My laptop is capable of running most games on high settings at decent frame rates. However, if the power cable gets unplugged while I'm playing, the game immediately starts slowing down, even if I'm using the High Performance power plan.
Why does this happen? Is the battery not able to keep up with the power demands of the GPU? Is there any way to fix this?
Running a high-performance GPU at full speed while on battery can damage the battery or require more power than the battery can safely supply
High-performance mobile GPUs can require significant amounts of power to operate at full speed. The GTX 765M requires 75 W, while top-of-the-line mobile GPUs like the GTX 780M and GTX 980M can consume up to 122 W.
The GPU is not the only power-hungry part in a laptop. A modern Intel performance mobile CPU typically draws about 47 W at full power. In addition, you need to power other system components, such as the display, disk, and USB peripherals. When you add it all up, you might need anywhere from 140 W to 200 W to operate a gaming laptop under full load depending on your system configuration.
A typical battery in a gaming laptop can store about 60-80 Wh of energy. Most Li-ion batteries are not designed to be discharged faster than twice their Wh rating per hour (2C). In addition, sustained discharge at rates exceeding 1C can significantly reduce the overall service life of the battery. Continuously pulling 150 W or more from a typical 77 Wh battery is not a great idea and your battery could overheat and fail or even catch fire. While it's likely the battery's own protection circuitry would shut down the battery if overloaded or overheated, a device should never subject its battery to an unsafe load at any time during operation.
To avoid overloading the battery, the GPU will typically throttle to a lower clock speed. The GTX 780M on my personal laptop will not run faster than about 400 Mhz when on battery. Lower clock speeds reduce power consumption not only by having transistors switch less rapidly, but also by allowing lower core voltages—power consumption and heat dissipation scale with the square of voltage.
Most graphics cards (and even integrated cards) will have a graphics control panel (Catalyst, Intel, etc) - open the graphics control panel and dig around for power-related options. I know I've seen such things myself in both Intel and AMD panels.
Pretty simple—because the battery can't provide enough power, so the GPU and CPU will run at lower clock speeds. Sadly, there's nothing you can do about it.
I had a laptop with a GeForce GT 540M GPU and I could play games unplugged without any problems. However, when I later upgraded to a laptop with a GT 650M, I couldn't play anything while unplugged—the GPU clock speed was just too low.
Most Windows laptops have quite a few more options than just power saver, balanced and high performance.
In Windows 7 and later, you can access the "advanced" power options, and it would present you with a list of all the various components in the laptop and how much power they should get while plugged in as opposed to while on battery. I think you will find that even high performance mode makes some sacrifices to spare the battery, and unless you have a truly amazing battery that is probably a good thing. You'll probably mostly want to adjust the CPU, maybe the hard drive.
The battery cannot provide enough power for your laptop at full performance given that it is probably a Li-ion battery. If this is really a major problem for you, you may want to consider a Li-po battery (lithium polymer), as they are designed to have a higher output current, but it may not run for as long as a Li-ion. Here is a link for some Li-po laptop batteries.