should be fine. just make sure the battery doesn't get too warm. all "properly designed" laptops will have a overheat sensor that will turn off the power if things get too hot.

The battery is dead anyway - I guess I'll leave it out while using the laptop (if the design allows operation with no battery installed)...

it's best to match the volts and as long as it has as much or more amps your okay. having to many volts can hurt the charging and power circuitry of the laptop.

No, do NOT plug a 19V adapter into a 16V laptop. Some laptops may be able to handle it, but many won't.

It's usually safe to go a little bit above the rating, like a 14V adapter is usually fine with a 13.5V laptop, but a 3V difference is a pretty big jump.

Well, it turns out that the Acer power connector is not identical to IBM's - they look alike, but the central pin (the +) is thicker on the ThinkPad, so the output power connector of the charger won't plug in.

Resuming, therefore, the search for the right charger...

several companies makes aftermarket replacement chargers... belkin and apc are some of them
they come with different connectors for you to use to match up to your laptop and has selectable voltages