It Turns Out You Can’t Use Apple’s Original 29W Charger with the 15W MagSafe Duo

While we’ve already observed that MagSafe charging is a confusing mess right now, it turns out that it actually gets even worse with the arrival of the new MagSafe Duo, which not only doesn’t ship with a USB-C charger, of course, but apparently has some even stickier compatibility requirements than we ever could have imagined.

Even before the MagSafe Duo began shipping to customers, folks watching closely observed that Apple’s new 20-watt USB-C charger would only get you 11 watts of charging power. To go faster, you needed to pick up at least a 27-watt brick, and even that would only give you 14 watts — slightly less than the standalone MagSafe charger gives you with a mere 20-watt adapter.

Confused yet? Well, it doesn’t stop there. It turns out that Apple’s own 29-watt USB-C power adapter can’t be used properly with the MagSafe Duo at all.

That’s right, even though Apple notes that only a 20-watt adapter is required to use the MagSafe Duo (albeit at an 11-watt speed), if you have one of Apple’s old 29-watt adapters laying around, you won’t even get that much performance.

Instead, what will happen is that the MagSafe Duo will still work without the “Duo” aspect — you’ll be able to charge an Apple Watch or an iPhone, but not both.

What’s Going on Here?

While this seemingly defies logic on the surface, there is actually a technical reason for this, and it’s rooted in how confusing USB-C Power Delivery (USB-PD) actually is under the hood.

It’s something that Apple has been able to largely avoid until now, but the fact is that there are actually multiple voltages used by USB-PD, ranging from 5 volts at 2 amps up to 20 volts at 5 amps, with various steps in between depending on the version of the spec that’s in use.

Since it’s also not strictly necessary for every USB-C adapter to support all of the different power combinations, it’s possible to run into incompatibilities in terms of the maximum power output.

To be absolutely clear, using the wrong USB-C adapter won’t cause any problems, since every USB-C device has to support the 5V/2A rating as a baseline — assuming of course that it’s from a reputable company; all bets are off if you’re getting non-certified or “counterfeit” chargers.

However, since that only works out to 10 amps, you won’t get optimal charging if a higher wattage can’t be matched up between both devices — for example, if your USB-C power adapter can only output 9V/3A but your target device can only charge using 15 volts.

Unfortunately, the wattage rating doesn’t tell you anything in this case, since wattage is simply the voltage multiplied by the amperage. So for instance, both 5V/3A and a 9V/1.67A work out to 15W, and it’s impossible to know voltages are being used without consulting the more detailed specs for the adapter.

However, this appears to be exactly what’s happening with Apple’s 29W USB-C power adapter, which technically offered up 14.5V at 2A (29W) and 5.2V at 2.4A (12W).

On the other hand, the MagSafe Duo only supports 5V and 9V charging, and of course it requires the 9V version for maximum performance since USB-C 5V tops out at 15W. A 9V/3A (27W) adapter will offer up the peak 14W charging rate., while Apple’s new 20W adapter can output 9V at 2.2A, and the older 18W adapter is rated for 9V at 2A.

While Apple notes that “higher wattage adapters at or above 9V/3A” can also be used, the emphasis here is on wattage; in other words, you could use a 9V/4A adapter (if such a thing existed, as it’s not in the USB-PD specs), but you can’t use a 15-volt or 20-volt adapter, so even though Apple’s 29W USB-C adapter can put out 29 watts, it’s doing that at 15 volts — a voltage that’s not supported by the MagSafe Duo.

Now to be fair, Apple released this original 27W adapter back when 9V charging wasn’t part of the USB-PD standards, and it’s now been discontinued for over two years. However considering that not only did fast charging come to the iPhone 8 and iPhone X lineup the year before, but it had actually been available on the iPad Pro since 2016, there’s a good chance that a lot of Apple users already have at least one of these 29W adapters kicking around — especially since it was the only USB-C adapter that Apple sold back then.

However, it’s now easy to see why Apple quietly replaced it in June 2018 with a newer 30W version. Even though that seemed like a relatively modest increase in power, what it actually brought was support for 9V charging — something that was added in the newer USB-PD 3.0 spec — albeit only at 3A/27W; the “30W” rating comes from the fact that it supports 15V/2A and 20V/1.5A charging as well, which remains supported on Apple’s iPhones and iPads.

So to summarize, here’s the rundown of Apple’s USB chargers and their various power output levels:

(*discontinued)

So the bottom line here is that you won’t be able to use one of Apple’s original USB-C chargers with your MagSafe Duo — at least not if you want to be able to use the “Duo” aspect of it to charge both your Apple Watch and your iPhone 12 at the same time. The good news, however, is that pretty much every other one of Apple’s USB-C charger will work just fine, including those that come with Apple’s MacBooks, since they all support the 9V charging spec.