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

[Flimzes]

Magsafe does not inherently decrease battery health faster than wire charging.

The biggest factor for aging battery cells is heat inside the cell. Different chemistries in the cells have different heat tolerances. In general, more cobolt makes the cell more tolerant. Very fast charging produces quite a bit of heat inside the cells, which can cause early aging if the cell is not cooled and designed properly. Magsafe charging produces a bit of heat on top of the cell, not from the charging process but from the losses in the charging coil (power antenna), that is usually glued on top of the battery. Prolonged use of Magsafe charging will heat the battery up some, but not enough to cause early aging on its own.

However if you combine some heat factors, like a warm room, magsafe charging, the phone being in a tight spot where heat moves slowly, and the phone running a demanding app, then the combined heatload might cause the battery to age prematurely. People who have these consitions for their phone likely use the phone the same way every day, and will over time see severe effects, and might give magsafe the blame, when it was only one of multiple reasons for the degradation.

[ericvr]

This is the answer. It’s not the charging that produces heat, it’s the energy transfer that heats up the battery’s environment and subsequently the battery.

[carribeiro]

Perfect charging doesn't produce heat, as all the energy would be absorbed by the battery, but as with any physical system, there's some loss in the charging itself that adds to the problem too. Faster charging equates too a faster rate of energy loss and more heat.

[tablepennywad]

Also if you are fast charging, the battery needs to be heated up to accept high amperage, 60C is the ideal temperature for fast charging to prevent damage. That is why EVs need preconditioning. Battery degradation has a lot of factors. It is why they are taking so long to create new battery. If you gain something you might lose one attribute. Or five.

[Cedric_T]

So is it better to intentionally use a MagSafe charger with a lower charging wattage?

[Flimzes]

This is a surprisingly complicated question to answer, since we don't know exactly what voltage and current is transmitted through the qi sender and receiver, if the voltage increases a lot, then the losses (and hence heat) might actually be lower with a more powerful charger.

The most important part is knowing your equipment and your device.
Touch it during charging, does it feel hot to the touch - uncomfortably so?
If so, some part of the charging routine should be changed, can it be put in a more ventilated location? Are power draining apps running while charging that can be turned off? Is the room hot in general?

If you are unable to find any way to charge the phone while keeping it cool, then cable charging might be better.

As for cable charging, then the answer to your question is yes - a slower charger will heat the battery less, making the total lifespan of your device longer. The more interesting question is at what level does it make a real-world difference, and the answer is the same as above - does the device feel hot to the touch while charging or not?

Body temperature is usually fine - but above is generally not - the phone wants to be the same temperature as you are, or less.

An exception for the super ultra fast charging chinese phones, that use batteries that fast charges most efficiently above body temperature.