Slow Charging Explanation – Explanation for Wrong Charging Wattage / Slow Charging

chargingmagsafepower

I have a 2013 Retina MBP and a fairly new 85 W Magsafe 2 charger. But when I connect this charger to my Macbook and check in Battery Health, it claims that it is receiving only 49 W.

Does anyone know why the laptop should be drawing such a low supply from the Magsafe 2?

Best Answer

85 W = maximum power used by your charger on the power supply side on AC.

49 W = actual power received by your MacBook internal power managment on DC.

Where is evaporating the missing 36 W?

  • Within AC -> DC transformation.
  • Joule effect within all conductors. This explains why your charger is heating (and incidentaly heating the atmosphere of our hosting Earth). This consumption is high because it is depending on the square of I (RI²) and on low voltage I is high.
  • A part is diverted by the PMC to power different parts of the computer based on its energy saving state.
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