Batteries have a life usually expressed in charge cycles.
Does that include everytime you plug in AC, even if battery fully charged?
If not, how discharged must a battery be for the recharge to count
(I actually work on assumption that batteries are good for about a year, but I am curious)
The charge cycle spec for lithium-ion is for a discharge to about 20% capacity and then back to fully charged. I don't know if there is a precise formula for how much the life is extended by not going all the way to 20%, but it must have some effect. My experience on my Inspiron 5160 with the nine-cell 96 watt-hour battery is that both have lasted about four years thus far. When they were new, they would power the computer for about three and a fraction hours. Now the capacity is down enough that I get about two hours. I don't allow them to discharge fully very often, which is probably why they still work.
There is a post that has a link to a good paper on battery life and care, but with the forum change my link no longer works. You could try a search for battery life to see if you can bring it up; perhaps you will have more success than I.
Dell Forum member since 2005