The right thing to do would have been to come up with a new plug, but to also provide an adapter (or 2nd jack, but adapter is fine) to plug in the old 16.6v power supply.
The new thinkpad could easily have said, "Ah, I see only 16 volts." Then, if it saw that the battery was highly discharged, it could either have reduced charging current to it, or announced to the user "Sorry, we can't charge an empty battery and run your computer at the same time. Thus your battery is currently not being charged. If you wish to charge the battery, you may turn off the computer until the battery charge light blinks yellow, and then you can operate again. Or operate without charging as long as you like."
Note that the charging light blinking yellow would happen at something like 50%, essentially as soon as the charge current has dropped to a level the old 70 watt supply can handle while running the computer.
Other options would have included shutting off USB power, or limiting screen brightness or keeping the CPU at a lower speed or whatever else might keep the power budget in line.
Truth is, we are long past the point that power supplies and devices should be smart, and communicate about power needs, so that you only need one type of power supply to run just about anything. Laptops should plug directly into the smart plugs of the sort found in Kensington/Targus/iGo, though of course those are not standardized themselves. If Lenovo had chosen one, the world would have standardized very quickly.
But in this case, the laptop could easily know -- 16 volts means old supply, 20v means new.
Of course, old laptops could not physically plug into the 20v supply presuming they can't handle it, because the adapter would only work one way.
Modern devices all use cheap buck converter chips that actually take a wide range of voltages and convert to what's needed. I would be amazed if the TP doesn't do this, though I admit I don't want to try putting 20v into my older model to see what it does.
But any modern laptop could be set to easily run on anything from 12 volts to 36 volts with today's chips. (12v is pushing it a bit with a 10.8 volt battery, though it is such a worthwhile ability that having both buck and boost should be considered.)
The only question would be current. You want to know how much current the supply can handle. In many cases you can "test" it by drawing more current until the supply shuts down, but sometimes that would be because it blew a fuse or melted, which would not be good.
I found that when I ran my 4.5A thinkpads off older 3.5amp supplies, they worked fine except with a totally empty battery. In that case the supply would shut down due to the overcurrent and the thinkpad would beep, and so I would know to put it to sleep for a while. And that's without any planning for this contingency.
I wrote this up in more detail here:
http://ideas.4brad.com/laptops-could-ge ... ays-stupid
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