How a battery works


How a battery works

Nickel-metal-hydride battery

By removing the cadmium and using nickel hydride instead, these batteries are made to hold more energy, but they can't be recharged more than a few hundred times. They are generally referred to as NiMH.

Porous separator

This permeable material or membrane separates the battery's two electrodes and allows current to flow from the positive to the negative electrode.


This refers to the amount of energy a battery contains. The typical notebook battery has between 2,000 and 6,000 milliamp hours (mAh) of capacity. See milliamp hours.

Charge cycle

This describes the complete charge and discharge cycle of the battery. Fully draining the battery then recharging it is one charge cycle.


The process by which the chemicals in a battery lose their ability to hold a full charge. See memory effect.


This describes using the power stored in a battery by chemically depleting the charge.


This chemical carries electrons while the battery is being used.

Energy density

This term describes how much energy a battery contains, based on its watt-hour capacity divided by its weight; many external batteries have between 100 and 200 watt-hours of energy.

Fuel cell

This refers to any of various devices that convert chemical energy directly into electrical energy. They are different from batteries because they use liquid fuel to produce electrical energy, whereas batteries use reversible chemical reactions.

Lithium-ion battery

These batteries use lithium for the negative electrode and offer high energy density and the ability to undergo repeated charge cycles.

Lithium-ion-polymer battery

Similar to a lithium-ion battery, a lithium-ion-polymer battery uses a conductive plastic and is more malleable than traditional lithium-ion batteries. Lithium-ion polymer can be moulded into different shapes, which can be critically important to the makers of small devices, such as mobile phones.

Memory effect (a.k.a. memory degradation)

Not to be confused with computer memory, this is the loss of the ability to fully recharge a battery, which happens over a long period of battery use.

Milliamp hour

This is the main battery capacity rating, equal to one-thousandth of an amp-hour, generally referred to by its acronym: mAh. The typical notebook battery has between 2,000 and 6,000 milliamp hours of capacity.

Negative electrode

This is the conductive part of the battery to which electrons flow.

Nickel-cadmium battery

Also known as NiCd, this is the original battery technology used in notebooks. In using cadmium as the negative electrode, these batteries have a relatively low energy density and suffer from memory effects.

Positive electrode

This is the conductive part of the battery; electrons flow away from it.

Rechargeable battery

This is a battery that can be used repeatedly by adding power to it when the cells are drained. These batteries typically can go through a few hundred charge cycles before they start to lose the ability to hold a charge.


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