It depends on the battery type. Also, I think you have a typo as 1.2V is one normal battery cell (like a AA or AAA).
The best advice is to invest in a smart charger. It doesn't have to be fancy, airsoftparts.ca sells a very simple, yet good one for $35 if you're in Canada. In any case if you must use that charger, you should never leave the battery in it for too long. They can overheat and explode. Use a timer or something to automatically shut it off.
A simple search on this forum would have yielded this (and many other) very useful threads:
http://www.airsoftcanada.com/showthread.php?t=25656&