Apple’s latest support document on how to properly set up and use the MagSafe Duo charger confirms that the company’s now-retired 29W power adapter is incompatible with the charger.
When clarifying the proper method to use the MagSafe Duo charger with the iPhone 12 and Apple Watch, Apple states that it is recommended that the charger is used with a 20W or greater USB-C power adapter. As reported by MacRumors, an asterisk included on the support page states “The Apple 29W USB-C Power Adapter isn’t compatible with the MagSafe Duo Charger.”
If a user were to use a 29W power adapter with the MagSafe Duo charger, it would only charge one device rather than both at the same time. This is likely due to the fact that the 29W adapter does not support 5V/3A or 9V/1.67A power ratings. The support document mentions that to charge both devices, the charger requires at least 15W 5V/3A or 9V/1.67A. Although, Apple does not clarify whether or not there is more to it.
The 29W power adapter was discontinued back in 2018, when Apple replaced it with the 30W adapter. The 30W power adapter is fully compatible with the MagSafe Duo and is capable of charging the iPhone 12 and Apple Watch simultaneously.
Apple has also stated that the MagSafe Duo is limited to delivering up to 12W when used with the iPhone 12 Mini. Charging with Lightning accessories attached such as headphones will result in a limited 7.5W charge to “comply with regulatory standards”.