I carry a small 2-AA flashlight, and a few times, I have accidentally left it on in my holder. Each time it dies completely, so that no light is emitted, but if I turn it back off and wait for a few minutes, the light works weakly again. The longer I leave it off, the longer the dead battery lasts. I've noticed similar patterns in my mobile phone battery.
Why does this happen?
Answer
The actual process is dependent on the type of battery we are talking about. In a lead acid battery,
The cell voltage will rise somewhat every time the discharge is stopped. This is due to the diffusion of the acid from the main body of electrolyte into the plates, resulting in an increased concentration in the plates. If the discharge has been continuous, especially if at a high rate, this rise in voltage will bring the cell up to its normal voltage very quickly on account of the more rapid diffusion of acid which will then take place.
from here.
In general, you can think of it as a normalizing of the chemicals involved. There isn't any more "life" in the battery, the life remaining is just in the correct place to give you a little use.
Also, read this answer for basic battery info
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