A battery stores chemical energy and releases it as electricity on demand. Inside, two electrodes sit in an electrolyte. Chemical reactions push electrons from one side to the other through an external circuit. That flow of electrons is the current that powers a phone, a car, or a grid.

Batteries come in many shapes. Lithium-ion dominates consumer electronics and electric vehicles. Lead-acid still starts most gasoline cars. Flow batteries store large amounts of energy for the grid. Sodium-ion and solid-state designs are emerging, promising lower cost or higher safety.

Key metrics matter. Energy density determines how much power a battery can hold per kilogram. Power density determines how fast it can discharge. Cycle life determines how many times it can be recharged. Cost per kilowatt-hour drives adoption. Engineers trade these off depending on the application. A phone needs light weight. A grid battery needs durability and low cost.

Common types

• Lithium-ion
• Lead-acid
• Nickel-metal hydride
• Flow batteries
• Solid-state (emerging)