Why can't I limit charging to 80% on my iPhone?

You are misinterpreting the message. It is a new feature on iPhone, when people use fast charging (20w or more) that the phone will rapidly charge to 80%, then cool down until just before you get up in the morning, where the phone will then charge to 100%.

Read this link which explains Optimized Battery Charging and Optimized Charge Limit --> About Charge Limit and Optimized Battery Charging on iPhone - Apple Support

Mine limits the charge to marked charge levels between 80 and 95 %. It’s stop charging at the level the slider is set to. Have you read the Apple Support article Lobsterghost1 shared? Here’s the link again for your convenience.

About Charge Limit and Optimized Battery Charging on iPhone - Apple Support

Woofly90 wrote:

I have that feature. It’s inadequate because if I need to recharge during the day the battery can go over 80% unless I monitor it.

i don’t want to babysit my phone.

You don't need to babysit your phone. Having it go to 100% won't hurt it. Once it reaches 100%, it will stop charging and drain a little. It sounds as if you're overthinking this.

See also:

When to charge your iPhone or iPad - Apple Community

Woofly90 wrote:

So apparently I don’t have IPhone 15 or above. Maybe it’s time to upgrade. In spite of what some are now saying about not bothering to be careful about charging, I’ve heard sbout the 80% charge limit for years, first on Apple phones and later on electric cars (which have actual battery cooling management)

I don't know from electric cars, but I agree with Bob that charge limitation is a bit of a gimmick. It appeals to people who feel the need to micromanage their battery. Turn on optimized charging. Don't leave the phone in your car in the summer. Don't drain the battery to 0% and then leave it that way for days or weeks. And stop worrying about it.

You can certainly buy a new phone if you want. Far be it from me to interfere with a good excuse to buy new toys. But if getting charge limitations is the only thing you think you need, I wouldn't bother.

Woofly90 wrote:

So apparently I don’t have IPhone 15 or above. Maybe it’s time to upgrade. In spite of what some are now saying about not bothering to be careful about charging, I’ve heard sbout the 80% charge limit for years, first on Apple phones and later on electric cars (which have actual battery cooling management)

Sometimes what you hear about is wrong. Amazing, isn't it? Electric cars are totally different from iPhones. For one thing, you can leave an iPhone plugged in while you use it. Try that with an electric car, unless you have a Very long extension cord. Where the fallacy comes from is that Lithium batteries do deteriorate if left at 100% for weeks or months, but not for hours or days. The reason electric cars suggest stopping at 80% is the charge rate slows to a trickle above 80%, but they also say to charge to 100% if you are planning a long trip.

Your iPhone does have the very best charge management feature - Optimized Charging. It’s better that 80%, because it will go up to 100% just before you are ready to use it, giving you 20% more runtime during the day. And since you are using it the iPhone battery will only stay at 100% for minutes.

Apple has enabled a feature called Charge limit that allows you to disable charging at points between 80% and 100%. Please see screenshot below,

Even with charge limit set to below 100% iPhones will still occasionally charge to 100% to calibrate the algorithm used to determine charge. If you have a newer iPhone enable it. Older iPhone do not have the necessary hardware to enable this feature.

The feature works as Apple designed. My wife uses it on her iPhone 15 PM.

Are you reading what were telling you? The Apple charging algorithm will occasionally charge to 100%. Do you understand that? If it’s charging to 100% it’s because Apple has determined for the overall health of the battery it needs a complete charge. This also allows the algorithm to learn the true capacity of the battery. If the algorithm doesn’t know what 100% is, how can it stop at 80%?

Jeff, That option appears not to exist on some SE models, which the OP seems to have. My SE 2020 is fully updated to 18.4.1 and does not offer the slider shown in your screenshot:

Woofly90 wrote:

I have that feature. It’s inadequate because if I need to recharge during the day the battery can go over 80% unless I monitor it.

i don’t want to babysit my phone.

You're going to drive yourself crazy trying to babysit the charge on your iPhone. I failed to notice you have an iPhone SE, which I believe does not support Charge Limit. There is simply no reason to worry about limiting your charge to 80%. I own iPhone 16 Pro Max and I don't use the Charge Limit. I use Optimized Battery charging, which guarantees my phone when plugged in at night, will charge to 100% when I get up. Most of us senior members on this forum don't use Charge Limit. It's honestly, more of a gimmick, which won't make your battery last longer, nor will it stop Battery Health Decline. If you need to charge you phone, charge your phone and stop worrying about it reaching 100%.

I’ve heard sbout the 80% charge limit for years

If Charge Limits were really that important, Apple would have introduced this feature on all iPhones starting 15 years ago.

Somehow, billions of iPhone users have gotten by without Charge Limits for many many years.

Only two models of the iPhone.....the iPhone 15 and iPhone 16 even have a setting for Charge Limits....and these models have only been introduced in the last 2 years. Apple could have added this feature for all phones in a simple software upgrade, but they have chosen not to do this.

Frankly, Charge Limits are more of a gimmick than anything else, but that might be another discussion. Even if your iPhone has a setting fo Charge Limits, the battery is still going to charge to 100% more often than you might think. You just don't know that it is happening.

I believe it available on all iPhone 15 models and iPhone 16 models. It was introduced in iOS 18.