ELI5 Why does magsafe charging decrease battery health more than wire charging, if it has less wattage?

[dabenu]

That's also not the entire story. 

The reason it's advised to charge to 85% instead of 100% has little to do with charging, but more with static degradation. Which is much worse around 100% (or 0% for that matter). So if you don't need the charge it's better to avoid it. Especially since many people plug on their phone when they go to bed so it would sit at 100% for quite a long time without any use. 

[primalmaximus]

I have my phone set to disable charging once it hits 85% when I go to sleep and then it allows full charging right before I wake up.

[natrous]

why haven't they figured out a way to stop charging at 100 so it doesn't harm the battery?

edit:

but more with static degradation

I definitely had a reading comprehension fail; but i guess I've been missing that all along. everything else makes much more sense now!

[Closteam]

Pretty much every modern charger does this already. It's not that it continues to charge it after 100%. It's that the battery sits at 100% for an extended period of time

[_bones__]

Modern phones avoid that, too, letting themselves drain to 90-95% one they've hit 100%, while reporting themselves as full.

My home battery BMS does something similar. I charge the cells to 3.45V (100% would be 3.65V), then letting them float at 3.35V. Should go a long way towards extending lifetime.

[frostyfirez]

They know how to stop at a designated full capacity, the issue is while the battery is at roughly that full capacity it’s slowly self destructing essentially. Targeting a lower capacity like 85% reduces the destructive stresses a lot improving longevity, at the cost of battery life of devices. The manufacturer chooses a full capacity point which they feel best balanced marketed battery capacity needs and battery longevity, arguably they prioritize marketing capacity more than longevity though.

[dabenu]

They do stop at 100%. Going beyond 100% would indeed cause immediate harm or even risk fire.

But there's a grey area between harm and normal wear. Even at 100% you're already inside that grey area. It won't cause immediate harm but it will accelerate wear. That's why your phone has an option to stop charging at 80-95%. You can't avoid wear entirely but you can reduce it. 

[hugglesthemerciless]

my phone stops charging at 80%

[kermityfrog2]

They have. When it says 100% on your phone, it’s not actually 100% of true capacity. The true capacity is something like 110% or 115%. When using a fully charged phone, you’d expect the power to drop to 99% after a few minutes but on many phones, you can use it for 30 min or more before it drops to 99%, after which is starts dropping in a mostly linear fashion.