If you read the specs for a MagSafe2 Apple Charger, it says output 16.6V 3.5A, which can be useful when you are looking for a 12V source power for i.e. a battery.

But, if you just cut the Magsafe2 cable and measure the voltage, you’ll read only around 0.3V.

What’s the trick?

This charger has an internal 16-bit microprocessor. You need to put a resistance between both terminals (i.e. 22K Ohm), to tell the microprocessor that there is a resistive load between terminals (same does your mac when you plug it).

When the microprocessor registers this load, after 1 second, it will deliver the 16V.

Now you can use this charger to regulate the output (maybe using an lm2596) and charge a 12V battery…

More information: http://www.righto.com/2013/06/teardown-and-exploration-of-magsafe.html