Are your using a laptop? If yes here is a great tool for your battery, shows minutes that you have till the battery goes empty. BatteryBar is a simple, straight-forward, battery meter that displays your battery status in your taskbar.
When you’re running on battery, BatteryBar shows the percentage bar in:
green (> 40%)
red (< 25%).
The bar displays the time left on your battery in Hours:Minutes
When charging, the percentage bar is shown in blue to indicate charging, and the estimated time to full charge is shown. The bar turns black and shows "On A/C" when fully charged.