A brand new feature that was introduced in BlackBerry OS 7.1 is Battery Saving Mode. This simple option lets you set a battery threshold which will then automatically adjust settings to conserve battery. So instead of keeping a close eye on your device when your battery hits a low point (say 20%), with this feature enabled, the device will automatically adjust things like screen brightnes and time out in order to not waste battery power. Battery Saving Mode isn't a miracle worker, but it will keep your BlackBerry hanging on a bit longer until you can find your way to a charger. A very cool feature that I'm sure you won't mind having.
If you're running OS 7.1, Battery Saving Mode should be enabled by default, but you can head to Options > Device > Battery Saving Mode to enable/disable this feature and change the battery level. Keep in mind this is just one way to save battery on your device. You can always manually change some settings to conserve battery power. Things like turning off Wifi and lowering screen brightness can go a long way. For more on this, check out ten tips and tricks to maximize your BlackBerry battery life. If all else fails, just carry a spare battery or charger and you'll never have to worry about a dead device!