Nowadays the default battery is just "consumable" (I don't know if this is the correct English term), as printer cartridges per example.
Each time you empty the battery, you make the battery to die : the next charge will not fill the battery as much as the previous one.
I hope the charging system from dell stops charging the battery when it's full...
I bought a 9 cells battery and I avoid to empty it under 50%.
I use Linux with a tick-less kernel and I optimize the system with
powertop. I don't know if something like that exists for Windows.
On all systems, lowering the back-light will lower electric consumption and so the impact on the battery; but don't lower your comfort, your eyes depend on it.
Denis.