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How a Battery Works

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.

Most Automotive 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 means 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 Household batteries.

How an Automotive Lead Acid 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, acurrent 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 tolead 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?


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.

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.


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.


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.


The electrodes are initially made from a mixture of lead oxide and lead sulphate. 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.

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.

Battery Service Problems


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 general sign that the charging system is faulty. The common diode fault in the rectifier will see 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.

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 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 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 the alternator voltage is in the region of 13.6 – 13.8 Volts .

Basic Battery Terminology

Ampere (Amp, A)

The unit of measure of the electron flow, or current, through a circuit.

Ampere (Amp) Hours

A unit of measure for a battery’s electrical storage capacity, obtained by multiplying the current in amperes by the time in hours of discharge. (Example: A battery which delivers 5 amperes for 20 hours delivers 5 amperes x 20 hours = 100 Amp-Hr of capacity.) The base measurement is done at 25°C and is measured down to a cut off voltage of 10.5°C

Volt / Voltage

Voltage is what causes current to flow. Voltage is the pressure from an electrical circuit's power source that pushes charged electrons (current) through a conducting loop, enabling them to do work such as illuminating a light. In brief, voltage = pressure, and it is measured in volts (V). The term recognizes Italian physicist Alessandro Volta (1745-1827), inventor of the voltaic pile—the forerunner of today's household 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 in a 12-volt lead-acid battery.


The ability of a fully charged battery to deliver a specified quantity of electricity (Amp-Hr, AH) at a given rate (Amp, A) over a definite period of time (Hr).

Cold Cranking Performance (Amps)

Number of amperes a lead-acid battery at 0°F (-17.8°C) can deliver for 30 seconds and maintain at least 1.2 volts per cell which is 7.2v in a 12v battery. In simple terms the higher the CCA, the easier it is to start the vehicle.Temperature plays a key role in a battery's ability to deliver CCA.


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

Deep Discharge

State in which a cell is fully discharged using low current, so that the voltage falls below the final discharging voltage.


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

Final discharging voltage

Indicates the lowest allowable voltage level that a battery or cell can be discharged to. Discharging to below this cutoff voltage (deep discharge) can impair or (through pole reversal) destroy the electrochemical cell in the case of various battery types (e.g. lead, Ni/Cd, NiMH). FOr a 12V battery this is usually 10.5V

Reserve Capacity

The length of time in minutes tha a fully charged battery at 25°C can deliver a current of 25 Amps until the voltage drops to 10.5V. RC provides an indication of the time that a vehicle with a normal electrical load will run if the charging system ( alternator) fails

Battery Ratings

Volt - Voltage

AH - Ampere Hours

A ( EN) - Cold Cranking Amperes ( CCA)

W - Water Loss - rated W1 - W5

The ability to retain water content. The better the battery performs the less water it will loose in service. W1 ( traditional Lead-acid Battery) - W5 ( Premium next generation battery technology)

C - Charge Retention: Rates C1 - C2

The ability of the battery to retain stored chemical energy ( charge) when not in use. C1 ( Tradionla lead-acid) - C2 ( Modern Calcium)

V - Vibration Level: Rated V1 - V4

The battery's physical ability to resist the potentially damaging effects of vibration. V1( Tradional car and light Van) - V4 (Extreme heavy duty plant & commercial vehicle)

E - Endurance: Rated E1 - E4 ( conventional batteries only)

The ability to withstand repeated charge/discharge cycles to 50% depth of discharge without failure. E1 (Tradional car and light Van) - E4 (Extreme heavy duty plant & commercial vehicle)

M - Micro - Cycle: Rated M1 - M3 ( EFB & AGM types only)

The ability of the battery to provide power to restart the engine after frequent stop phases, its ability to recover state of charge afterwards and to resist ageing effects due to shallow pulse loads. M1 ( Basic start - stop) - M3 ( High end start - stop and hybrid)

Watt Hours

The amount of power the battery can supply when fully charged. You can calculate your requirements by working out the consumption of the accessories the battery will power and how long you usually use them for.

Cyclic Life

The number of times the battery can be discharged to 50% then fully recharged. The higher the cyclic life, the more work the battery can do during its life in service.

MCA - Marine Cranking Amps

The amount of cranking power the battery can provide to start the engine. It is the number of Amperes it can delivery for 30 seconds. Unlike CCA/A ( EN), MCA is measured at a temperature of 0°C as this is more relevant to marine use.

Battery Types

AGM Start Stop Starter Battery

Absorbant Glass Mat Battery. The sulphuric acid in the battery is absorbed into a very fine fibreglass mat, making the battery spill-proof. AGM batteries can charge up to 5 times faster than standard batteries and can deep cycle up to 80% without damaging the battery. A conventional battery can usually manage around 50%. AGM Batteries have up to 5 tiem the amount of cycle life in the battery than conventional batteries.

EFB Start Stop Starter Battery

Enhanced Flooded Batteries are a cost effective solution for entry level start stop vehicles. They are an enhanced version of a standard battery. The primary benefits are inproved charging acceptance and greater cyclic durability when operating at a reduced level of charge. As an approximation EFB batteries will provide 270,000 engine starts compared with 30,000 from a standard flooded porduct.

Standard Ignition - Starter Battery

Yuasa has 4 main types

  • 1000 - basic power demands - 3 Year Warranty
  • 3000 - standard power demands - 4 Year Warranty
  • 5000 - high power demands - 5 Year Warranty
  • Classic & Vintage

AD - Basic power demands - 3 Year Warranty

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