Electricity Basics

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All matter is composed atoms. Atoms have a nucleus with electrons orbiting around it. The nucleus is composed of protons and neutrons (not shown). In their neutral state, atoms have an equal number of electrons and protons. Electrons have a negative charge (-). Protons have a positive charge (+). Neutrons are neutral. The negative charge of the electrons is balanced by the positive charge of the protons. Electrons are bound in their orbit by the attraction of the protons.


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The old saying, “opposites attract,” is true when dealing with electric charges. Charged bodies have an invisible electric field around them. When two like-charged bodies are brought together, their electric fields repel one body from the other. When two unlike-charged bodies are brought together, their electric fields attract one body to the other.

The electric field around a charged body forms invisible lines of force. These invisible lines of force cause the attraction or repulsion.

During the 18th century a French scientist, Charles A. Coulomb, studied fields of force that surround charged bodies. Coulomb discovered that charged bodies attract or repel each other with a force that is directly proportional to the product of the charges and inversely proportional to the square of the distance between them. Today we call this Coulomb’s Law of Charges. Simply put, the force of attraction or repulsion depends on the strength of the charges and the distance between them.


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An electric current is produced when free electrons move from one atom to the next. Materials that permit many electrons to move freely are called conductors. Copper, gold, silver, and aluminum are examples of materials that are good conductors. Copper is widely used as a conductor because it is one of the best conductors and is relatively inexpensive.


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Electricity is the flow of electrons in a conductor from one atom to the next atom in the same general direction. This flow of electrons is referred to as current and is designated by the symbol “I”. Current is measured in amperes, which is often shortened to “amps”. The letter “A” is the symbol for amps.



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Elements are defined by the number of electrons in orbit around the nucleus of an atom and by the number of protons in the nucleus. A hydrogen atom, for example, has only one electron and one proton. An aluminum atom has 13 electrons and 13 protons. An atom with an equal number of electrons and protons is said to be electrically neutral.

Electrons in the outer band of an atom are easily displaced by the application of some external force. Electrons which are forced out of their orbits can result in a lack of electrons where they leave and an excess of electrons where they come to rest.

A material with more protons than electrons has a net positive charge and a material with more electrons than protons has a net negative charge. A positive or negative charge is caused by an absence or excess of electrons, because the number of protons in an atom normally remains constant.


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Electrons in the outer band can become free of their orbit by the application of some external force such as movement through a magnetic field, friction, or chemical action. These are referred to as free electrons.

A free electron leaves a void which can be filled by an electron forced out of orbit from another atom. As free electrons move from one atom to the next an electron flow is produced. This is the basis of electricity.


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Materials that allow few free electrons are called insulators. Materials such as plastic, rubber, glass, mica, and ceramic are good insulators.
An electrical cable is one example of how conductors and insulators are used together. Electrons flow along a copper conductor to provide energy to an electric device such as a radio, lamp, or motor. An insulator around the outside of the copper conductor is provided to keep electrons in the conductor.


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The relationship between Voltage, Current and Resistance was discovered by the German physicist Georg Ohm, (1787 – 1854). Georg Ohm found that, at a constant temperature, the electrical current flowing through a fixed linear resistance is directly proportional to the voltage applied across it, and also inversely proportional to the resistance. This relationship between the Voltage, Current and Resistance forms the bases of Ohms Law and is shown below.


Where: “V” equals Voltage in Volts (V), “I” equals Current in Amps (A) and “R” equals Resistance in Ohms (Ω).

By knowing any two values of the Voltage, Current or Resistance quantities we can use Ohms Law to find the third missing value.

To find Voltage (V): V=I x R, where V (Volts) = I (Amps) x R (Ohms)

For example, if the Amperage is 2A and the Resistance is 6Ω, then the Voltage is 12V.

To find Current (I): I = V ÷ R, where I (Amps) = V (Volts) ÷ R (Ohms)

For example, if the Voltage is 12V and the Resistance is 6Ω, then the Amperage is 2A.

To find Resistance (R): R = V ÷ I, where R (Ohms) = V (Volts) ÷ I (Amps)

For example, if the Voltage is 12V and the Amperage is 2A, then the Resistance is 6Ω.

To help you better understand this relationship between Voltage, Current and Resistance, below is a pictorial representation known as Ohms Law Triangle.



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Electrical Power, (P) in a circuit is the amount of energy that is absorbed or produced within the circuit. A source of energy such as a voltage will produce or deliver power while the connected load absorbs it. The quantity symbol for power is P and is the product of voltage multiplied by the current with the unit of measurement being the Watt (W). The Watt is equivalent to one Joule per seconds and was discovered by James Watt.


By using Ohm’s law and substituting for V, I and R the formula for electrical power can be found as (also known as Watts Law):

To find Power (P): P = V x I, where P (Watts) = V (Volts) x I (Amps)

For example, if you have a battery charger rated at 12V with a current rating of 2A, then the wattage of the battery charger will be 24W.

Power can also be found by:

To find Power (P): P = V² ÷ R, where P (Watts) = V² (Volts) ÷ R (Ohms)

For example, if the voltage is 12V and the Resistance is 6Ω, then the Wattage is 24W. Mathematically, 12V squared equals 144V, and then divided by 6Ω, equals 24W.

To find Power (P): P = I² x R, where P (Watts) = I² (Amps) x R (Ohms)

For example, if the Amperage is 2A and the Resistance is 6Ω, then the Wattage is 24W. Mathematically, 2A squared equals 4A, and then multiplied by 6Ω, equals 24W.

To help you better understand Power better, below is a pictorial representation known as Watts Law Triangle or the Power Triangle.

One other point about Power, if the calculated power is positive in value for any formula the component absorbs the power, but if the calculated power is negative in value the component produces power, in other words it is a source of electrical energy. Also, we now know that the unit of power is the WATT but some electrical devices such as electric motors have a power rating in Horsepower or hp. The relationship between horsepower and watts is given as: 1hp = 746W.



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A third factor that plays a role in an electrical circuit is resistance. Resistance is the property of a circuit, component, or material that opposes current flow. All material impedes the flow of electrical current to some extent. The amount of resistance depends upon composition, length, cross-section and temperature of the resistive material. For any specific material at a constant temperature, the resistance of a conductor increases with an increase of length or a decrease of cross-section.

Resistance is designated by the symbol “R.” The unit of measurement for resistance is ohms, symbolized by the Greek letter omega. While all circuit components have resistance, a resistor is a component manufactured to provide a designated resistance that is often shown in color coded bands around the resistor.



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The force required to make electricity flow through a conductor is called a difference in potential (pd), electromotive force (emf), or voltage. Voltage is designated by the letter “E” or the letter “V.” The unit of measurement for voltage is volts which is also designated by the letter “V.”

A voltage can be generated in various ways. A battery uses an electrochemical process. A car’s alternator and a power plant generator utilize a magnetic induction process. All voltage sources share the characteristic of an excess of electrons at one terminal and a shortage at the other terminal. This results in a difference of potential between the two terminals.

For a DC voltage source, the polarity of the terminals does not change, so the resulting current constantly flows in the same direction. The terminals of an AC voltage source periodically change polarity, causing the current flow direction to change with each switch in polarity.



Lead Acid Battery Information

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The lead acid battery was invented in 1859 by French physicist Gaston Planté. A lead-acid battery is an electrical storage device that uses a reversible chemical reaction to store energy. Lead acid batteries are the oldest type of rechargeable battery. Despite having low energy-to-weight ratio a correspondingly low energy-to-volume ratio; lead acid batteries have the ability to supply high surge currents (or large power-to-weight ratio).


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Generally, lead acid batteries are classified into two (2) groups, starting batteries and deep cycle batteries. Starting batteries, commonly called SLI, which stands for Starting, Lighting and Ignition are designed to deliver quick bursts of energy. Deep cycle batteries are designed to withstand continuous discharge cycles.

Starting batteries are commonly used to start combustion engines. Because they are designed to delivery short, but high bursts of current, these batteries feature a greater number of thin lead battery plates in order to discharge energy quickly over a short period of time. Starting batteries are not designed to handle multiple discharges. In fact, SLI batteries will only tolerate being completed discharged a handful of times before damaging the battery and decreasing the battery life.

Deep cycle batteries feature thicker lead battery plates which help make these types of batteries more resilient to deep discharges. However, deep cycle batteries cannot provide quick bursts of current like starting batteries, which make the less likely to be used for starting combustible engines. Deep cycle batteries can still be used as starter batteries, but a higher battery capacity (Ah rating) should be selected. Some batteries, such as “marine”, are classified as dual pro purpose, meaning these batteries can be used for both starting and deep cycle applications. Typically, dual purpose batteries have thinner battery plates than “true” deep cycle batteries, which make these batteries more prone to battery damage and shortened battery life.


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Lead acid batteries are produced in various types and chemistries, such as Wet Cell, Maintenance Free, Absorbed Glass Matt and Gel Cell.

Wet Cell – also referred to as “Flooded” batteries, contain electrolyte that must be continually maintenance to avoid battery damage or premature failure. Wet Cell lead acid batteries have a removable battery cap that allows the user to replenish the water in the electrolyte that has been dissipated as “gas” during the charge and discharge cycle. The gas generated by the electrolysis of water is a combination oxygen and hydrogen, which can be an explosive mixture. When Wet Cell lead acid batteries “gas”, the mixture is highly corrosive and typically result in battery corrosion on the attached battery terminals and battery cables. In most Wet Cell lead acid batteries 60% of the battery weight is comprised of the lead battery plates and 40% of the weight is the electrolyte. Electrolyte, or battery acid, contains 65% water and 35% sulfuric acid.

After extended use, a Wet Cell lead acid battery will begin to lose its ability to produce sufficient electricity. This is a result of the material on the positive battery plates flaking off during the discharging and charging cycles. As the battery plates continue to shed material, the battery plates become smaller (less surface area) and forms sediment at the bottom of the battery. As sediment continues to build, the battery will eventually short out and the battery will be completely destroyed. Wet Cell lead acid batteries are also more prone to damage or premature failure during extreme heat, excessive use, excessive vibration and over-charging.

Maintenance-Free (MF) – similar to Wet Cell lead acid batteries, except they require less routine maintenance. Maintenance-Free lead acid batteries also use a calcium alloy of lead instead of an antimony alloy, which reduces the amount of electrolysis. The amount of free-standing electrolyte above the plates is designed to be much higher, which means that there’s enough electrolyte to keep the plates covered even after a few seasons of normal use. Thus, during normal battery operation, there should be no need to add water to these types of batteries

Absorbed Glass Matt (AGM) – also known as “starved electrolyte” batteries, is a class of Value Regulated Lead Acid (VRLA) batteries in which the electrolyte is absorbed into a matt of glass fibers. Because of this design, AGM batteries do not spill electrolyte (even when damaged), nor do AGM batteries require regular addition of water into the electrolyte, nor are they susceptible to shock and vibration.

AGM batteries are a recombinant battery, which means the oxygen and hydrogen inside the battery will recombine, creating water. However, AGM batteries do have a pressure relief valve (ergo “Valve Regulated”) in the event that the rate of hydrogen evolution becomes dangerously high.

AGM batteries perform particularly in extreme ambient temperatures, both hot and cold. AGM batteries typically will not freeze in cold weather applications because of the lack of liquid electrolyte. Conversely, AGM batteries also perform well in hot weather applications because of their plate proximity and pure lead battery plates, which provides a lower internal resistance.

AGM batteries have a lower self-discharge rate when compared to flooded lead acid batteries. AGM batteries typically have a self-discharge of 1-3% per month, whereas, flooded lead acid batteries can self discharge up to 20% per month (or 1% per day).

Gel Cell – is similar to AGM batteries because the electrolyte is sealed by using a thickening agent (such as fumed silica) to immobilize the electrolyte. Similar to AGM batteries, Gel Cell batteries are a non-spillable and recombinant battery.

Both Gel Cell and AGM batteries can dispense charge at higher rates than flooded batteries due to their lower Peukerts exponent. Typically, flooded batteries cannot deliver more than 25% of their rated amp-hour capacity in amps without decreasing its Available Capacity.

Gel Cell batteries have the highest charge efficiency other than AGM batteries. Flooded batteries convert 15-20% of its electrical energy into heat instead of potential power, Gel Cell batteries about 10-16% and AGM batteries about 4%.


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The battery voltage of a lead acid battery is directly proportional to the number of battery cells wired in series. If the nominal voltage of a typical lead acid battery is about 2V per battery cell, then there is roughly 6 battery cells for a standard lead acid battery.

In general, the nominal voltage of a lead acid battery is roughly 2.10V to 2.13V per battery cell. Thus, a fully charged lead acid battery will have a voltage of 12.6V to 12.8V, whereas, a fully discharged lead acid battery would have a voltage of 11.8V to 12.0V. As you can see, the difference between a fully charged battery and a fully discharged battery is only roughly 1V. However, these values are different from battery chemistry to battery chemistry. Below are some estimates of battery voltages based on chemistry:



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A batteries State of Charge measures the available capacity of the battery, which is expressed as a percentage. State of Charge is rated from 0-100%, with 100% representing a fully charged battery and 0% representing a fully discharged battery.

A batteries State of Charge can be measured by reading either the terminal voltage or the specific gravity of the electrolyte. Terminal voltage can be measured with a volt meter, whereas, the specific gravity is measured by using a hydrometer. Battery hydrometers can only be used with flooded batteries, not sealed (Gel Cell or AGM) batteries.

Below is a SOC reference chart for terminal voltage and specific gravity readings for a 12V lead acid battery:



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The Depth of Discharge (DOD) for a lead acid battery is the measure of how deeply a battery is discharged. Similar to a batteries State of Charge, the Depth of Discharge is also measures as a percentage. A fully charged battery will have a DOD level of 0%, whereas, a fully discharged battery will have a DOD level of 100%.

Most lead acid batteries are adversely affected by the Depth of Discharge. Although some lead acid batteries are designed handle deep discharge cycles, such as deep cycle batteries, continued deep discharged may shorten the batteries life span. As a general rule, do not discharge your batteries more the 50% before recharging your batteries.


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The storage capacity of a lead acid battery is measure in Amp-Hours (Ah). The Amp-Hour is the amount of usable energy the battery will deliver at a constant rate of discharge over a period of 20 hours before the battery is completely discharged. Based on this definition, a 12V lead acid battery with a capacity rating of 100Ah, will last roughly 20 hours when connected to a 5A load.

Battery Amp-Hour ratings are important for selecting the appropriate battery and battery charger for a given application. When selecting a battery, determine the amount of load the battery system will need to handle. For example, let’s consider you have a bass boat with a trolling motor.


Based on some simple calculations, we can determine that you will need at least a 75Ah rated battery. However, as a general rule, add 20% (75 x 0.20) or 15Ah as a margin for error. As a result, you would need to select a battery that has Amp-Hour rating 90Ah (75Ah+15Ah).

Using this same example, you can select the appropriate battery charger and calculate the batter recharge times for recharging your batteries. Let’s say you have selected at 90Ah battery (typically a Group 27 battery, see Battery Group Sizes) and want to select a battery charger. First, calculate the required battery recharge time. To calculate the battery recharge time, take the Ah divided by the maximum amperage rating of the battery charger. In other words, Battery Recharge Time = Battery Ah Capacity / Maximum Amperage of Battery Charger. If we selected a 10A battery charger, the estimated recharge time is roughly 9 hours (90Ah divided by 10A). However, rarely are batteries completely depleted, so for practical purposes, use 50% of the Ah rating or 45Ah. Using this modified formula, the battery charge time would be 4.5 hours (45Ah divided by 10A). Thus, we can determine that selecting a 10A battery charger would be an appropriate choice because a 4.5 hour battery recharge time is very fast. However, it is always important to read the manual before selecting a battery charger. Battery charger are rated for different Ah capacities.

Another battery capacity rating is Reserve Minutes. Reserve Minutes is the number of minutes a battery can carry a 25A load (at 80°F) before becoming completely discharged. If you have a battery rated at 120 Reserve Minutes, you can theoretically sustain a 25A load for roughly 2 hours. However, for practical purposes, it is never a good idea to discharge your batteries more than 50%. Thus, a 120 Reserve Minute battery should only last around 1 hour.


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Battery manufacturers define the end-of-life of a battery when it can no longer hold a proper charge or when the available battery capacity is 80% or less than what the battery was rated for. The life of lead acid batteries depends on several factors:
Cycle Life – Cycle Life is a measure of how many charge and discharge cycles a battery can take before the battery plates deteriorate and short out. The greater the average depth-of-discharge, the shorter the cycle life (the opposite holds true).
Age – Over time, the chemicals inside the battery will be to deteriorate the battery plates.
Battery Chemistry – AGM and Gel Cell batteries typically have a longer battery life than flooded or Wet Cell batteries, because of their shock absorbance, self discharge rates and recombinant design.
Plate Thickness – Typically, the thicker the battery plates, the more abuse the battery can withstand.
Sulphation – Sulphation is the result of batteries continually being under-charged. As a result, a layer of lead sulphate can form into the battery cells and inhibit the electro-chemical reaction that allows you to charge/discharge batteries.


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Lead acid batteries come in different sizes, known as battery group size. Battery group sized are rated by the Battery Council International (BCI), which refers to the physical size of the battery. For more information, visit the Battery Council International (BCI) at www.batterycouncil.org. Below is chart of some popular BCI group size lead acid batteries: