IIRC the Apple adapters are variable voltage and will increase up to "18.5V" depending on the load current, but the laptop itself is perfectly happy to receive full voltage --- and slightly more --- all the time.

In fact, laptops that have a 19V adapter will be fine with 20V too, and may actually run slightly cooler because of the lower current draw. I suspect all the funny voltages is just to create differentiation and a sense of lock-in, but the underlying engineering and tolerances on components mean that a 19V adapter may actually be in the range of 18-21V depending on load. It's amusing to find laptop adapters claiming 18.25, 18.75, 19.6, and a bunch of other "why!?" voltages which are technically all in the same tolerance range.

I've written about this before too: https://news.ycombinator.com/item?id=7594383

Some further discussion on MacBook-PSU-related details here: http://www.eevblog.com/forum/repair/replacing-apple-magsafe-...

http://www.eevblog.com/forum/projects/apple-magsafe-power-br... (last post is enlightening)

The real obstacle with the "one adapter for all laptops" is the proprietary DRM-ish ID schemes that a lot of them have, where they may complain/warnin and/or won't charge the battery with just pure voltage input.