Apple offers a wide variety of USB-C chargers that correspond to the many models of MacBook, MacBook Air, and MacBook Pro it’s made that remain available or that it offers replacements for. You might think because these are so specific, like a 29W or 140W charger, that you must use exactly that wattage adapter for a given device.
Fortunately, that’s not the case. While true before the widespread adoption of the USB-C plug type by Apple and everybody else, USB-C makes it even easier as there’s only a single kind of jack you can plug into any port. Power management is a fundamental part of USB generally; USB-C improved on it by allowing for power to flow either direction on a cable (and by adopting the most advanced parts of the USB trade group’s power delivery specification).
Different host devices (like iPhones and MacBooks) and adapters have specific combinations of power they can offer. But in all cases, any USB-C adapter can provide power to any host device with a USB-C port. You can’t go wrong: no adapter will provide more power than a host can accept.
For instance, if you plug a 10W adapter into a MacBook Pro that can charge up to 140W, it only passes power at 10W. Likewise, a 140W adapter won’t blow up a maximum 29W chargeable device! In many cases, you can use a relatively low-wattage adapter with a laptop and keep the battery from depleting or charge it slightly. You need to match full wattage to full wattage or higher to ensure a device charges completely, even while in use.
This Mac 911 article is in response to a question submitted by Macworld reader Buddy.
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Source : Macworld
