The active material in the positive plates of a battery is lead dioxide and that in the negative plates is metallic sponge lead. When an electrical circuit is created, these materials react with sulphuric acid during charging and discharging according to the following chemical reaction

PbO2 + Pb + 2H2SO4 = 2PbSO4 + 2H2O

AMPERE (Amp., A.)
The unit of measure of the electron flow rate, or current, through a circuit

AMPERE-HOUR (Amp.-Hr., Ah.)
A unit of measurement for a battery’s electrical storage capacity, obtained by multiplying the current in amperes by the time of the hours of discharge. (For example, a battery which delivers 5 amperes for 20 hours delivers 5A x 20Hr = 100Ah of capacity)

The ability of a fully charged battery to deliver a specified quantity of electricity (Ah) at a given rate (A) over a definite period of time (Hr). The capacity of a battery depends on a number of factors such as: active material weight, density of the active material, adhesion of the active material to the grid, number, design and dimensions of the plates, plate spacing, design of separators, specific gravity and quantity of available electrolyte, grid alloys, final limiting voltage, discharge rate, temperature, internal and external resistance, age, and life history of the battery.

The basic electrochemical current-producing unit in a battery, consisting of a set of positive plates, negative plates, electrolyte, separators and casing. There are six cells to a 12 volt lead acid battery.

An electrical circuit is the path followed by a flow of electrons. A closed circuit is a complete path. An open circuit has a broken, or disconnected, path.

A circuit which only has one path for the flow of current. Batteries arranged in series are connected with the negative of the first to the positive of the second, negative of the second to the positive of the third and so on. If two 12 volt batteries of 50 Ah capacity each are connected in series, the circuit voltage is equal to the sum of the two battery voltages, or 24 volts, and the ampere-hour capacity of the combination is 50 Ah.

A circuit provides more than one path for the flow of current. A parallel arrangement of batteries (of like voltages and capacities) has all positive terminals connected to a conductor and all negative terminals connected to another conductor. If two 12 volt batteries of 50 Ah capacity each are connected in parallel, the circuit voltage is 12 volts and the ampere-hour capacity of the combination is 100 Ah.

The number of amperes a lead-acid battery at zero degrees Fahrenheit (-17.8 degrees centigrade) can deliver for 30 seconds and maintain at least 1.2 volts per cell.

The destructive chemical reaction of a liquid electrolyte with a reactive material. (e.g. dilute sulphuric acid on iron, producing corrosion products such as rust.) Battery terminals are subject to corrosion if they are not properly maintained.

The rate of flow of electricity, or the movement of electrons along a conductor. It is comparable to the flow of a stream of water. The SI unit of measure for current is the ampere (A)

A current that varies periodically in magnitude and direction. A battery does not deliver alternating current.

An electrical current flowing in an electrical circuit in one direction only. A battery delivers direct current (DC) and must be recharged with direct current in the opposite direction of the discharge.

In a battery, one discharge plus one recharge equals one cycle.

When a battery is delivering current, it is said to be discharging.

In a lead-acid battery, the electrolyte is sulphuric acid diluted with water. It is a conductor that supplies water and sulphate for the electrochemical reaction.

PbO2 + Pb + 2H2SO4 = 2PbSO4 + 2H2O

In a battery a set of positive and negative plates assembled with separators.

In battery manufacturing, formation is the process of charging the battery for the first time. Electrochemically, formation changes the lead oxide paste on the positive grids into lead dioxide and the lead oxide paste on the negative grids to metallic sponge lead.

A lead alloy framework that supports the active material of a battery plate and conducts current.

The reference potential of a circuit. In automotive use, the result of attaching one battery cable to the body or frame of a vehicle which is used as a path for completing a circuit in lieu of a direct wire from a component. Today, over 99% of automotive and LTV applications, use the negative terminal of the battery as the ground.

A float type device used to determine the state of charge of a battery by measuring the specific gravity of the electrolyte. (i.e. the concentration of sulphuric acid in the electrolyte).

An instrument which draws current (discharges) from a battery using an electrical load while measuring voltage. It determines the battery’s ability to perform under actual discharge conditions.

A battery which does not require periodic water addition under normal conditions. Also known as a maintenance free battery.

Designating, or pertaining to, electrical potential. The negative battery terminal is the point from which electrons flow during discharge.

The SI unit of electrical resistance. Also a unit of electrical impedance within an electrical circuit.

Expresses the relationship between volts (v) and amperes (A) in an electrical circuit with resistance (R). It can be expressed as follows

V = IR

Volts (v) = amperes (I) x Ohms (R). If any two of the three values are known, the third can be calculated using the above calculation.

The voltage of a battery when it is not delivering or receiving power. It is 2.11 volts for a fully charged battery cell, or 12.66 for a fully charged 12 volt battery (6.33 for a 6 volt battery).

Designating, or pertaining to, a kind of electrical potential; opposite of negative. A point or terminal on a battery having lower relative electrical potential.

This type of battery can store and deliver electrical energy, but cannot be recharged.

The time in minutes that a new, fully charged, battery will deliver 25 amperes at 80 degrees Fahrenheit and maintain a terminal voltage equal to, or higher than, 1.75 volts per cell. This rating represents the time the battery will continue to operate essential accessories if the alternator or generator of a vehicle fails.

The opposition to the free flow of current in a circuit. It is commonly measured on Ohms.

A battery which can store and deliver electrical energy and can be recharged by passing direct current through it in a direction opposite to that of discharge.

A divider between the positive and negative plates of an element which allows the flow of current to pass through it. Separators are made from numerous materials, such as polyethylene, polyvinyl chloride, rubber, glass fibre, cellulose, etc.

An unintended current by-pass in an electrical device or wiring, generally very low in resistance and thus causing a large current to flow. In a battery, a cell short circuit may be permanent enough to discharge the cell and render the battery useless.

The density of a liquid compared to the density of water. The specific gravity of the electrolyte is the weight of the electrolyte compared to the weight of an equal volume of pure water.

The amount of electrical energy stored in a battery at any given time expressed as percentage of the energy when fully charged.

The SI unit of measure for electrical potential.

The net difference in the electrical potential (voltage) when measured across a resistance or impedance (ohms). Its relationship to current is described in Ohm's Law.

The SI unit for measuring electrical power. (i.e. the rate of doing work, in moving electrons by, or against, an electrical potential.

Formula: Watts = Amperes x Volts

WATT-HOUR (Watt-Hr., WH)

The unit of measure for electrical energy expressed a Watts x Hours.


Battery Basics

How a Battery Works
A storage battery is an electromechanical device. It stores chemical energy which can be released as electrical energy upon demand. When a battery is connected to an external load, such as a starter, the chemical energy is converted into electrical energy and current flows through the circuit.

Storage batteries are used in a huge variety of fields, such as automotive – as starter batteries, power for lighting units, leisure, marine, agricultural, UPS, load levelling and much more.

How it operates
When two unlike materials such as the positive and negative plates are immersed in sulphuric acid (electrolyte) a battery is created and voltage is developed. The voltage developed depends upon the types of materials used in the plates and the electrolyte used. The voltage is approximately 2.1 volts per cell in a typical lead acid battery. Electrical energy is produced by the chemical reaction between the different materials and the electrolyte. When the chemical reaction starts, electrical energy flows from the battery as soon as there is a circuit between the positive and negative terminals. The electrical current flows as electrons through the outside circuit and as charged ions between the plates inside the battery.

Discharge Cycle
When a battery is connected to an external load, current flows and it starts to become discharged. The lead dioxide (PbO2) in the positive plate is a compound of lead (Pb) and oxygen (O2). Sulphuric Acid (H2SO4) is a compound of Hydrogen (H2) and the sulphate radical (SO4). As the battery discharges, lead (Pb) in the active material of the positive plate combines with the sulphate (SO4) of the sulphuric acid and forms lead sulphate (PbSO4) in the positive plate. Oxygen (O) in the active material of the positive plate combines with Hydrogen (H2) from the sulphuric acid to form water (H2O) which reduces the concentration of acid in the electrolyte.

Concurrently, a similar reaction is occurring at the negative plate. Lead (Pb) from the negative active material combines with sulphate (SO4) from the sulphuric acid to form lead sulphate (PbSO4) in the negative plate.

As the discharge progresses, the sulphuric acid in the electrolyte is being diluted by the water being created thereby reducing the specific gravity of the electrolyte. The specific gravity of the electrolyte can be measured by a hydrometer which provides an accurate and convenient method for determining the state of charge of a battery.

During discharge, the active material of both positive and negative plates is being converted to lead sulphate. The plates become more alike and the sulphuric acid becomes weaker – consequently the voltage drops since it depends of the difference between the two plates and the concentration of the acid. Eventually the battery can no longer deliver electricity at the required rate and is said to be discharged.

A discharged storage battery can be recharged (pass electrical current through it in the opposite direction of the discharge) and its active materials will be restored to their original composition. When fully recharged the battery is again ready to deliver its full power. This discharge / charge cycle can be repeated over and over until eventually plate or separator deterioration or some other factor causes the battery to fail.