Less than full

I am in the habit of filling my car's gas tank when it is 50% empty.  I don't want to be surprised by running out.  It can be cold and miserable outside, even dangerous.

I recently read in at least two different articles that my internet devices such as cellphone and tablets will last longer if I charge them only to 80% of their capacity.  How much "longer"?  Such a practice was said to MAYBE double the usable length of the device's battery.