When a reusable battery loses its stored charge, it can be recharged by applying a charging current that converts chemicals in the battery into stored electricity. The battery stores this charge until needed again, when the reverse chemical reaction releases the electricity stored in the battery. Charging current is what allows the battery to be used repeatedly, and how the current affects the battery depends on the chemicals used in it.
Lead-acid batteries are widely used in transportation equipment, solar power storage, and other applications requiring large electrical storage capacity. These batteries are made from a series of lead plates kept in a sulfuric acid and water mixture. A chemical reaction occurs between the lead and acid, and electrical current is produced. Each cell in a lead-acid battery creates about 2.2 volts, so a 12-volt battery will have six cells and a full charge slightly over 13 volts.
When a lead-acid battery is discharged repeatedly or ages, the lead and acid reaction creates lead sulfate, which eventually can coat the lead plates and cause the battery to fail. Proper charging current can reverse some of this reaction, called sulfation. Technology developed in the late 20th century, called pulse charging or pulse width modulation, can reverse sulfation to a great degree and restore good electrical capacity to older batteries.
Charging current must be controlled or regulated carefully, because excess power sent to a battery will cause it to overheat. Hot batteries not only have lower charge capacity, but also can fail if the water boils away or evaporates due to excess heat. Many chargers use charge controllers to lower the current flow as the battery charges, and some can check battery temperature to prevent overheating.
Smaller rechargeable batteries, including nickel metal-hydride and lithium-ion batteries, can, in some cases, be recharged. Nickel-hydride batteries are sensitive to charging current, and if a weaker battery is placed in a charger with stronger batteries, they may not accept the charge properly. Many of the chargers contain circuits that charge each battery separately, rather than combining them in one circuit. Separate charging allows each battery to receive a specific current to optimize its recharge.
Charging current also refers to the electrical power required to charge a capacitor. A capacitor is a solid-state device containing two plates made of a material that can conduct or pass electrons. The two plates are separated by a dielectric material, which resists electron flow to some degree. When the capacitor is charging, current flows to one plate, creating an excess negative charge. At the same time, the opposite plate is developing a positive charge.
This stored electrical charge acts as a battery, and can be stored for long periods of time. When a switch connects the capacitor to an electrical circuit, the electrons pass through the dielectric and into the positively charged plate, creating a flow of electricity. The electric current will flow until the capacitor is discharged, at which time it can be recharged repeatedly. Capacitors are used widely in electronics to provide different functions, including voltage and power control.