You'd be better off with this drop-in (bulb) as it is made for the lower voltages that a single battery has. The drop-in listed about is for a two battery configuration, so it'll be dimmer with only one. For one battery you need a drop-in that's rated 0.8V-4.2V or 3v-6V for maximum brightness.

http://www.solarforce-sales.com/prod...RB&s=40&id=200 (This will draw more power as it's a much brighter LED)