Car Care

Understanding The Automobile Battery

Storage battery | Constructions | Primary Function | Visual Checks | Installing a Dry-Charge Battery | Battery Leakage | Specific Gravity Test | Temperature Corrected Hydrometer | Battery Capacity | Battery Capacity Test | Three Minute Charge Test | Maintenance Free Battery | Battery Rating Methods | Amp-Hour Rating | Cold Cranking Rating | Reserve Capacity

The Storage Battery

Batteries used in automobiles are known as storage batteries. This term storage battery is sometimes misinterpreted. A battery does not store electricity but does store energy in a chemical form. It accomplishes its task by a chemical process, which takes place inside a battery when it is connected to a complete circuit.

Battery Construction and Operating Principles

Basically stated, a battery is composed of two dissimilar metals in the presence of acid. The battery is constructed of a series of positive and negative plates. These plates are insulated from each other by means of separators. All the positive plates are interconnected and all the negative plates are interconnected. These interconnected series of positive and negative plates are arranged alternately and submerged in the container filled with a sulphuric acid and water solution known as "electrolyte".

When a battery is in a full state of charge, the negative plates are basically sponge lead, the positive plates are lead peroxide, and the electrolyte has a maximum acid content and a minimum water content.

As the battery is discharging, the chemical action taking place reduces the acid content in the electrolyte and increases the water content, while both negative and positive plates are gradually changing to lead sulphate.

When the battery is in a state of discharge, the electrolyte is very weak since it now has a minimum acid content and a maximum water content and both plates are predominantly lead sulphate. The battery now ceases to function because the plates are now basically two similar metals in the presence of water instead of two dissimilar metals in the presence of acid.

During the charging process the chemical action that occurred during the battery discharge, is reversed. The lead sulphate on the plates is gradually decomposed, changing the negative plates back to sponge lead and the positive plates back to lead peroxide. The acid is redeposited in the electrolyte returning it to full strength. The battery is now again capable of performing all its functions. Top

Primary function

The primary function of a battery is to provide power to operate the starter motor. It must also supply the ignition current during the starting period and accomplish this even under adverse conditions of temperature and other factors. The battery can also serve, for a limited time, as a source of current to satisfy the electrical demands of the vehicle which are in excess of the output of the generator/alternator.

Battery Visual Checks

Battery care and testing are relatively simple. A basic knowledge of how a battery is constructed, how it works, along with a few pieces of test equipment and simple test procedures, will provide any serviceman with the essentials he needs to provide excellent service in this phase of the tune-up. The first test of a battery is a visual inspection. If a battery is cracked or otherwise defective, it must be discarded. If the electrolyte level in the battery is low, or if the ground connections or insulated connections are defective, the battery cannot operate efficiently. It is also very important that the battery be kept clean. Dirt and moisture can serve as a conductor and slowly discharge the battery over a period of time. Top

Installing a new Dry-Charge Battery

When activating a dry-charge battery, follow the battery manufacturer's service procedure and be sure to fill each cell properly with electrolyte supplied. Apply a warm-up charge of 15 amperes for 10 minutes after activating the battery, when so instructed. Observe charging cautions. Dry-charge batteries will be damaged if moisture enters the battery while it is stored. Always store dry-charge batteries in the coolest, and driest location possible. After activating a dry-charge battery, check the specific gravity. The gravity reading should be 1.260 or slightly higher. If the electrolyte level drops shortly after the initial fill, due to the plates and separators absorbing some of the solution, add more electrolyte to bring the solution up to the proper level. When so instructed, charge the battery at 15 amperes for 10 minutes before installing the battery to assure a full charge. Top

Battery Leakage Test

By using a voltmeter, a tune-up specialist can show his customer that a battery with a dirty top is actually leaking current and may become self-discharged. A leakage test is performed by clipping the negative voltmeter lead to the battery negative terminal. The positive voltmeter lead should be moved over the insulated surface of the top of the battery. Any reading is an indication of electrical leakage. This undesirable electrical path is composed largely of electrolyte which has been expelled from the battery through the filler cap vents by charging action of the generator/alternator. Electrolyte is, of course, a conductor of electricity.

If meter readings indicate any electrical leakage, the battery top should be washed with a solution composed of one spoonful of baking soda mixed in a pint of water. After the bubbling action, induced by acid neutralisation stops, rinse the battery top with clean water and dry the battery. Corrosion accumulation around the cable clamps should be removed with a brush and washed with the same solution. Top

Battery Specific Gravity Test

By using a hydrometer, the specific gravity of the electrolyte solution in a battery can be determined. The battery specific gravity is an indication of the battery state of charge. If the state of charge is low, the hydrometer will read low. If the state of charge is high, the hydrometer will read high. As an example, a reading from 1.260 to 1.280 indicates a fully charged battery. A reading from 1.200 to 1.220 indicates a battery is in a discharged condition and cannot give satisfactory service. The definition of specific gravity is the weight of a liquid compared to the weight of an equal volume of water. The specific gravity of chemically pure water at 80deg F.(28deg C.) is "one". Therefore, by knowing the specific gravity of sulphuric acid, we can accurately measure the ratio of sulphuric acid to water in the battery electrolyte solution. Top

Temperature Corrected Hydrometer

Hydrometer floats are calibrated to indicate correctly only at 80 deg F.(28deg C.) temperature. If used at any other temperature, a correction factor must be applied. The reason for this lies in the fact that a liquid expands when it is heated and shrinks when it is cooled. This will cause a change in the density of the electrolyte solution which will raise or lower the specific gravity reading. A thermometer is built into the temperature compensating-type hydrometer. The scale of this thermometer indicates the temperature of the solution. This reading should be used so that the proper temperature correction factor can be applied. The table is based on an electrolyte temperature of 80 deg F(28deg C). For other temperatures, correct the indicated reading by adding 4 points (.004) for each 10 deg above 80 deg F. and subtracting 4 points for each 10 deg that the electrolyte temperature is below 80 deg F.

For example: A specific gravity reading of 1.230 is obtained at a solution temperature of 10 deg F. If the electrolyte temperature is disregarded, the reading of 1.230 may be considered as low but acceptable. When the reading is temperature corrected, the true reading of 1.202 (7x4=28 from 1.230) reveals that the battery is actually very low and definitely in need of charging.

A specific gravity reading of 1.235 is obtained at a solution temperature of 120 deg F, The reading itself may be interpreted as being rather low but when temperature corrected the reading is actually 1.251 (4x4=16 added to 1.235). this specific gravity may be high enough for the battery to be restored to full charge by the car's generator/alternator.

These examples indicate the importance of temperature correcting specific gravity readings to accurately interpret the true state of battery charge. To accurately test the true condition of the battery, a light load test or a capacity test should be conducted after the specific gravity has been tested. Top

Battery Capacity

Most engine starting failures are caused by the inability of a battery to maintain a voltage high enough to provide effective ignition while cranking a cold engine.

Battery Capacity Test

The function of the battery capacity test is to duplicate the battery drain of a cold engine start while observing the battery's ability to maintain voltage. A battery that passes the capacity test will provide dependable performance. The Battery/Starter Tester has an ammeter, a voltmeter and a carbon pile, which is a battery loading device. The charged battery is discharged at a rate of three times its ampere-hour rating for 15 seconds while its voltage is observed. The voltage of a 12-volt battery should not drop below 9.0 volts or that of a 6-volt battery below 4.5 volts. A reading below this specification indicates a defective battery that should be replaced. Top

Three Minute Charge Test

A battery that is less than fully charged may be tested with a fast battery charger and a "Three Minute Charge Test". A fast battery charger is used in conjunction with a Battery/Starter Tester for this test. Fast charge the battery three minutes at not more than 40 amps for a 12 volt battery and 75 amps for a 6 volt battery. With the charger in operation, observe the voltage of the battery. If the voltage exceeds 15.5 volts for a 12 volt battery, or 7.75 volts for a 6 volt battery, the battery is sulphated or worn out. This indicates that the plates will no longer accept a charge under normal conditions and the battery should be discarded.

BE SURE to observe all precautions relative to working around a battery while it is being charged. Explosive hydrogen gas is liberated from the electrolyte while the battery is being charged. Sparks from a lighted cigarette or from charger clamps being disconnected while current is still flowing may cause an explosion that will destroy the battery and possibly inflict personal injury.

Also BE SURE that all precautions that are relative to working on a battery installed in a car that is equipped with an alternator are observed. These precautions will be fully covered in the section on alternators charging systems. These precautions also pertain to a car equipped with transistorised ignition or transistorised fuel injection systems.

A battery with a one-piece, hard-top cover must be tested with the capacity test since individual cell tests cannot be conducted. Individual cell tests require that the meter test prods contact the cell connectors, by piercing the sealing compound if necessary. Under no circumstance should there be an attempt to pierce the one-piece battery cover with meter prods to conduct an individual cell test. Top

The Maintenance Free Battery

A newer type of battery is called "maintenance free," will be seen in late model cars. It can easily be identified by its lack of filler caps, the top of the battery is a solid cover. These batteries are designed to operate without periodic additions of water throughout their normal service life.

Since the addition of water is the only maintenance normally performed on batteries, the elimination of this service results in a truly maintenance-free battery.

Such batteries are still classed as lead-acid batteries and still function in the same manner as the conventional variety. The only difference is in the lead alloy used to make the plates. Conventional batteries use a lead-antimony alloy for plate construction because antimony increases the strength and casting qualities of lead. Pure lead, by itself, is not a suitable plate material.

Although antimony adds the necessary mechanical properties to lead, it also affects the electrical properties of the battery. It tends to increase a battery's normal self-discharge rate and also to lower its "gassing" potential. Self-discharge is what causes an otherwise normal battery to gradually run down if left without charging for long periods, especially in temperatures over 60deg F. Normally, this is not a problem for batteries in regular service, but can be for new batteries left in stock or for vehicles left unattended for long periods. Eliminating antimony minimises this internal self-discharge.

"Gassing" is what happens to the electrolyte when a battery is under normal charge. This is what causes the water in the electrolyte to gradually "boil away". Actually, the water does not boil but, instead, is broken down by the charging voltage into its two elements - hydrogen and oxygen. By eliminating antimony from the plates, the voltage at which water breaks down, or "gasses," is raised. Therefore, at normal charging voltage, the original water in the electrolyte of a maintenance-free battery can last several years or more before it finally boils away.

In place of antimony, the lead in a maintenance-free battery is alloyed with calcium to give it the necessary manufacturing characteristics. Calcium, however, is a more difficult material to process and results in higher battery costs. But the result is a battery with a very low self-discharge rate and a greatly reduced tendency to boil off its water.

Although such batteries do not require servicing as do conventional ones, there are times when they must be tested to see if they are still serviceable. This is most commonly done with a battery test as described previously. If the battery voltage under load remains above the specified minimum (typically 9.6 volts), the battery is considered serviceable. However, if it falls below the minimum. It could be either defective or merely in a discharged state. Before it can be condemned, further steps are needed.

If the battery has a built-in "hydrometer," evidenced by a small window in the cover, check to see if a green dot appears in the window. If it does, it means that the battery is sufficiently charged (over 1.225 sp. gr.) to be load tested. If it now fails the load test, it means the battery is defective. However, if it fails the load test without the presence of the green dot, the battery must be recharged until the dot appears. It is then given a second test.

Batteries without a built-in hydrometer, if they fail the load test, must be fast charged for a period of time, specified by the manufacturer. If they fail the load test after the specified charge period, the battery is considered defective. Top

Battery Rating Methods

Over the years, many methods have been devised to specify the capacity or electrical size of the batteries. Presently, only three methods are commonly used: 1. The amp-hour method 2. Cold cranking performance 3. Reserve capacity In addition to electrical rating methods, batteries are also arranged according to their physical size by "group numbers." Batteries with the same group number have the same dimensions and are physically interchangeable. However, they may have widely varying electrical capacities and for this reason are not always interchangeable.

The Amp-hour Method

The amp-hour method has been used for many years, although it is being gradually replaced by the cold cranking and reserve capacity ratings. A battery's amp-hour rating is determined by discharging a fresh, fully charged battery at a constant rate so selected that at the end of 20 hours the voltage will have fallen to 1.75 volts per cell (or 10.5 volts for a 12 volt battery). This discharge current, time 20 hours, gives the battery's amp-hour rating. For example, if the required discharge current was 3.0 amperes, the battery would be rated at 3 x 20 or 60 amp-hours. It should be noted that this does not mean that such a battery can be discharged at 60 amperes for one hour, or any other combination, with the same results. When replacing batteries, always replace with the specified, or higher, amp-hour battery. Top

Cold Cranking Performance

This is a more resent rating method designed to show a battery's cold weather ability. A cold cranking rating shows how many amperes can be drawn from a battery at 0deg F for 30 seconds before its voltage drops below 1.2 volts per cell (or 7.2 volts for a 12 volt battery). As a rule-of-thumb, a battery's cold cranking rating in amperes should approximate the engine's displacement in cubic inches. Most new batteries have this rating, and sometimes the amp-hour rating, imprinted on the battery cover. There is no convenient way to convert between amp-hour ratings and cold cranking ratings.

Reserve Capacity

The reserve capacity is specified in minutes and indicates for how long a vehicle can be driven with battery only in the event of charging system failure. The rating is established by noting how long it takes a fully charged battery (at 80deg F) to drop to 1.75 volts per cell (or 10.5 volts for a 12 volt battery) at a constant 25 ampere discharge rate. This discharge rate represents a typical night-time electrical load with headlights and heater. Thus a battery with an 80 minute reserve capacity rating could keep a vehicle, with a defective charging system, running for at least one hour and 20 minutes. Operating time without the headlights and heater, of course, would be even longer. It must be remembered, however, that this rating assumes the battery to be fully charged initially. A partly charged battery may not provide the full specified operating time. Top

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