Batteries are an essential part of portable electronic devices, providing a source of power that allows the device to run for extended periods of time without needing to be directly connected to an electrical outlet. Batteries generate enough power by converting stored energy into electricity that devices can use. The power requirements of the device, the capacity and chemistry of the battery combine to determine how long the device can operate on a single battery charge. The chemical reactions that take place inside the battery generate electricity to power the device. In the case of lithium-ion batteries, these chemical reactions take place at the positive (cathode) and negative (anode) electrodes of the battery. Lithium-ion batteries are popular and commonly used in many devices because of their high energy density, long life, and low self-discharge rate.

When a lithium-ion battery is charged, lithium ions migrate from a cathode, usually made of lithium cobalt oxide, to an anode, usually made of graphite. During this process, the energy generated by the battery is stored in the anode in the form of electrochemical potential energy. During discharge, lithium ions migrate back to the cathode through the electrolyte solution, and the potential energy is released in the form of electron flow.

The amount of power a battery can produce depends on its capacity, measured in milliampere-hours (mAh) or watt-hours (Wh). The capacity of the battery determines how long the device can run on a single charge. For example, a device with a battery capacity of 4000mAh is likely to last longer than one with a capacity of only 2500mAh.

The chemical composition of a battery also plays a crucial role in determining how much power it can generate. Different battery chemistries have different energy densities, discharge rates, and cycle lives. Lithium-ion batteries, for example, have an energy density of around 100-265Wh/kg, which means they store and generate more electricity per unit weight than other rechargeable batteries like metal hydride nickel or lead-acid batteries.

One factor that limits a battery's power output is its internal resistance, which can cause a voltage drop under load. The resistance inside the battery increases over time and can cause the battery to produce less charge. A battery with lower internal resistance will have a higher power output, especially with a heavier load.