it is possible to use that battery for an external extra battery with your internal. i have definantly been down this road, and have tried it
i tried it with my sony picturebook. thier batteries are like 11.1V, so i hoped the 12v would be enough to charge the internal while i taped... wrong. the charger for the picturebook was 16V, so this pack didnt have enough current to actually charge my battery, it just kind of extended it a bit.... but it got me by for a YMSB show, both sets with a 2x capacity picturebook battery and it..
you might want to check out thier other batterys with a higher voltage, something similar to your charger voltage if you want something that will prolong your battery life a great deal
my suggestion, check and see if your lappy is compatible with a Valence:
http://www.valence.com/there are 5AH and 10AH versions of these... and they recently released a new one but i dont know the specs on it, they can get kind of pricey, but i tell you what, i LOVE my valence and wouldnt trade it for anything
as far as charging that battery goes, i try to discharge it fairly good before i charge it. BUT, sometimes that isnt possible. i would charge it while you can check on it, and see when the pack gets warm. these chargers do not have enough output to harm the pack, the extra juice will just be output as heat, and not very much heat at all for that matter. when i plug mine in I just let them go till they get kinda warm... they wont get warm untill they are fully charger