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Comment by ezfe | Hacker News Reader

I don't think the current setup is notably different from what you're proposing.

The phone does use the MagSafe battery first, it just uses it through the internal battery, but that is just a matter of semantics. It shouldn't have a notable impact on battery health.

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The critical importance here is if you plan on keeping your phone for a long time, a degraded internal battery is fundamentally the limit under the current scheme.

Or to put it another way, imagine you had a battery that only lasted a single minute, and a magsafe battery that lasts a full day. How long is your phone going to last? What will happen is that both your internal battery and magsafe will be drained, with the magsafe being drained the most. Eventually your internal battery will die, and so will your phone.

The other issue with using the internal battery (pass through, or charge) is that you're wearing it down more.

Maybe the behavior is different in an iPhone 14?

Li-ion battery degradation is minimal, or even negligible when they aren't cycled across different capacity ranges AFAIK. What that means is that current charging the battery while it is getting discharged at a similar rate (which usually means a constant battert voltage) doesn't induce wear in the same way as charging the battery from a capacity of 10% to 100% (or even from 75% to 80%) does.

But that could be different when that constant charge/discharge is happening at high capacity levels, lithium-ion batteries are weirder when they are at their maximim voltage. My knowledge is rusty at this point so let me know if I got any of this wrong.

> It shouldn't have a notable impact on battery health.

Is that actually true? Lithium batteries seem kind of fussy.

Batteries don't like staying at 0% or 100%. They don't like extreme temperatures. And lastly, they don't like cycles.

I'm not entirely sure how the internal power routing works, but my understanding is that it's not adding cycles to your battery if you're using the power as it's being added, even if it's going through the battery.

You'll likely see more degradation from the wireless charging waste heat effecting the battery.

But doesn't using one battery through another effectively mean that you're using both batteries in terms of wear and tear? If the phone drains the internal battery and the magsafe battery is charging the internal battery, to me that sounds like both batteries are in use experiencing normal wear. If that's not how it works, I'd be curious to hear how it does work.

Why wouldn't it?

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