Many times, I have run low on battery without a charger in sight, you can but the brightness very low and play around with those settings but it won’t help much. Smatphones these days such as the Nokia N8 have a feature called power save mode which saves power in every possible way. Follow after the break on how to activate power saving mode.
In 3 clicks, you can activate power save mode.
Click on the battery icon at the top of the homescreen
Click on the empty battery on the right.
Click on Activate Power Saving Mode.
Then when you wish to deactivate power saving mode, just follow the same steps but the last step will say Deactivate Power Saving Mode rather then Activate Power Saving Mode.