Battery cell
Cells are individual cylindrical compartments in a battery that produce power. As many as 12 cells are used in a notebook battery.
Capacity
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.
Degradation
The process by which the chemicals in a battery lose their ability to hold a full charge. See memory effect.
Discharge
This describes using the power stored in a battery by chemically depleting the charge.
Electrolyte
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.
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.
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.
Watt-hour
A watt-hour is a measurement of the amount of energy held in a battery that can power a one-watt device for one hour. Many external batteries have between 100 and 200 watt-hours of energy.
Dear reader,
Don't get confused by the descriptions above about the flow of electrons to and from the positive and negative electrodes.
A negative electrode is negative because it carries a surplus of negative charge. As electrons are negatively charged, this means that the negative electrode has an excess of electrons.
Similarly, the positive electrode is positive because it has a lack of electrons - and it is the tendancy of the electrons to move in such a way as to balance this out that causes electric current.
In other words, electrons will flow out of the negative electrode into the external circuit (in this case, the laptop's electonics), through this circuit and into the positive electrode.
The place to which the description refers is from the electrode into the electrolyte - the conductive paste that is between the electrodes inside the battery. Electrons move from the electrolyte onto the negative electrode, and from the positive electrode into the electrolyte.
The direction of flow (from the negative to the positive) is called electron current.
However, because our brains like to think in terms of positives, and because the early experimenters with electricity did not realise that the current was being carried by negatively charged particles, they defined current in terms of flow of positively charged particles from the positive to the negative. This is called conventional current.
A way of visualising this in electron flow is the opposing flow of "holes" into which an electron can move. As an electron moves forwards in the direction of electron current, the (effectively positively charged) hole moves backwards - in the direction of conventional current.
The effect is identical - charge is carried around a circuit through charged particles.