Chromebooks have a crazy long battery life, but that doesn't mean you won't run out eventually. If you need a few extra hours on your Chromebook, log in as a guest.
Weblog "OMG! Chrome!" shares a few tips on maximising the battery life on Chromebooks. Guest mode is probably the easiest:
Chrome guest sessions are the fastest way to just use your device without uninstalling or disabling a tonne of extensions, apps and background services supping away.
If you'd like some features to linger you may prefer to create a separate streamlined account just for this purpose.
Check out the link for other ways to save your Chromebook battery.
Simple Ways To Get More Battery Life From Your Chromebook [OMG! Chrome!]