Well one could humour oneself by noting out that the screen is indeed possible to turn off and by doing so you conserve energy and thus increasing the battery time, what function a portable without a power supply (thus on the road?) fills is however beyond me
You can also dim the backlight to a very low level. That definitely helps a lot.
You do realize the battery isn't designed to be user-replaced, don't you?
If you carry an extra battery you better also carry a tool set to take the computer apart to replace it! If you're field-replacing the battery you would also cause a lot of other problems such as system clock resetting, etc.
In the end it'd probably be more convenient to carry some kind of solar panel or gas generator to recharge the existing battery.