When you mention 6 hour battery life, I think immediately of laptops with ULV (ultra-low voltage) processors, or netbooks. Then, you mentioned a 17" screen so that grandpa can see better, which I understand... however, ULV CPU's usually come in laptops with smaller screen sizes, such as 13". Netbooks come with processors such as the Intel Atom, with screen sizes of 10.1" or so, & around 6 hours of battery life, which is standard. It's going to be difficult to find you a laptop with those exacting requirements. However, I think that on a 17" notebook you can buy a bigger battery, like a 9-cell or possibly a 12-cell lithium-ion, although the bigger batteries would probably protrude from the laptop chassis. The 17" laptops probably have at least an i3 2nd Gen. processor, which has a TDP of 35W. Whilea 35W TDP is low, I doubt that an i3 powering a 17" screen would last 6 hours, at least with the stock battery, likely a 6 cell. Buy a battery with a high mAh (milliamp hours) rating, like 6600, that should last a while.
This is an example of a high capacity notebook battery. In particular, this battery has 12 cells, & is rated at 14.8 volts/ 6600 mAh. That's a lot.Edited by N7-OC - 11/24/11 at 9:04pm