Will undervolting underclocking a 7800 GTX improve battery life?

Not sure about battery life, but less heat for sure

cheers ...

I'm gonna guess yes. But i don't think it will be worth it. You'll see minimal improvement (5-10) minutes at best. Those older notebooks did not carry a charge for long. They weren't designed/ meant to.

By selecting the correct powermizer setting you can lower the vcard to 1 volt instead of 1.32 volts. That will help battery power.

I'd attempt to adjust mem voltage too with software if possible. Along with that, change screen brightness and wifi takes up alot of power (driver settings can usually allow for power savings at the cost of speed)
Also, try undervolting your cpu and downclocking it, you can use some battery extender programs too.

Yes, underclocking and undervolting will increase battery life but not significantly. The problem with these cards is that they cant underclock much without becoming unstable. You can reduce the 2D core frequency to somewhere around 100 MHz most likely but the 2D memory will likely need to be 150 MHz or higher (for your card). You need to monitor these frequencies when you're done to make sure you didn't go too far. Signs of going too far are lockups and the appearance of artifacts on the screen. Another sign that is harder to recognize is that your card may throttle up from 2D to Low Performance 3D just from dragging some basic windows across the desktop. The general rule I like for saving power is that the card should always be in 2D unless there is a video or a game running. In order to make sure your card is not doing this, then you will need to view the realtime clocks with a program like rivatuner or gpu-z.

Also, if you don't use the card for gaming at all you might want to either disable the 3D clocks (via active performance levels) or significantly reduce the 3D clocks and use the same voltage that low performance 3D uses for 3D voltage. An easier way to disable 3D mode would be to use powermizer but you know I always do things the hard way and dont like adding in extra variables.

Undervolting with these cards via the bios has never really been confirmed or not. I'd just recommend staying away from modifying the default voltage values or adding a new voltage. Just use the current voltages already supplied. Following this method, you would likely not be able to decrease the 2D voltage but as I've already mentioned, it probably would not work anyway.