Sulphuric Acid. This is the electrolyte or liquid contained in a battery’s cells


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

Adding electrolyte to a dry battery.

Absorbed Glass Mat

A battery that does not contain any free liquid electrolyte. The electrolyte is absorbed in glass mat material located in each of the battery’s cells. AGM and VRLA batteries are the same design

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)

A hard brittle silver-white metal with a high lustre from the arsenic family. Chemical formula Sb, atomic number 51.

A metallic element, highly resistant to corrosion, used as protective plating on battery components. Chemical formula Cd, atomic number 48.

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.

A test that discharges a battery using a constant current at room temperature until voltage drops to 1.75 volts per cell.

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.

A battery cell’s maximum ability to deliver current (amps). The positive plates contain a maximum amount of lead oxide and a minimum of lead sulphate and the negative plates contain a maximum of sponge lead and a minimum of sulphate. The electrolyte is at maximum specific gravity.

A battery assembled with dry, charged, plates and no electrolyte.

A fully charged battery containing electrolyte (ready to be installed)

The process of converting electrical energy to stored chemical energy

The current (amps) in amperes at which a battery is charged.

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.

A battery charger that produces a constant current (amps) during the charging process

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.

Any specified amperage rate at which a battery is discharged

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.

Recharge voltage rate that is slightly higher than the open circuit voltage (OCV) of a battery

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.

Fabric made from glass fibres with a polymeric binder, such as styrene or acrylic, which is used to help retain positive active material. Glass mats also absorb electrolyte in an AGM battery.

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).

A chemical element, the principal constituent of a lead acid battery. Chemical formula Pb, atomic number 82.

A metal alloy commonly used in battery castings or plates.

A lead base alloy sometimes used for battery components in place of antimonial lead alloys.

A brown lead oxide which is the positive material in a fully formed positive battery plate.

The chief component of the active material of a fully formed negative battery cell plate.

A compound that results from the chemical reaction of sulphuric acid on oxides of lead within a battery cell.

The principal acid compound of Sulphur. Sulphuric Acid in diluted form is the electrolyte of a lead acid battery. Chemical formula H2SO4.

A low rate continuous charge approximately equal to a battery’s internal losses and capable of maintaining a battery at a fully charged state.

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.

One thousandth of an ampere (amp)

A charge in which the charging voltage is held constant while a fixed resistance is inserted in the battery charging circuit causing a rising voltage as charging progresses.

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

The grid and active material that current flows to from the external circuit when a battery is discharging.

The battery terminal from which current flows through an external circuit to the positive terminal when a battery discharges.

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 flooded lead acid 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.

The battery terminal that current flows toward in an external circuit when the battery is discharging.

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

Amp Hours of discharge that can be removed from a fully charged battery at a specific constant rate.

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 in 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.

Gradual loss of electrical energy when a battery is stored.

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 difference in electrical potential that exists between the terminals of a battery or any two points in an electrical circuit.

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.

Valve Regulated Lead Acid. A sealed battery that features a safety valve designed to release excessive internal pressure while maintaining sufficient pressure for recombination of oxygen and hydrogen into water. VRLA and AGM refer to the same type of battery design.

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. In a Lead Acid battery. the voltage developed is approximately 2.1 volts per cell. 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 converted to lead sulphate. The plates become more alike and the sulphuric acid becomes weaker – consequently the voltage drops since it depends on 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. Methods of recharging a storage battery include the use of an alternator or a dynamo in an internal combustion engine or an external charging unit (“a battery charger”). As the battery becomes charged, the electricity begins to hydrolyse the water in the electrolyte into its constituent components of hydrogen and oxygen, which is released as gas. This is why a battery gasses when it is charged. The loss of the hydrogen and oxygen through gassing is why it is sometimes necessary to replenish a battery’s electrolyte with distilled water. In a Maintenance Free battery, the gases are not lost and the gases are recombined with the electrolyte.

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 over time.



Modern car charging systems allow only a small current to flow into the battery when it is fully charged. If there is a fault in the alternator, a much higher current will pass through the battery all the time that the car is running. This current will cause the battery to lose water rapidly, destroying the maintenance free characteristics of the battery, and will also reduce the life of the battery by damaging the positive grids. A dark brown/black colour on the bottoms of the vent-plugs is a strong sign of overcharge. If an alternator (non Start-Stop vehicle) has a voltage above 14.8 Volts at normal temperatures, this is a generally sign that the charging system is faulty. If a common diode fault in the rectifier sees charging voltages of 16.0V at the battery, the alternator should be repaired immediately to prevent any further damage to the battery.

Note: with latest Start-Stop vehicles with Brake Energy Regeneration, higher voltages (15.2V) are used to maximise charging efficiencies and reduce alternator charging periods.


Modern charging systems keep a battery in a high state-of-charge whist a car is running under most operating conditions. However, the battery will discharge under abnormal conditions or if the car is allowed to stand with a load on, for example, with the headlights on. In addition, on most modern cars when parked there is normally a constant drain on the battery caused by such components as the computer, alarm system, clock etc, and this will cause the battery to become discharged. Depending on the vehicle, this can take weeks or months. Vehicle batteries are designed to accept some cycles of discharge and recharge, but are not designed for applications in which there are constant cycles of charge and discharge (deep cycling). Leisure batteries have been designed for these types of application, and have a special construction to enable them to be deep cycled on a continuing basis. Continual deep cycling of vehicle batteries will cause failure as the positive active material will gradually fall to the bottom of the battery, reducing the ability of the plates to store electricity. A large number of small black/brown particles in the electrolyte are a strong indication that the battery has been deep cycled.


Sulphation is a normal part of the operation of a battery and occurs whenever a battery is discharged. When the battery is recharged, the sulphation (lead sulphate) is changed back into active material. If a battery is left flat for a period of time, the lead sulphate slowly crystallises into a form that cannot be changed back into active material on charging. Therefore, if this has happened, after charge, the battery will not return fully to give its original performance. If the sulphation is bad enough, the battery will be compromised enough so that a car will not start.


Undercharge occurs if the battery is not receiving enough charge to return it to a full state-of-charge; this will slowly cause sulphation. This fault can occur if the car is being used only occasionally for short journeys, or for Start-Stop urban motoring. Undercharge will also occur if an alternator or dynamo is not supplying enough charge.


Replacing the battery on a modern vehicle is no longer a straightforward fit-and-forget task.

On vehicles with advanced technology such as Start-Stop, battery replacement can be complex and time consuming.

It is essential the battery on these vehicles is replaced with one of the correct specification. If a vehicle is fitted with an AGM Start-Stop battery then the replacement must also be AGM Start-Stop. The same applies to EFB products.

Most modern vehicles come fitted with a Battery Management System (BMS). This must be reset through the on-board diagnostics (OBD) port to tell the vehicle when the battery has been replaced.

Failure to fit the correct battery or reset the BMS can cause issues such as battery failure and complete loss of Start-Stop functionality.

On many new vehicles the battery is not fitted under the bonnet and locating it can take time. The replacement process can also be extremely time consuming, with some vehicles requiring up to an hour and a half of work.