Electrical System for All Caterpillar Products Caterpillar

Introduction to Battery

The battery stores energy for the complete electrical system. The battery produces current upon demand for the machine electrical devices.

Batteries

Illustration 1	g01072957

A battery stores electrical energy in chemical form to be released as electrical energy for the machine electrical system. This includes the starting, charging, and accessory circuits. This battery current is produced by a chemical reaction between the active materials of the battery plates and the sulfuric acid in the electrolyte. The battery is a voltage stabilizer for the system. The battery acts as an accumulator or a reservoir of power.

After a period of use, the battery becomes discharged and will no longer produce a flow of current. The battery can be recharged with direct current, in the opposite direction that current flows out of the battery. In normal operation, the battery is kept charged by current input from the alternator.

For good operation, the battery must do the following:

• Supply current for starting the engine.

• Supply current when the demand exceeds the output of the charging system.

• Stabilize the voltage in the system during operation.

Battery Construction

A battery is made up of a number of individual elements in a hard rubber case or plastic case. The basic units of each cell are positive and negative plates, as shown in Illustration 1. Negative plates have a lead surface, which is gray in color, while the positive plates have a lead peroxide surface which is brown in color. The negative plates and positive plates are connected into plate groups. In some batteries, there is always one more plate in the negative group than in the positive group. This allows the negative plates to form two outsides when the groups are interconnected. Other batteries have the same number of positive plates and negative plates.

Each plate in the interlaced group is kept apart from a neighbor by porous separators, which allow a free flow of electrolyte around the active plates. The complete assembly is called an element. Elements in different cells are connected in series in order to increase voltage. The cells are separate from one another, so there is no flow of electrolyte between the cells. Each cell will produce approximately 2.2 volts. So if 6 cells are connected together in series, the battery will produce approximately 13.2 volts.

Illustration 2	g01072958

The electrolyte in a fully charged battery is a concentrated solution of sulfuric acid in water. The electrolyte has a specific gravity of about 1.270 at 27 °C (80 °F), which means the electrolyte weighs 1.270 times more than water. The solution is about 36% sulfuric acid H₂SO₄ and 64% water H₂0.

Battery Water

The necessity for pure water in batteries has always been a controversial subject. It is true that water with impurities affects the life and performance of a battery. Whether or not the effect of impure water is truly significant will depend on how high the mineral content of your water supply is. Generally, you do not have to use distilled water rather than tap water, but it will be better for the battery if you do use distilled water.

Battery Terminals

Batteries have negative posts and positive posts. The positive post is larger in order to help prevent the battery from being connected in reverse polarity. The positive terminal has a (+) marked on the top and the negative post a (-) marked on the top. Other identifying marks that are on or near the posts, are "POS" and "NEG". There are colored plastic rings that are placed on the posts, red for positive and black for negative.

Battery Vent Caps

Illustration 3	g01072960

Vent caps are located in each cell cover. Some batteries have individual vent caps for each cell. Some batteries have gang units which connect three cell vents together in a single unit. Vent caps cover access holes through which the electrolyte level can be checked and water added. The access holes provide a vent for the escape of gases that are formed when the battery is charging.

Battery Potential

Illustration 4	g01072962

Each cell in a storage battery has a potential of about 2 volts. Six volt batteries contain three cells that are connected in series. Twelve volt batteries contain six cells in series (Illustration 4, top diagram). For higher voltages, combinations of batteries are used. Illustration 4 (bottom diagram) shows two twelve volt batteries that are connected in series in order to provide 24 volts.

How a Battery Works

Illustration 5	g01072967

The battery produces current by a chemical reaction between the active materials of the unlike plates and the sulfuric acid of the electrolyte. While this chemical reaction is taking place, the battery is discharging. The battery is discharged after all of the active material have reacted. Then, the battery must be recharged before use.

Note that batteries of the same voltage can produce different amounts of current. The reason for this is that the amount of current a battery can produce is dependent on the number and size of the battery plates. The more plates there are, the more chemical reactions can take place between the electrolyte and the plates, therefore, the greater the amount of current produced. If two 12 volt batteries have a different number of plates, the battery with the greater number will supply more current flow and will have higher capacity.

Operating Cycles

Illustration 6	g01072970

A battery has two operating cycles:

• Discharging

• Charging

Discharging Cycle

When a battery is supplying current, the battery is discharging. The chemical changes in a discharging battery are as follows:

• Positive plates are made of lead peroxide PbO₂. The lead PB reacts with the sulfated radical SO₄ in the electrolyte H₂SO₄ to form lead sulfate PbSO₄. At the same time the oxygen O₂ in the lead peroxide joins with the hydrogen H in the electrolyte to form water H₂0.

• Negative plates are made of lead PB. The lead also combines with the sulfated radicals in the electrolyte to form lead sulfate PbSO₄.

• In the discharging process, lead sulfate forms on both the positive plates and negative plates making the two plates similar. These deposits account for the loss of cell voltage, because voltage depends on the positive and negative plates being different. As the battery progressively discharges, more lead sulfate is formed at the plates and more water is formed in the electrolyte. Note that although SO₄ radical leaves the electrolyte, it never leaves the battery. Therefore, never add any additional sulfuric acid H₂SO₄ to a battery. The extra SO₄ would only cause the battery to selfdischarge at a higher than normal rate. Water is the only substance in a battery that must be replaced.

Charging Cycle

Illustration 7	g01072971

The chemical reactions that take place in the battery cell during the charging cycle (Illustration 7), are essentially the reverse of those which occur during the discharging cycle. The sulfate radical leaves the plates and goes back to the electrolyte. This replenished the strength of the sulfuric acid. Oxygen from the water in the discharged electrolyte joins with the lead at the positive plate to form lead peroxide.

The Battery and the Charging Circuit

Illustration 8	g01072972

Batteries operate in a charging circuit with an alternator. The battery supplies current to the circuits and the battery becomes discharged. The alternator sends current to the battery in order to recharge it. Operation of the charging circuit varies with the engine speed. When the engine is shut off, the battery alone supplies current to the accessory circuits. At low speeds, both the battery and the alternator may supply current. At higher speeds, the alternator should take over and supply enough current in order to operate the accessories. The alternator will also recharge the battery. The voltage regulator limits the voltage from the alternator to a safe value which does not overcharge the battery at high speeds.

Electrolysis

Illustration 9	g01072977

When an electric current flows through water, the water molecules split into their component parts: hydrogen and oxygen. These two gases bubble to the surface and evaporate into the air. The water level goes down correspondingly. This process is called electrolysis. Electrolysis occurs whenever you charge a battery. When the current flows through an electrolyte, electrolysis takes place and the water level decreases.

Variation in Battery Efficiency or Terminal Voltage

Battery voltage is not constant. A 12 V battery does not deliver 12 V at all times. The main factors which affect the terminal voltage of a battery include temperature and operating cycle.

Temperature

A battery produces current by chemical reactions through sulfuric acid that is acting in the positive plates and the negative plates. At lower temperatures, the chemicals do not react as fast. Therefore, the battery has a lower voltage. Temperature will affect the terminal voltage of the battery. As temperature goes down, the battery becomes less efficient, while the cranking requirements of the engine will increase. At 27 °C (80 °F) a battery is 100 percent efficient. The battery has full cranking power. At -30 °C (-22 °F) a battery is only 30 percent efficient.

Types of Batteries

There are basically two types of batteries that are used in automotive equipment and heavy equipment applications:

• Conventional

• Maintenance free.

Conventional Batteries

Conventional batteries may be dry-charged or wet-charged. A dry-charged battery contains fully charged elements, but the dry-charged battery contains no electrolyte. Once the dry-charged battery is activated by being filled with electrolyte, the dry-charged battery is essentially the same as a wet-charged battery. A dry-charged battery retains a full state of charge as long as moisture is not allowed to enter the cells. If the dry-charged battery is stored in a cool, dry place, the battery will not lose part of the charge on the shelf prior to being used.

The activation of a dry charged battery is usually done at the warehouse where the battery is purchased by the dealer. To make sure the correct electrolyte is used and the battery is properly activated, many manufacturers furnish a packaged electrolyte for their dry charged batteries along with instructions for activation. These instructions must be carefully followed.

Wet-charged batteries contain fully charged elements and are filled with electrolyte at the factory. A wet-charged battery will not maintain a state of charge during storage. A wet-charged battery must be recharged periodically. During storage, even though a battery is not active, a slow reaction takes place between the electrolyte and the plates that causes the battery to lose the charge. This reaction is called self discharge. The rate at which self discharge occurs varies directly with the temperature of the electrolyte.

A fully-charged battery that is stored at a temperature of 38 °C (100 °F) will be completely discharged after a storage period of 90 days. The same battery that is stored at 15 °C (59 °F) will be slightly discharged after 90 days. Wet-charged batteries should be stored in the coolest place possible, without being so cold that the electrolyte freezes.

Note that a wet-charged battery which is kept fully-charged will not freeze unless the temperature goes below -60 °C (-76 °F), whereas a discharged battery with a specific gravity of 1.100 will freeze at -8 °C (17 °F). Wet-charged batteries which are stored for a long period of time without recharging may be permanently damaged by the formulation of hard, dense lead sulfate crystals on the plates. In order to prevent the crystals from forming, wet-charged batteries that are in storage should be brought to full charge every 30 days.

Maintenance-Free Batteries

Illustration 10	g01072981

The maintenance-free battery was developed in an effort to reduce battery maintenance, and to make batteries more dependable and longer lasting. A maintenance-free battery is similar in shape to a conventional battery, but the maintenance-free battery has no filler caps, so the electrolyte is completely sealed inside. Some of these batteries contain a state of charge indicator.

The indicator is a built in hydrometer that has a small green ball. This ball floats when the specific gravity of the electrolyte is 1.225 or higher. The indicator can also be used as a quick, easy way of telling if the battery is charged or discharged. The indicator must be read according to the manufacturer recommendations.

Characteristics of Maintenance-Free Batteries

Since the electrolyte is sealed in, the battery has a lifetime supply. The battery level does not have to be checked. Problems of over filling or under filling the cells are eliminated. Gases are produced during the discharge and the charging process. The gases that rise to the top of the case are trapped by the liquid gas separator. The gases cool and condense, and then the gases drain back into the electrolyte reservoir. Internal pressure that may occur is released through a small vent hole in the flame arrester vent located in the side cover.

Maintenance-free batteries and coventional batteries have plate groups, but the groups are constructed differently. Another difference is that the plates are enclosed in envelopes that act as the separators. These envelopes collect sediment as the plates come apart with age. The envelopes are bonded together. The envelopes permit the element to be placed on the bottom of the case.

In contrast, the element that is in a conventional battery is raised in the case to give room for sediment to collect without touching the plates. Having the element rest on the bottom of the tank allows for more electrolyte to cover the plates. The battery efficiency is improved.

Another important design difference in maintenance-free batteries is the material that is used to construct the grid for each cell plate. In a conventional battery the grid is made from lead antimony. In a maintenance-free battery, the grid is made from lead calcium. It is this difference in grid material that gives the maintenance-free battery the characteristic of not using water. The lead calcium grid significantly reduces the gassing and subsequent loss of water that is compared to a battery with lead antimony plates.

Deep Cycle Battery

A variation of the standard automotive and heavy equipment type lead acid battery is the deep cycle battery. This is also a lead acid battery, but the battery is specially constructed for use in applications that may not incorporate a charging system to support the electrical system and keep the battery charged.

A deep cycle battery is also used in applications where the battery is used to operate electrical systems when the engine is not running, such as in a motor home.

The deep cycle battery has a denser active material and thicker plates. This helps keep the active material in the grid during repeated deep discharge and recharge cycles. Glass separators may be used to reinforce the plates, reduce vibration damage, or shedding of the active material from the grid. The deep cycle battery can be discharged fully and recharged many times without harm. A standard automotive and heavy equipment battery would soon break down under these deep cycle conditions.

Battery Ratings

The following factors influence battery capacity ( the amount of current a battery can produce):

• The number, size and thickness of the plates.

• The quality and strength of the electrolyte.

Batteries used the ampere hour rating method for many years until new capacity ratings for batteries were adopted in 1971 by the Society of Automotive Engineers (SAE) and the Battery Council International (BCI).

Three current methods that are used for rating automobile size batteries are cold cranking performance, cranking performance and reserve capacity.

Cold Cranking Performance

The basic job of a battery is to start an engine. This involves a high discharge rate in amperes for a short period of time. It is more difficult for a battery to deliver power when the battery is cold. The engine requires more power to turn over when the engine is cold. The following definition is for the cold cranking rating:

• The discharge load in amperes which a new, fully charged battery at -18 °C (-0 °F) can continuously deliver for 30 seconds and maintain a voltage of 1.2 volts per cell.

Many low cost batteries can deliver only 200 amps, while more powerful batteries will deliver up to 1000 amps under the same conditions. The cold cranking performance of the battery must match the power requirements of the engine it has to start. If an engine under cold conditions required 400 amps to start, obviously the cheaper battery delivering only 200 amps would be inadequate.

Cranking Performance

Cranking performance at 0 °C (32 °F), is a new rating recently recognized by BCI. Cranking performance is the discharge load in amperes which a new, fully charged battery at 0 °C (32 °F) can continuously deliver for 30 seconds and maintain a voltage of 1.2 volts per cell.

Reserve Capacity

Reserve capacity is defined as the ability of a battery to sustain a minimum machine electrical load in the event of a charging system failure. It is also a comparative measure of the battery's ability to provide power for machines that have small parasitic electrical loads for long periods of time, and still have enough capacity to crank the engine. The reserve capacity rating is defined below:

• The number of minutes which a new, fully-charged battery at 26.7 °C (80 °F) can be continuously discharged at 25 amperes and maintain a terminal voltage equal to or greater than 1.75 volts per cell.

Battery Use and Replacement

Be sure to replace the battery with another battery that is equal in capacity to the original. A smaller battery, although it may initially seem to be adequate, will eventually fail as a result of excessive cycling which shortens battery life. A larger battery than the original may be needed if accessories such as an air conditioning unit are added to the vehicle's electrical circuit.

A high output alternator may be needed in cases where electrical loads are excessive. This high output alternator will help keep the battery charged and will increase the battery service life.

Battery Charging

Illustration 11	g01072982

When a battery is in use the battery will alternate between fully-charged and fully-discharged. When you test a battery and determine that the battery requires charging, you will have to decide how the battery is to be recharged.

Battery Chargers

While an engine is running, the battery charge is maintained by the charging system. Occasionally, the battery charge may wear down. If not attended to, the battery will not have enough power to start the engine. When a battery's state of charge is low, the battery should be recharged. Recharging can be done on the battery, while the battery is in the vehicle or after the battery has been removed. There are a number of different battery chargers.

Constant Current Chargers

A constant current charger supplies a constant or a set amount of current to the battery. The recommended charging rate is 1 amp per positive battery plate per cell. For example, if a battery has five positive plates per cell, the battery should be charged at 5 amps. Most batteries which are slow charged with a constant current charger will take 5 to 6 amps.

Constant Voltage Chargers

A constant voltage charger supplies the battery with a constant voltage during the charging period, for example, 15 volts for a 12 volt battery. This charger will charge the battery at a fairly high amperage when the battery is low. As the battery builds up charge, the amperage tapers off almost to nothing as the battery becomes fully-charged. Constant voltage chargers are much more common than constant current chargers.

Charging Conventional Batteries

Time is usually the main factor when you decide whether to fast charge or to slow charge a battery. Obviously, it is better to slow charge a battery, because you get a more thorough charging job. However, you do not always have the time (24 to 48 hours) to do a slow charge and in such cases fast charges must be done.

Constant Current Slow Chargers

A slow charger can be either constant current or constant voltage (constant voltage is more common). The maximum amount of voltage that a charger will produce is printed on the charger. For example, a 60 volt charger could be used for five 12 volt batteries (total 60 volts) or ten 6 volt batteries (total 60 volts).

The term slow charging refers to a charge rate of 10 Amps or less. When there are a number of batteries of different sizes on the charger, average out the charge rate. On some of the new chargers, you do not have to bother counting or averaging out the new positive plates. These chargers have a yellow, green and red band on the charge rate indicator, and it is recommended the control be set in the green range.

To connect a constant current charger, start with the black lead (negative) from the charger and connect the lead to the positive post of the last battery. Using good jumpers, connect the batteries, positive to negative to complete the series circuit.

Recheck all the connections by turning the connections slightly on the posts. Finally, turn the charger on and adjust the charger to the correct charge rate.

The state of charge of a battery that is being charged should be checked with a hydrometer twice a day, when possible. The total charging time will vary depending on the strength of the charge. At the end of 48 hours batteries should be fully-charged. If a battery becomes fully-charged and the specific gravity is 1.275 or over before 48 hours are up, remove it.

Constant Voltage Slow Chargers

Constant voltage chargers are connected to the batteries in parallel. The maximum number of batteries a charger can handle will be marked on the charger.

The voltage control is set at a specified voltage, such as 15 volts for a 12 volt battery. The charge rate is automatically sensed by the charger. The charge rate will be high when the discharged battery is first connected to the charger. The charge rate will gradually taper off as the battery becomes fully charged. When connecting batteries in parallel to a constant voltage charger, start with the black lead (negative) from the charger and connect it to the negative (-) post of the first battery. Using good jumper cables, connect the batteries negative to negative and positive to positive.

As with a constant current charger, check the specific gravity of the charging batteries twice a day and remove the batteries when they are fully charged.

Fast Chargers

Fast chargers will give a battery a high charge for a short period of time, usually no more than one hour. Fast chargers are portable in contrast to slow chargers. Slow chargers are usually mounted to a wall or sit in a permanent position on a bench. Portable fast chargers can charge a battery while the battery is still in the machine. Generally, only one battery at a time is charged on a fast charger. Many modern fast chargers also have a capacity to slow charge a battery.

Precautions When Fast Charging

Whenever a battery is charged, especially fast charged, never allow the electrolyte to exceed 51 °C (123 °F).

Watch the color of the electrolyte when you are fast charging batteries. As a battery, ages the electrolyte will become discolored by sediment. During a fast charge the sediment is stirred up. The sediment can get trapped between the plates. This can cause a short. Check the color of the electrolyte during charging with the hydrometer. If sediment begins to appear, reduce the charging rate.

Correct Battery Charging Practice

Before you connect conventional batteries to a charger make sure that the battery tops are clean and the electrolyte is up to the correct level.

All chargers need 110 V alternating current supply.

Always make sure that the charger is turned OFF before you connect the charger to a battery.

When you connect any charger, observe the correct polarity. Always be sure to connect negative to negative and positive to positive. Most chargers are polarity protected.

Check the charger voltage settings before you turn the charger ON. On a constant voltage slow charger, set the voltage to match the number of volts in the batteries that you are charging. On a constant current charger, set the voltage for 6 or 12 V depending on which battery you are charging.

Charging Time

When you are slow charging a battery, do a specific gravity check twice a day to see if the battery is fully charged. Do not continue to charge a battery if tests indicate that the battery has reached full charge. Set the fast charge time for no longer than one hour. Watch the battery to make sure that the battery does not overheat.

Always turn the charger off before you disconnect the charger in order to prevent any sparks from accidentally igniting explosive hydrogen gases that are given off during charging. Never charge a battery in a place where there may be any chance of sparks, such as in an area where welding or grinding is done.

Charging Maintenance-Free Batteries

Maintenance-free batteries are charged by using conventional battery charging equipment. The fast and slow charging rates for maintenance-free batteries are lower and the times of charging are proportionately longer.

Jump Starting

Illustration 12	g01072983

When a charger is not available, a common practice to start a vehicle with a dead battery, is to use jumper cables and a battery pack. Before you connect jumper cables, be sure all the electrical accessories such as lights, radio and wipers are OFF.

Observe battery voltage when you are jump starting a vehicle. Jump a 6 volt battery with a second 6 volt battery or jump a 12 volt battery with a second 12 volt battery. This is important because of the danger of arcing when you connect the jumper cables which could cause a battery to explode.

On heavy duty starting systems that use two 12 volt batteries in series to provide 24 volts for cranking, special precautions must be observed to prevent damage to the electrical components while you are jump starting. Check the Service Manual recommendations before you attempt to jump start any machine with this battery. You will require two sets of jumper cables and two 12 volt batteries.

Identify polarity before you connect jumper cables. Connect the jumper cables negative to negative and positive to positive (since you are just replacing the existing power source).

Connect the jumper cables in the following order:

1. Connect one cable clamp to the positive terminal of the dead battery.

2. Connect the other end to the positive terminal of the booster battery.

3. Connect the second clamp to the negative terminal of the booster battery.

4. Then, connect the other end to the engine block of the vehicle with the dead battery.

When you remove the cables, reverse the procedure for connecting the cables. Keep the clamps separated until the cables are disconnected from the source in order to prevent arcing.

Battery Maintenance

The battery is the heart of the electrical system. No accurate tests can be performed on any part of the electrical system unless the battery is properly serviced and fully charged.

Battery Testing

In order to determine what is wrong with a battery, you have to test it. Preform the following tests on batteries:

• Specific gravity (chemical test)

• Load test

Specific Gravity Test Conventional Battery

Specific gravity is the weight of a liquid that is compared to water. When you perform a specific gravity test on a battery you are determining the state of charge in the battery that is based on the percentage of acid to water in the electrolyte. The strength of the electrolyte varies directly with the state of charge of each cell. The higher the specific gravity, the greater the capability of the battery to produce an electrical potential. Specific gravity tests are done by using a hydrometer.

Hydrometer

Illustration 13	g01072984

Hydrometers are calibrated in order to measure specific gravity correctly at an electrolyte temperature of 27 °C (80 °F). To determine a corrected specific gravity reading when the temperature of the electrolyte is other than 27 °C (80 °F): Add to the hydrometer reading four gravity points (0.004) for each 5.5 °C (41 °F) above 27 °C (80 °F). Subtract four gravity points (0.004) for each 5.5 °C (41 °F) below 27 °C (80 °F). This compensates for expansion and contraction of the electrolyte above or below the standard.

The specific gravity of each battery cell should be tested by using the hydrometer. If water has been recently added to a battery, a hydrometer will not give an accurate reading of the battery's state of charge. Charge the battery long enough to ensure complete mixing of the water and electrolyte. Then check the battery cells with the hydrometer.

Fully charged specific gravity varies in different types of batteries. Typical readings are as follows:

Table 1

State of charge	Specific Gravity
100%	1.280
75%	1.250
50%	1.220
25%	1.190
0%	1.130

The electrolyte should be clear. A cloudy brown color indicates that plate material is shedding and that the battery is failing.

When the specific gravity reading is below 1.250, the battery may be in satisfactory condition but the state of charge is low. Charge the battery before making further tests.

When the specific gravity reading is above 1.280, the battery may be in satisfactory condition but the battery is above full charge. In use, the specific gravity should return quickly to the normal range. Make further tests in order to determine the battery's condition.

The amount of variation in the specific gravities of the cells should be within 30 to 50 points (0.030 to 0.050). If cell variation exceeds this amount, an unsatisfactory condition is indicated. This may be due to unequal consumption of electrolyte in the cells that are caused by an internal defect, short circuit, improper activation, or deterioration from extended use. The battery should normally be replaced, however, a battery should not be condemned based on specific gravity readings alone. Further testing should be done.

Specific Gravity Test Maintenance Free Battery

Look at the state of charge indicator (if equipped) that is on the battery in order to decide whether the battery needs charging before testing.

Green dot visible

If the green dot on the battery's state of charge indicator is visible, the battery charge and fluid level are within range. On some occasions, after prolonged cranking, the green dot may still be visible, but the battery will not have sufficient cranking power. Should this occur, charge the battery.

Green dot not visible

Charge the battery according to the manufacturer's specifications.

Yellow indicator

On some occasions, the test indicator may turn light yellow which indicates a low electrolyte level. In this instance the battery should not be tested, charged or jump started because, there is a very real possibility that the battery may explode.

Using a digital voltmeter, check battery voltage at the battery terminals. If the battery voltage is below 12.0 volts, charge the battery.

Use a battery load tester to remove the battery surface charge. Adjust the load tester to 50 percent of the battery's cold cranking amps (CCA) for five seconds. Allow the battery to rest for 5 minutes before testing.

Check the battery voltage at the battery terminals. Voltage must be over 12.4 V (which indicates at least 75% charge) before a load test can be performed. If the voltage is under 12.4 V (which indicates below 75% charged), charge the battery and test it again.

Battery Load Test

Illustration 14	g01079627

A load test gives the best indication of a battery's condition. If the state of charge is 75% or better, a load test (capacity test) can be done on the battery. If, however, that state of charge is below 75%, you should charge the battery.

Typical load test procedures:

1. Connect the tester's ammeter and voltmeter leads to the appropriate post on the battery. The load control knob must be in the Off position.

2. Turn the control knob clockwise until the ammeter reading is one-half the cold cranking rate of the battery or as specified by the battery manufacturer.

3. Maintain the load for 15 seconds, then note the voltmeter reading and turn the control knob back to OFF position.

If the voltmeter reading is within the green band, 9.6 volts for a 12 volt battery or 4.8 V for a 6 V battery or is higher, the battery has good output capacity. However, although the battery may pass the load test, it may still require some charging to bring it back up to peak performance.

When cold, a battery has a lower discharge capacity. If a cold battery fails to pass the capacity test, let it stand until 27 °C (80 °F), then retest.

Open circuit voltage test

The open circuit voltage test can be used on maintenance free batteries to indicate state of charge if the battery does not have a state of charge indicator. To perform this test the battery must not have been heavily discharged or charged recently.