(Internet Archived version with no paywall: https://archive.ph/YqHsD )

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Thank you! They deleted my post for being inaccurate, but looks like the info all lines up it's just a bit more complicated than "it's better.". Appreciate the link!