Undervolting is one of the more advanced tweaks you can do to your Android device. It’s an experimental mod that literally reduces the voltage the device’s processor draws, with the effect that it can reduce the amount of heat produced (leading to a more stable device) and potentially even increase battery life.
The voltage can be set at different levels for different clock speeds.
Thus, if you want to try and minimise battery usage when your phone or tablet is in standby mode you can focus your undervolting efforts on the slowest clock speeds, or if you’re a power user you might tweak the voltages at the higher clock speeds to try and eek out a little extra performance under more intensive tasks.
You need a custom kernel in order to adjust the voltage. We’re the franco kernel, which can be installed and tweaked through the franco.Kernel updater app from the Play Store. This is one of the most widely compatible custom kernels, and is very stable. While it is possible to adjust voltage within this app, we’re using Voltage Control Extreme, which has a friendlier UI for this particular task.
Undervolting is a relatively safe process, although if you set the voltages too low your device may become unstable or even crash altogether, so you should take small steps to begin with.
It’s also worth bearing in mind that it only affects the processor, and the greatest battery drain – the screen – will remain as power hungry as ever. Ideally undervolting should be used as a part of a wider series of tweaks to squeeze out as much power as you possibly can from your device’s battery.
Before you begin you must perform a Nandroid backup. As with most hacking topics this is an essential way to enable you to restore your device should anything go wrong.
If your device becomes unstable or unresponsive due to selecting a too-low voltage hold down the power button or pull the battery in order to reset it. If you set the voltages so low that your device will not boot at all then you will need to restore a Nandroid backup or reflash the kernel in order to return to the default values.
What you need
Before you begin you will need to root your device and perform a Nandroid backup. You will also need a custom kernel that supports undervolting. For the first steps here we’re using Franco’s kernel with the franco.Kernel updater app from the Play Store.
Flash the kernel
Having installed the app open it and flash the latest version of the kernel. Go to franco.Kernel updater and select Download latest stable kernel, then choose the option to flash it and reboot your device. The entire process takes less than a minute.
Re-open the app and select Frequencies/Voltages followed by Voltages from the bottom of the list. Hitting the -25mV option reduces voltages by a small amount, and is a good starting point. You can also tap a frequency and input your own number. Remember – take small steps.
Tick the Set voltages on boot box to ensure your settings are applied. Exit the app and reboot your device. When it restarts it should be using the new lower voltages that you have set. You probably won’t notice any difference, but can check in the franco app to make sure the settings are in use.
Now test a few apps and games, and put the device into standby, to make sure that it all seems to be working. It’s a good idea to leave some time between tweaks, just to be sure they’re okay, but once you’re happy with the changes, go ahead and experiment further.
If you can’t use Franco’s kernel then you might need to manually flash one that works with your device. Head to the forum for your handset at xda-developers.com to locate a suitable kernel. Download and flash it in the same way you would flash a custom ROM.
Using Voltage Control
With your new kernel flashed download the Voltage Control Extreme app from the Play Store. This gives you a range of controls for the kernel, including over and underclocking as well as access to the voltage settings. You will need a compatible device and kernel, and most popular ones are supported.
Adjust the voltage
The principle of using this app is the same as with the Franco one. Swipe across to the Voltages screen and you’ll see the different clock speeds you can adjust the voltage for. Select one and move the slider down to your required level. The CPU History screen gives you an idea of which speeds are most used.
As before save your changes and ensure they are applied when you boot your device, then reboot. Test a few apps and use the device normally for an hour or so to ensure there are no problems. You can now return to the app and push those voltages lower if you wish.
Finally, if you’re a particularly hardcore user you might want to go the other way and overvolt. This supplies more power to the CPU, and could be used in conjunction with overclocking. Be warned, though, this may shorten the lifespan of the battery and processor, so should only be tried if you really know what you’re doing.