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The battery, at all times, must be kept clean and dry. The electrolyte, which is used in a battery, is Sulphuric Acid that is both conductive and corrosive. If acid is evident, it will cause an electrical discharge path, which slowly discharges the battery. It also damages the paintwork and metal parts of vehicle. Evidence of acid is generally due to the overfilling of the battery or possibly a damaged casing and should be investigated and remedied immediately.

The Sulphuric Acid should be neutralized by washing the affected area and outside of the battery with a solution of bicarbonate soda and warm water (10 grams to 1 litre). Care should be taken so that none of the solution is allowed to enter the battery. The affected areas should then be washed with clean cold water and dried as best as possible. Affected areas may be prepared and repainted or a thin layer of Petroleum jelly applied to prevent oxidation. Care should be taken to ensure that the battery is properly located and secured in its correct position.

Battery Posts, Terminals and Cables

Battery terminals tend to corrode due to electrolysis and the acid fumes which are evident. These should be cleaned by soaking them in bicarbonate solution for a while. The metallic contact area of the terminals and battery posts should be cleaned with a terminal cleaner or emery cloth to ensure good contact. Once cleaned, a thin layer of Petroleum jelly should be applied around the visible metal area of the terminal and post. This will assist in halting the build up of corrosion.

All cables and connections between the battery, engine block, body, alternator and starter motor should be checked to ensure they are tight, clean and not frayed or damaged. Dirty terminals and poor earth connections can cause similar symptoms to those presented by a discharged or faulty battery.

Topping Up

The electrolyte levels of the battery should be checked regularly. These should be corrected to the level indicated or 10-15 mm above the plates, which are visible through the vents. Only approved water (distilled or deionized) must be added. Sulphuric acid should be added where electrolyte has been lost due to spillage or over topping. The battery dealer should be consulted where this is required.

The electrolyte levels tend to decrease on a discharged battery and increase as the battery becomes fully charged. Topping up should, therefore, be carried out on a fully charged battery to avoid the possibility of acid spillage. If the state of charge of the battery is not known then water, sufficient only to cover the plates, should be added and further amounts added following a charging period. Most modern batteries require very little topping-up. If more topping-up is required more frequently than normal, overcharging may be indicated and should be investigated. More frequent topping-up and a reduction in the fully charged specific gravity reading of the battery are often an indication of an older battery reaching the end of its useful life.

Charging System

The condition and tension of the alternator/fan belt should be checked on a routine basis. A high pitched squeal when accelerating is an indication of a loose or worn belt and will adversely affect the charging of the battery. The battery will tend to run down over a period if the alternator is not charging adequately. Discharged batteries are often erroneously replaced due to faulty charging systems.

When replacing a vehicle's battery or having it charged due to some problem always have the rate checked as a precaution. The charge rate on most vehicles is between 13.8 and 14.2 volts. The voltage can only be tested with a fully charged battery in the vehicle. More comprehensive testing should be referred to your dealer or a recognized auto electrician. Where the battery requires frequent topping-up, overcharging or excessive under-bonnet temperatures are indicated and should be investigated.


Battery testing is usually carried out by means of a Hydrometer, which is used to determine the specific gravity (S.G.) of the electrolyte. The S.G's indicate the state of charge of the individual cells of the battery. As a battery discharges the S.G. decreases.

  • Fully charged 1.250 1.260 = 12.6 volts
  • 75% charged 1.220 1.250 = 12.4 volts
  • 50% charged 1.200 1.220 = 12.2 volts
  • 25% charged 1.150 1.200 = 12.0volts
  • Discharged 1.100 1.150 = 11.9 volts

When testing, readings should be taken from each cell and recorded.

a) If the readings are uniform and high, e.g. 1.230 and upward, a reasonably charged battery is indicated. If the battery does not crank the engine there could be a loose connection in the battery, but more than likely a system fault is indicated. b) Uniform low readings below 1.200 are indicative of inadequate charging or an electrical fault, which is discharging the battery. c) Good readings on five cells, e.g. 1.230 - 1.250, and low readings on one cell, e.g. 1.180, would generally indicate a faulty cell. Any difference in readings of 0.05 points or more should be investigated.

There are many examples, however, thus, unless the age of the battery or visible mechanical damage warrants replacement, the battery should be charged and retested before discarding.

Some batteries are totally sealed in which case a voltmeter should be used to establish the state of charge. The voltage indicated in table above will assist in assessing the state of charge.

Due to the nature of the vehicles charging system and factors such as temperature the battery is generally only 70 - 80% charged. Occasional charging outside the vehicle recommended in some instances to avoid breakdowns and prolong battery life.

Load Testing

Hydrometer testing can be accompanied by load testing. This is performed by means of a test instrument fitted with volt and ampere meters, which indicate the battery voltage and current draw of the battery under load. The load tester basically simulates the current draw applied to the battery when cranking the vehicle's engine.

A load test should be carried out after the Hydrometer testing and then only if the S.G. indicates that the battery is more than 50% charged.

Procedure: Disconnect the battery terminals, removing the earth cable first to avoid a short. Remove the vent plugs and cover the open cells with a damp cloth. Connect the leads of the load tester to the battery, taking note of the polarity.

A load of three times the battery's capacity should be applied for 10 - 15 seconds. Apply the load observing the readings indicated on the voltmeter. A battery which is in good condition will maintain a voltage> 9 volts. Remove the cloth and carefully observe the reaction in the cells. A faulty cell will generally bubble vigorously and the voltage indicated on the voltmeter will drop below 7 volts. A battery with these symptoms should be replaced.

A load may also be applied to the battery by disconnecting the vehicle coil wire (HT lead) and cranking the engine for 10-15 seconds. A healthy battery will easily crank the engine, whereas, on a faulty battery, the cranking will quickly fall away and may stop altogether. The procedure may differ slightly depending on the type of load tester; therefore the manufacturer's instructions should be followed carefully.

Battery Charging

The charging of batteries out of the vehicle is recommended to fully restore batteries that have become discharged due to faulty electrical systems and to correctly charge them for testing. Batteries used for caravans or as portable power supplies do not always have built in charging facilities and also require regular independent charging.

A battery should never be stored in a discharged condition as this leads to permanent sulphation. Caravan and boating batteries should be recharged before being stored for a period, fresh charge should given approximately every 12 weeks and before using again.

There are various types of chargers available and manufacturer's instructions should be followed carefully.

The battery should be correctly connected to the charger. The vent plugs should be removed and the cells checked to ensure that the plates are covered by electrolyte. Topping-up to the correct levels should be done at the end of charging, as the levels will rise during the charge.

The time taken to fully charge the battery depends on the following:

  • The initial state of charge of the battery
  • The ampere hour capacity of the battery.
  • The charge rate (A) at which the battery is recharged

An indication would be the capacity of the battery versus the ampere input e.g. a 50 AHr battery which is totally discharged would take 10 hours to charge at 5A. Due to inefficiency losses the battery should be recharged to 110% - 115% of its capacity. Thus a 50 AHr battery charged at 5A would be charged for 11.5 hours and receive the equivalent of 57.5A Hr to correctly recharge.

Furthermore, most chargers are taper chargers and the initial current input tapers as the battery voltage rises. Therefore total current acceptance cannot be accurately calculated under normal circumstances.

The safe and most efficient charging rate is a current in amperes equal to 1/1 0 of the battery's 20 hour capacity rating. The charge rate may be increased under certain circumstances, but should be reduced once the battery reaches the gassing stage or voltage, which is discussed below.

The temperature of the battery should also be monitored. If the battery appears to be getting hot whilst on charge the current should be reduced or the change paused to allow the battery to cool.

The battery naturally begins to gas once it reaches 13.98 volts on charge. This is an indication that the battery is accepting charge and is nearing its fully charged state. The S.G's will rise rapidly from this stage and the charge should continue until the S.G's cease to rise.

The most suitable manner to ascertain the state of charge of the battery is to take regular S.G readings during the charge. Once the S.G readings have reached an acceptable level (refer to table under testing) and remain constant after three consecutive hourly readings, the charge may be terminated.

Extreme caution should be exercised to ensure that no naked flames or sparks are caused around batteries, especially when charging as the gases produced are highly explosive.

Refer to the relevant section in this guide for the various charge rates and safety instructions