I did some research and figured out how you can adjust the "idle" time of your computer. Basically this way you can adjust it so that the computer doesn't ever dim the display or adjust how long it takes before the display dims.
You will need dconf tools, and to be on the safe side you should make sure all your packages are updated first.
So in your terminal type:
sudo apt-get update
Then install dconf-tools:
sudo apt-get install dconf-tools
Once that is finished, in the terminal type the following to launch the program:
You should see an expandable list on the left hand side of this new window use them to navigate the following path:
org > gnome > settings-daemon > plugins > power
Once you've clicked power you should get a list of options on the right hand side of the window. Scrolll to find "idle-dim-battery" and uncheck the box. This will completely prevent the screen from dimming while on battery. OR if you'd prefer, find "idle-dim-time" and change the value in seconds.
Hope this helps with the dimming problem.
Next, for video watching you can use a program called caffeine:
sudo add-apt-repository ppa:caffeine-developers/ppa
sudo app-get update
sudo apt-get install caffeine python-glade2
Lastly for power saving there are a lot of different options, I personally installed and use Jupiter, Laptop mode tools, and Powertop.
Best of luck and I hope this helps.
Workaround / Updated Answer
Should dconf not work, you can also try adjusting these settings using the terminal command "gsettings". This accomplishes the exact same thing as Dconf editor except without the graphical user interface (GUI).
To entirely turn off the automatic dim on battery, copy and paste the following into your terminal:
gsettings set org.gnome.settings-daemon.plugins.power idle-dim-battery false
or to instead set a time in seconds until the computer automatically dims the screen enter the following and replace "X" with the number of seconds (i.e. 90 or 200 etc.)
gsettings set org.gnome.settings-daemon.plugins.power idle-dim-time X
Where, again, "X" is the number in seconds.
I personally tried these commands in my terminal and they did work. I still highly recommend using dconf editor as above and navigating to the same power options to verify that the changes did in fact take place.
Hope this works!