old MacBook power adapter with iPhone 12 Pro Max?

Fast charge your iPhone

Use fast charge with certain iPhone models. You can recharge your iPhone up to 50 percent battery in around 30 minutes.

You can fast charge your iPhone 8 and later up to 50 percent battery in around 30 minutes.1

Fast charging works when you use an Apple USB-C to Lightning cable and one of these adapters:

• Apple 18W, 20W,2 29W, 30W, 61W, 87W, or 96W USB-C Power Adapter
• A comparable third-party USB-C power adapter that supports USB Power Delivery (USB-PD)

If you're not sure which Apple Power Adapter you have, check the wattage on the top or bottom of the adapter:

1. Testing conducted by Apple in August 2017 using preproduction iPhone X, iPhone 8, and iPhone 8 Plus units and software, and in August 2018 using preproduction iPhone XS, iPhone XS Max, and iPhone XR units and software, with accessory Apple USB-C Power Adapters (18W Model A1720, 29W Model A1540, 30W Model A1882, 61W Model A1718, 87W Model A1719). Testing conducted by Apple in August 2019 using preproduction iPhone 11, iPhone 11 Pro, and iPhone 11 Pro Max units and software and accessory Apple USB-C Power Adapters (18W Model A1720, 29W Model A1540, 30W Model A1882, 61W Model A1947, and 87W Model A1719). Testing conducted by Apple in February 2020 using preproduction iPhone SE (2nd generation) units and software and accessory Apple USB-C Power Adapters (18W Model A1720 and 30W Model A1882). Testing conducted by Apple in September 2020 using preproduction iPhone 12, iPhone 12 mini, iPhone 12 Pro, and iPhone 12 Pro Max units and software and accessory Apple USB-C Power Adapter (20W Model A2305). Fast-charge testing conducted with drained iPhone units. Charge time varies with environmental factors; actual results will vary.

2. You need a 20W or higher power adapter to fast charge your iPhone 12.

It will not be any faster than the 20 watt Apple sells for $20.