windows 7 - Laptop display dims when running on battery power

I recently bought a Lenovo IdeaPad Y580 and it has one minor annoyance: It continues to dim the screen when I unplug it from the power cord. I have tried setting the screen brightness in the advanced power settings but no matter what I set this to, the laptop always gets dimmer when unplugged.

So how can I keep the display brightness at 15% at all times?

Answer

Unfortunately, all the power settings in windows may be complete voids due to LENOVO's own power management application,
Look for "Lenovo Energy Management" it should have some customisable settings.

If there are no such settings then Try disabling "Lenovo Energy Management", from Task Manager

If nothing works and if you are so desperate to achieve custom settings, Uninstall the Energy Management Software. (Not Recommended)