Why do dead batteries appear to recharge on their own after awhile?

The other night, I found myself crashing at a friend's house without a MicroUSB charger handy. As such, my phone (Nokia Lumia 920) died before I went to bed - attempting to boot would simply yield a "low battery" screen. When I awoke about 6 hours later, I decided (in a moment surely befitting the definition of insanity) to try turning the phone on again. The phone booted this time, showing about 12% battery remaining. Aside from the expected "low battery" warning messages, I was allowed to use the phone normally for awhile before I finally got back into my car and was able to charge it there.

I'm pretty sure I've seen this phenomenon in other small electronics (usually phones). A device will have drained its battery to the point of being un-bootable but, after a few hours or so, I'll later try again and find it coming to life as if nothing was wrong.

What causes this?

Answer

Batteries work by way of a redox reaction. The reaction can only occur so fast, thus limiting the current the battery can provide. As more current is drawn from the battery, the voltage decreases.

As the battery becomes depleted, there are less of these reactants available to drive electric current. However, if the battery is left to sit for a while, the reaction can proceed a bit, building up an excess of electrons at one terminal, and a lack at the other.

As soon as the battery terminals are connected with a conductor, a current results as the electric charge imbalance attempts to reach balance. However, pretty soon all that imbalance that was developed as the battery sat on the shelf has been used, and the chemical reaction must continue to create more imbalance to continue driving the current from the battery. If the battery is nearly dead, this reaction can't happen very fast, as nearly all of the reactants in the battery have already reacted.

So, the battery on the shelf isn't "recharging", because it's not gaining any more energy. You are simply giving the chemicals inside the battery more time to react, converting more of the chemical energy that was already there into electrical energy.