The power supplied by a battery is the result of a chemical reaction in which electrons flow between two different metals. One of these-for instance zinc-binds electrons less tightly to its atoms than the other-for instance copper. If the metals are immersed in and electrically conductive liquid, which provides a connection between them, the electrons are able to travel and form a surplus on the copper. This surplus then flows as a current as soon as an external circuit connected to the battery provides an opportunity to do so.
Cold slows down every kind of chemical reaction, which is why fewer electrons are released in batteries when the temperature drops. However, warming a battery up often returns it to near full power. This is why many photographers carry a spare camera battery in an inside pocket during winter. Although special additives have made car batteries less sensitive to the cold, they can stop working altogether in winter-especially if they are very old.