Louis;
It's great that your boss gave you the OK to use your E-cig, but a word of advice. I suggest to keep it out of sight as much as possible. It only takes one person to complain that you were smoking and your boss will have no choice but to cancel his permission to keep the peace.
As far as the battery issue goes, get some more. You'll vape em dead before they can fully charge. I would say to have 4 or 5. I'm using heavy duty battery mods, and I carry 4 batteries with me. I always leave mine on the charger for 8 hours, and no problems yet. Here's some good battery information.
http://www.batteryuniversity.com/index.htm