Electrolytes are liquid substances that act as a medium to conduct electricity. They are referenced in a number of different contexts, the most common including health and fitness, and various electronics and automotive disciplines. An electrolyte is full of ions, which are atoms that have some sort of net electric charge, either positive or negative. One that is dilute has a relatively small amount of ions for its volume, while a concentrated one has a high amount of ions.
In basic fuel cells, such as those used in automotive technology, an electrolyte acts as the liquid that allows ions to travel between the cathode and anode to keep the power-generating process underway, while keeping the reactive oxygen and hydrogen apart. In the type of fuel cell known as a proton exchange membrane cell, the substance specifically moves protons (positively-charged hydrogen ions) to the cathode from the anode where they are produced, with the end result being the production of water and electricity.
Batteries also employ an electrolyte of some sort, both to conduct electricity between the battery plates and to store energy on the plates themselves. The type used in a battery depends on the battery's type and purpose. Most car batteries, for example, use one that contains sulfuric acid, which is why they require careful handling. Alkaline batteries use an alkaline solution. Lithium batteries use a special organic electrolyte that freezes at much lower temperatures than more traditional water-based ones.
In human health, electrolytes aid in a number of vital bodily processes. Many heart and nerve functions, muscle control and coordination, and the body's ability to absorb fluids all depend on a healthy balance of electrolytes. The most common types found in the human body are sodium, potassium, magnesium, chloride, and calcium. Various hormones in the body help regulate intake of these substances, and the kidneys filter them out when they reach excessive levels. Unhealthy levels of consumption of any or all of them can cause serious health issues. The most common imbalances are hyper- and hyponatremia, or excessive and insufficient levels of sodium, and hyper- and hypokalemia, or excessive and insufficient levels of potassium.
Many sports drinks contain added potassium and sodium to help restore the body's proper electrolyte balance after intense physical exertion. After exercising heavily, it can be dangerous to ingest large quantities of water, because some sodium and other salts are lost as a person sweats. The water can dilute those that remain and cause physical problems. Additionally, a number of enhanced drinks exist specifically for children, and these may be helpful for regular use, particularly to restore a child's electrolyte balance after an illness that has included vomiting or diarrhea.