Why do batteries loose omph when it gets cold?

[TSN79]

Why do batteries loose "omph" when it gets cold?

Simple question really;
It's well known that car batteries (and others) don't work as well when the temperature approaches -20C. I'm just wondering why? What's the physical reason? Also, if you bring the battery back to room temperature, will it regain its "strenght" to where it was before it got cold?

 

[mgb_phys]

Chemical reactions slow down at lower temperatures.
It also increases the internal resistance so although the amount of energy stored in the battery is the same - the amount you can get out drops.

Providing that the electrolyte doesn't freeze and burst the battery then lead acid typically recover, in fact storing them at low temperatures reduces the self discharge rate.