Why do batteries regain some of their charge after a short rest?
Here are the facts:
I am a civilian MedEvac helicopter pilot. The other night, I went to hospital A to pick up a patient to take her to hospital B’s cardiac catheterization lab.
We land & shut down. By the way, thank you, cell phone provider, for putting a tower right on the only decent approach to the hospital pad! And thanks for making in 299 feet tall, instead of 300, so that it doesn’t have to have any lights (coming soon to a BBQ pit near you).
ANYWAY, we load the patient & I start the helicopter up. Just before the start is completed, the patient has a real, live, heart attack, I shut down the helicopter and the RN gives her the paddles. She comes back to life and everyone is happy. Except the helicopter battery is reading 22.0 volts. 24.0 is required for start, otherwise the computer does odd things and you risk making a giant “foom” sound from delayed ignition. And yes, aircraft batteries suck. It is the only part you only need when you are on the ground, so it is a good place to save weight.
Twenty minutes later, the battery reads 24.2V. I turn off EVERYTHING except one fuel pump, and start that puppy up. It wasn’t the fastest start, but it worked.
What is happening with a battery mysteriously regaining charge after a rest?
(Not-so-WAG) Batteries generate electricity through a chemical process. After being severely discharged, the active areas near the surface of the plates get depleted of the required chemicals. Letting the battery sit for a while allows fresh chemical to diffuse into the active region.