Battery Basics: Voltage

Understanding voltage is a key element to troubleshooting electrical circuits.  Voltage is defined as "electric potential or potential expressed in volts" by Merriam Webster.  Voltage is nothing more than a measurement in the difference between two points.  If we measure the voltage of a starter battery using a Digital Multi Meter (DMM) we would see approximately 12.8 volts DC (VDC) for a charged battery.  That battery is able to exert 12.8 volts of electrical pressure.  A completely dead battery would measure at 0 volts DC and is incapable of exerting any electrical pressure, thus the term "dead".  To understand how the difference in electrical potential is created, a brief explanation of what happens on the atomic level is needed:

Atoms are made up of neutrons and protons in their core.  Electrons orbit around the core.  Normally an atom will be balanced and have an equal number of protons and electrons, which results in a neutral charge.

If an atom is not balanced it is called an "ion".  If an ion has more electrons than propns it would be considered to have "negative" potential or charge.  If an ion has fewer electrons than protons it would have a "positive" potential or charge. 

Atoms can be forced and held into either of these states by chemical or electromagnetic means.  When ions have a very strong attraction to one another, it can be measured as voltage.  The higher the voltage, the higher the electrical pressure.

Inside of a typical lead-acid cranking battery there are actually 6 separate batteries, referred to as cells.  Each cell can chemically produce approximately 2.2 VDC.  These cells are wired in series so that the total difference between the battery terminals is 12.8 VDC.