WHAT IS A BATTERY?

A battery is a device for storing electrical energy in a chemical form, and then releasing it as direct current in a controlled way.

All types of batteries contain a positive and a negative electrode immersed in an electrolyte, the whole assembly being within a container.

All batteries are lead-acid batteries, which means that they have positive and negative electrodes made of lead compounds in a dilute sulphuric acid electrolyte.

Lead-acid batteries are secondary batteries, which mean that they can be recharged after they have been discharged. Primary batteries can be discharged only once and then have to be thrown away; examples are some types of torch and radio batteries.

HOW A BATTERY WORKS

The positive electrode is made of lead dioxide and the negative electrode is made of porous lead. When an electrical load (for example, lights or a starter-motor) is connected across the battery, a current flows through the electrolyte in the battery and through the external load. This causes the battery to discharge, which results in the chemical composition of both the electrodes changing to lead sulphate.

A battery can be charged by putting a current through the battery from an outside source of electricity such as an alternator, dynamo or charging unit. This converts the lead sulphate back to the original materials of lead dioxide and porous lead.

As the battery becomes charged, the electricity begins to decompose (hydrolyse) the water in the electrolyte into its constituent elements of hydrogen and oxygen, which are released as gas. This is why a battery gases when it is charged.

WHAT IS A BATTERY MADE OF?

Grids
As the positive and negative electrodes are made of weak materials, they need a mechanical support which is provided by a grid made from a lead alloy; lead on its own would be too soft. In addition to providing a support for the electrodes (the active material), the grid also conducts electricity from the electrodes to the outside load

Electrodes
The electrodes are initially made from a mixture of lead oxide and lead sulphate, and this is converted into lead dioxide in the positive plate and porous lead in the negative plate when the battery is initially charged.
The negative electrode also contains small amounts of additives to give the battery a good discharge performance at low temperatures to improve starting. The combination of grid and electrode is normally called a plate.

Electrolyte
The electrolyte is dilute sulphuric acid. This acts as a conductor to transport electrical ions between the positive and negative plates when the battery is being charged or discharged.

The acid also takes part in the discharge as the sulphate ions react chemically at the electrodes to produce lead sulphate.

Separator
The separator is an insulator placed between the positive and negative plates to prevent them shorting together.
The separator needs to be microporous with very small holes to allow the ions to flow through the separator from one plate to another. It also needs to be able to resist the high temperatures and strongly acidic oxidising conditions that occur in a battery.

Most modern separators are made of microporous polyethylene, which has the right properties to meet the demanding conditions within the battery.

Container and Lid
These are normally made of polypropylene, which is a light but strong plastic. Unlike some plastics, it does not become brittle when it is cold, and so can resist knocks during handling. It is not attacked by acid and it can also withstand the fluids (petrol, diesel, brake-fluid, antifreeze) normally found on a vehicle.

WHAT MAKES A BATTERY MAINTENANCE-FREE?

30 years ago, batteries lost water at a high rate, and motorists were advised to check the acid level as one of their weekly checks; modern maintenance free batteries need no water addition throughout their life under normal operating conditions. Incidentally, during the same period, battery life has doubled from 2 years to 4-5 years. In the past, battery grids were made of an alloy of lead with 10 per cent of antimony; the purpose of the antimony was to give rigidity as pure lead would be too soft on its own. Unfortunately, some of the antimony dissolved in the acid and resulted in the battery losing water.

With improvements in battery technology, we have been able to reduce the antimony content from 10 per cent to 1.5 per cent, and this reduction has resulted in batteries that are low maintenance, needing only yearly attention.
The latest improvement has been the use of 0.1 per cent of calcium as a hardening agent in grids in place of antimony; this causes less contamination of the acid and much reduced water loss, making the battery maintenance-free so no water needs to be added during its operational life.

SERVICE PROBLEMS

Overcharge
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, however, 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 has a voltage above about 14.7 – 14.8 Volts at normal temperatures, this is a sign that the charging system is faulty.

Deep Cycling
Modern charging systems keep the battery in a high state-of-charge while the 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, lights. On 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
As explained earlier, 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, this sulphation slowly changes its form into one that cannot be changed back into active material on charging, so, after charge, the battery will not return to give its original performance. If the sulphation is bad enough, the car will not start. This is the problem normally referred to as sulphation.

Undercharge
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 stop-start urban motoring. Undercharge will also occur if the alternator voltage is in the region of 13.6 - 13.8 Volts.

COLD CRANKING PERFORMANCE (AMPS) – SAE

The Cold Cranking Performance (CCA)
Measures the starting performance of the battery. In simple terms, the higher the CCA, the easier it will be to start the vehicle.

This is the starting test according to the SAE (Society of Automotive Engineers).

The test specifies that the battery at a temperature of –18°C will deliver a current equal to the Cold Cranking
Amps for 30 seconds with the voltage staying above 7.2 volts (3.6 volts for a 6 volt battery).

Battery performance drops off quickly with temperature, so this test is a good check of a battery’s starting ability. Some other manufacturers use other CCA standards, for example, DIN, the German standard, JIS, the Japanese standard, ETN, the European standard etc. These will give different values to the SAE value. Obviously, the more electrical accessories you turn off, the further you can drive the car.

AMPERE-HOUR CAPACITY AT 20 HOUR RATE (Ah)

The Ampere-Hour Capacity measures the total amount of electricity stored in a battery.

An Ampere-hour represents the amount of electricity when a current of 1 Ampere passes for 1 hour.

The Ampere-Hour Capacity varies with the rate at which the battery is discharged; the slower the discharge,
the greater the amount of electricity that the battery will deliver.

The Ampere-Hour Capacity is the amount of electricity that a battery will deliver during 20 hours before the voltage falls to 10.50V. For example, a 60Ah battery will deliver a current of 3A for 20 hours.

RECOMMENDED CHARGE RATE (Amps)

This is the recommended current for charging batteries with a constant current charger.

DIMENSIONS – LENGTH (mm)
This is the dimension over the longest part of the battery, including the holddown if fitted.

DIMENSIONS – WIDTH (mm)
This is the dimension over the widest part of the battery, including the holddown if fitted.