Magsafe 1 Charger to Replace Magsafe 2 Charger

MagSafe chargers can output variable voltage. When disconnected from a computer, they output a very low voltage. When connected, the computer requests the voltage it needs, using the center data pin. Your voltmeter is acting as a resistor in this circuit, which may be fooling the charger into thinking a computer is attached.

According to this link, the original MagSafe uses 6V when disconnected, and MagSafe2 uses 3V. So I can't explain your 0-0.5V fluctuation, except that maybe you're not making a good connection with the test leads.

http://www.righto.com/2013/06/teardown-and-exploration-of-magsafe.html?m=1

Apple says 60W. Page 7 of this pdf manual.

45W is just for the MacBook Air