OK, let's differentiate "runtime" from "lifespan". Runtime is the length of actual time the battery will supply power sufficient to run the device based on some specific percentage of capacity. From a full charge at 100% to completely depleted at 0%, this would be the maximum capacity. So in the case of a brand new battery that has been fully charged properly, and since it's bad practice to allow the battery to discharge below annoy 10% let's assume for explanation purposes that the expected runtime for that battery to the point at which it indicates that it's time to charge (10%), would be 18 hours (or 2 hours for each 10%). So then from that we can extrapolate that if you only charge to 80% and use to 20%, you are using 60% of the battery's capacity, and at 2 hours per 10%, that means a runtime of 12 hours.
Whether you charge to 100% and use to 40%, or you charge to 80% and use to 20 %, or you charge to 60% and use until the phone powers down on its own at 0%, you will get 12 hours of runtime in all three scenarios.
Now, lifespan is the time from when the battery leaves the factory as a brand new baby boy or girl, until the time when it can no longer keep up with others on the dance floor. In numerical terms, if a battery is supposed to be able to take a charge of 3,300 mAh (its rated capacity), then one would expect it will do that the very first time it's charged. Each subsequent charge will reduce that capacity ever so slightly but over time will result in the battery having a significantly reduced capacity. A 10% reduction in capacity would mean the battery will now only hold 2,970 mAh, and a 20% reduction means it will now only hold 2,640 mAh.
A battery that's at 90% of rated capacity will supply 90% of the runtime for the same percentage of capacity used (since the capacity is now 10% less than before). Using the brand new 3,300 mAh battery above as an example we can calculate that each hour consumes 165 mAh to a 10% charge level remaining (3300-10%=2,970/18=165). So then if we use that same formula with the battery that has the 10% reduced capacity, we arrive at 148.5 mAh available with each 10% of power available (2,970-10%=2,673/18=148.5) . Since we need 165 mAh to give us 2 hours of runtime, we will be falling short by 10% in time, or 6 minutes and (165/60=2.75 per minute) and (148.5/2.75=54 minutes per 10% consumed).
SO...a battery that is only able to hold 90% of its rated capacity will yield a reduced runtime of an equivalent 10% less for the same numerical percentage of power used, all other things being equal. Therefore that battery which in the first example gave you 12 hours for the 60% consumed from between 20% and 80% charge, would now only give you 10 hours and 48 minutes (12 hours*60 minutes=720 minutes, 10.8 hours*60 minutes=648 minutes - or 10 hours 48 minutes).
And if that battery were at only 80% of rated capacity or "end of useful life" as deemed by the manufacturers, it would only supply 8 hours and 36 minutes for 60% of capacity. A full charge to fully depleted for a new battery as illustrated at the top, would supply 20 hours total, whereas one that is no longer "useful" (only able to hold a charge of 80% of original rated capacity), would only be able to run 16 hours before being completely dead.
In the case of someone whose phone battery is easily replaceable, this may not be such a big deal...and they may actually notice the reduced runtime sooner than the 20% threshold manufacturers specify, so they might decide to actually replace that battery even sooner than when the manufacturer indicates is time. On the other hand with someone like me whose phone is a Razr MAXX, I would likely be more concerned about lifespan since replacing the battery is a big hassle.
Edited to correct for a divergence in my math by 10%.
