The battery in a car is one of the most important components that allows the car to function. Without it, the car would not start, run the lights, or even bump the radio. For the battery to work properly, there has to be a flow of electrons through the circuits in the vehicle from the positive terminal to the negative terminal which creates the usable current flow.
The way this is created in a car battery is by the use of two dissimilar metal plates, both typically consisting of lead except the positive being a lead dioxide with antimony and the negative being a porous lead called sponge lead with calcium. These metals are then submerged in an acid mixture consisting of 36% sulfuric acid and 64% water. When these terminals are connected through a circuit, a chemical reaction is created within the battery causing electrons to flow. During the chemical reaction in the battery the sulfuric acid is reduced while water is increased.
As we all know, water will freeze when the atmosphere reaches a certain temperature, especially in the winter time. Typically your battery will not freeze because the charging system is maintaining it at an optimal working condition. If the system fails or the battery becomes defective due to deterioration and old age, then it can become very dangerous because of the possibility of hydrogen gases becoming trapped in the ice. A simple spark from a friendly jump could cause the battery to explode as in the picture above.
In order to prevent any of this from happening, be sure to replace your battery or repair your charging system when signs of failure start to occur. A couple hundred bucks to get what you need fixed will be much cheaper than buying a whole new car like this guy.