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My MBP15 (8,2, Late 2012) doesn't normally charge or draw power from a 45W adapter (from a 2008 MBA), with one exception:

  1. First, plug in the 85W power adapter;
  2. Second, switch the adapters quickly.

Following this procedure the MBP15 draws power form a 45W without any glitch for light usage.

Q1: Why is this procedure necessary and how does it work?

Q2: Is there a way to skip step (1)?

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As far as I know, the 15-inch MacBook Pro is not supposed to use the 45W power adapter. Therefore, when you plug in the 85W and quickly swap in the 45W, you are essentially tricking the MacBook into thinking that it is still charging from the 85W power source.

I don't believe this to be dangerous to your MacBook in any way, but it is definitely a hack and it is unlikely that this can be done without using an 85W adapter.

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