I am in the IT field and have tons of laptops and laptop AC adapters
laying around. I have purchased a new laptop and need an additional
power adapter for it and I would like to use one of the older adapters
I have laying around instead of buying yet another one but I am afraid
I may ruin my battery by charging it with an adapter that is the wrong
specs. What should I look for to determine that the old charger would
work with my new laptop? The power adapter that came with my new
laptop says 19V, 6.3A and when I did a search for a replacement, I saw
things like 120W max even though there is now Watts max listed on the
adapter itself.  Which of these numbers are important when purchasing
a replacement or trying to use an existing adaptor that wont ruin my
laptop or battery life?

You need an adaptor which will provide same voltage, that is 19V
The other number (Amps) is dependent, meaning it will be OK when
Volts are right.

Watts, power can be used to determine the voltage, but not the max power.

Max power however  is important: Your laptop uses 19V* 6.3A = 120W

If your adaptor has less that  120W max, it will not ruin the battery,
but it will overheat, and that's bad too.

http://amasci.com/elect/elefaq.html
http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l3c.html


Hedgie