Batteries

A battery is a collection of cells.  A cell generates electricity by chemical reaction.  It consists of 2 metal conductor called electrodes immersed in a solution called electrolyte.  The chemical interaction between the electrodes and the electrolyte produce a separation of charge - one electrode become positively charged and one become negatively charged..  When a conductor connects up the terminals of the cell, a current flow from the negative terminal to the positive one.  The voltage generated depends on the choice of the types of electrodes and electrolyte.

A cell is represented schematically by 2 lines.  The longer on represents positive terminal and the shorter one is the negative one.

Primary cells cannot be recharged.  When the chemical reaction stops, no charge will be produce and the battery needs to be replaced.

Alkaline - 1.5v (AA, AAA, C, D cells)
Mercuric Oxide - 1.35v (for watches and calculator)
Silver Oxide - 1.6v (for hearing aids, watches)

Secondary cells can be recharged.  In other words, the chemical reaction can be reversed when the battery is connected to an external DC voltage.  The electrodes and electrolyte can be rejuvenated.

Nickel-cadmium - 1.2v (tools)
Nickel metal hydride - 1.2v (laptop, cell phone)
Lead acid - 2.1v (cars)
Lithium ion - 4.1v (cell phone, laptop, iPod)

The amount of current produced by cell depends on the its size and the quantity of the materials use.  Large electrodes and electrolyte produce more current but voltage stays the same.  The type of material, not the volume determine the voltage.

For example, both AA and the D cell made of same material produce 1.5v cell.  The D cell can last longer because of its larger size.

When connect batteries in series, the overall voltage increases proportionally.  When connect battaries in parallel, the current increases but the voltage stays the same.

Solar cell (photovoltaic PV cell) convert light into electrical energy.  A solar cell in space satellite is often used to charge the secondary cell (Nickel-cadmium) rather than operating the equipment directly.

Fuel cell combines oxygen and hydrogen to produce voltage and current.  Heat and water is the byproducts. Fuel cell generates about 0.5v to 0.9v and so many of them is needed to produce work.