I have a Macbook Pro Retina 15" and like last 1 month I also use two external displays, one is 22" Benq display connected to Macbook via HDMI and other one is 24" Dell display with 8K resolution connected via DP (monitor says DP but i guess it is thunderbolt), and you know what, my original Macbook pro 80W Magsafe 2 adapter just got broken (luckily at least not the power unit in the macbook), and because when i see the comparison of apple adapters like 45, 60, 80 watts and i see descriptions like - for 13" retina screens, or for 15" retina screens, I wonder if my adapter broke because i had connected two external displays (one is even 8k resolution) or if it is not related at all.
Connecting multiple displays was not the cause of the broken power adapter. It was not caused by electrically powering the displays because neither HDMI nor DP carries power for the displays. The displays had their own separate power plugs.
Thunderbolt (same port as DP) does carry power, but your Dell Display probably uses the traditional DP connection instead.
Plugging in additional displays causes the GPU to work harder, but not enough to cause the power adapter to fail. Laptop power adapters do fail occasionally, particularly at the places the cords bend.
There's a whole myriad of reasons why your MagSafe 2 adapter could have failed:
- Perhaps it was defective, to begin with?
- Was it connected directly to the outlet? Maybe it was made defective with a surge?
- Did it suffer any physical damage?
A 15" MBP Retina must use an 85 Watt power adapter (I actually just bought a new one) because not only does it need to drive the display, the MBP and charge the battery but also provide the necessary power to drive any USB / display connectivity.
However, driving those displays didn't do your power supply in as, I'm sure, those connected displays have their own power source.