Another possible cause would be found in the "Advanced Power Settings". Quickest way to tell you how to access is by copy and paste, so here goes.....
1. First click on the Batter or the Charger icon on your notification area and from the window choose "More power options" as shown in the below screenshot.
2. Then from the next window that you get, choose the "Power plan" that you want these settings to be applied and click on the Link next to it that says "Change plan settings" which should bring a new window similar to the below one.
3. From the next window that you get, look for the bottom section and click on that says "Change advanced power settings" as with below picture.
Once you click on this link you should be reveled by a new window that lets you configure a lot of advanced power settings concerning your hardware through the ACPI (advanced configuration and power interface, a standard) and using this you can change some big number of settings such as:
Note: If you click on the "Change settings that are..." (marked in Red) link, then it'll only display the settings that are not available in the default/simple configuration window...
*. Manually change the Hard-Disk turning off time.
*. Pause the desktop background slide-show.
*. Change whether the Wireless adapter should give you the maximum performance or maximum power efficiency.
*. Enable/disable wake timers, change auto hibernate time or enable the Hybrid sleep mode (turned on by default for Desktops), etc.
*. PCI Express cards related settings.
*. Change Advanced Processor related settings such as: Cooling policy, Minimum processor state or Maximum processor usage limiter (which can have a significant effect on your battery life if you know how to balance things a bit 😀), etc.
The only part here that you should worry with is the "Processor Power Management". Just make sure that the minimum for it isn't set at 100%. 10% is good enough.