Why does my Surface power brick keep blinking on and off?

If you try to charge your phone and your Surface from the same power brick, keep a close eye on the charger: See if the charging light is blinking.

Here’s the deal. There’s an issue with some phones in the way they negotiate current with Surface power brick. The two get into a current negotiation fight.

1. The power brick detects that there is a phone plugged in.
2. The power brick says, “Hey, I can give you 1.2mA, is that okay?” (Note: All numbers in this story are made up.)
3. The phone says, “I’m cool with that.”
4. The power brick says, “Okay, heres 1.2mA. Enjoy.”
5. The phone gets confused and draws 1.3mA instead of 1.2mA.
6. The power brick shuts down due to overcurrent. The charging light goes off.
7. Your Surface is now running on battery.
8. The phone says, “Okay, well, easy come easy go.” It stops drawing power.
9. The power brick says, “Whew, that overcurrent situation is over. I sense no load. The user must have unplugged the curling iron or whatever it was. Let me turn myself back on.” The charging light goes on.
10. The Surface is now running on AC power.
11. Go to step 1.

The result of this issue is that the PC is constantly switching between battery and AC, and the phone doesn’t charge. Basically, everybody loses.

The Lumia folks issued a firmware update to address this issue, but if you’re using some other phone model, then who knows when/if it’ll get fixed. But at least you know what’s going on.