Recycling batteries is one of the most important recycling practices. Recycling glass or paper is essential, due to the cost efficiency or forest protection. In addition to the reasons listed above, recycling batteries has one more crucial reason: batteries contain toxic chemicals. Releasing these chemicals into the environment (for example, when batteries end up in a landfill) can cause significant water and soil pollution. Scientific studies  have demonstrated that mercury, cadmium and lead of the batteries present the major environmental problems.
How do batteries work?
A battery stores energy in the form of chemical energy and transforms this chemical energy into electricity, when connected to an electrical circuit. An electrical circuit is basically a flow of electrons in one direction, or a flow of positive ions in the opposite direction. These charges (electrons or ions) flow through a conductive path, for example, a wire. Such a wire thus forms an electric circuit.
Any battery consists of three main parts: a cathode (+), and anode (-) and an electrolyte to separate them. When the battery is not connected to an electrical circuit, the electrolyte prevents cathode and anode from exchanging charges. However, when we install a battery into a circuit, there is an external path – this circuit – following which electrons can reach the cathode. Therefore, an electrical current starts flowing through the circuit. The chemical reactions in the battery define the amount of charges this battery can produce. Some batteries – the disposable ones – can only transform chemical energy into electrical energy. Therefore, they can only used once. Other batteries – rechargeable – also work in an opposite direction, when electrical energy from an external source can be transformed back into chemical energy. Therefore, such batteries are multiple-use, and are thus better for the environment.
Currently, all types of batteries can be recycled. Most of them are even 100% recyclable, meaning every part of the used battery ends up as a part of a new product (usually a battery as well). Some batteries – like car batteries – always end up at a recycling facility, due to the fact that recycling is much more energy- and cost-efficient. But the batteries that we use in our everyday life – for example, in a TV remote – still often finish their life cycle in a landfill.
Lead battery recycling
Let us briefly describe the procedure of battery recycling on an example of lead acid batteries. After their arrival at a recycling facility, the batteries are crashed into small pieces. The next step is to separate the different components of these small pieces. To do that, battery pieces undergo a special procedure, as a result of which lead and other heavy metals fall to the bottom of a special vat, while the plastic pieces float. Therefore, there are now three separate material components, all separated from each other: lead and other metals, plastic and acid.
The lead pieces proceed to hot furnaces, where they melt. When the lead melts, impurities float on its surface, which allows to clean the lead from these impurities. The cleaned lead then goes back to battery manufacturers. Plastic pieces also end up back at the battery manufacturer, to become parts of new batteries again. Old battery acid mostly undergoes a neutralization procedure, as a result of which it turns into water.
You can find more detailed information about recycling procedures for this and all the other battery types here.
 Bernardes, A. M.; Espinosa, D. C. R.; Tenorio, J. A. S. “Recycling of batteries: a review of current processes and technologies”. Journal of Power Sources. 130 (1–2): 291–298. doi:10.1016/j.jpowsour.2003.12.026.