7.1 Charging Circuit

The basic charging system consists of a battery, alternator, voltage regulator, ignition switch, and indicator light or indicator gauge or both. They must all work together to provide a source of electricity for the vehicle to operate. The charging system performs several functions:

Storage Battery

Figure 7-1 — Battery.

The storage battery is the heart of the charging circuit (Figure 7-1). It is an electrochemical device for producing and storing electricity. A vehicle battery has several important functions:

The type of battery used in automotive, construction, and weight-handling equipment is a lead-acid cell-type battery. This type of battery produces direct current (DC) electricity that flows in only one direction. When the battery is discharging, it changes chemical energy into electrical energy, thereby, releasing stored energy. During charging (current flowing into the battery from the charging system), electrical energy is converted into chemical energy. The battery can then store energy until the vehicle requires it.

Battery Construction

The lead-acid cell-type storage battery is built to withstand severe vibration, cold weather, engine heat, corrosive chemicals, high current discharge, and prolonged periods without use. To test and service batteries properly, you must understand battery construction. The construction of a basic lead-acid cell-type battery is as follows:

The battery element is made up of negative plates, positive plates, separators, and straps (Figure 7-2). The element fits into a cell compartment in the battery case. Most automotive batteries have six elements.

Figure 7-2 — Battery element.

Each cell compartment contains two kinds of chemically active lead plates, known as positive and negative plates. The battery plates are made of a stiff mesh framework coated with porous lead. These plates are insulated from each other by suitable separators and are submerged in a sulfuric acid solution (electrolyte).

Charged negative plates contain spongy (porous) lead (Pb), which is gray in color. Charged positive plates contain lead peroxide (PbO2), which has a chocolate brown color. These substances are known as the active materials of the plates. Calcium or antimony is normally added to the lead to increase battery performance and to decrease gassing. Since the lead on the plates is porous like a sponge, the battery acid easily penetrates into the material. This aids the chemical reaction and the production of electricity.

Lead battery straps or connectors run along the upper portion of the case to connect the plates. The battery terminals (post or side terminals) are constructed as part of one end of each strap.

To prevent the plates from touching each other and causing a short circuit, sheets of insulating material (micro-porous rubber, fibrous glass, or plastic impregnated material), called separators, are inserted between the plates. These separators are thin and porous so the electrolyte will flow easily between the plates. The side of the separator that is placed against the positive plate is grooved so the gas that forms during charging will rise to the surface more readily. These grooves also provide room for any material that flakes from the plates to drop to the sediment space below.

The battery case is made of hard rubber or a high-quality plastic. The case must withstand extreme vibration, temperature change, and the corrosive action of the electrolyte. The dividers in the case form individual containers for each element. A container with its element is one cell.

Stiff ridges or ribs are molded in the bottom of the case to form a support for the plates and a sediment recess for the flakes of active material that drop off the plates during the life of the battery. The sediment is thus kept clear of the plates so it will not cause a short circuit across them.

The battery cover is made of the same material as the container and is bonded to and seals the container. The cover provides openings for the two battery posts and a cap for each cell.

Battery caps either screw or snap into the openings in the battery cover. The battery caps (vent plugs) allow gas to escape and prevent the electrolyte from splashing

outside the battery. They also serve as spark arresters. The battery is filled through the vent plug openings. Maintenance-free batteries have a large cover that is not removed during normal service.


Hydrogen gas can collect at the top of a battery. If this gas is exposed to a flame or spark, it can explode.

Battery terminals provide a means of connecting the battery plates to the electrical system of the vehicle. Either two round post or two side terminals can be used.

Battery terminals are round metal posts extending through the top of the battery cover. They serve as connections for battery cable ends. The positive post will be larger than the negative post. It may be marked with red paint and a positive (+) symbol. The negative post is smaller, may be marked with black or green paint, and has a negative (-) symbol on or near it.

Side terminals are electrical connections located on the side of the battery. They have internal threads that accept a special bolt on the battery cable end. Side terminal polarity is identified by positive and negative symbols marked on the case.

The electrolyte solution in a fully charged battery is a solution of concentrated sulfuric acid in water. This solution is about 60 percent water and about 40 percent sulfuric acid.

The electrolyte in the lead-acid storage battery has a specific gravity of 1.28, which means that it is 1.28 times as heavy as water. The amount of sulfuric acid in the electrolyte changes with the amount of electrical charge; the specific gravity of the electrolyte also changes with the amount of electrical charge. A fully charged battery will have a specific gravity of 1.28 at 80°F. The figure will go higher with a temperature decrease, and lower with a temperature increase.

As a storage battery discharges, the sulfuric acid is depleted and the electrolyte is gradually converted into water. This action provides a guide in determining the state of discharge of the lead-acid cell. The electrolyte that is placed in a lead-acid battery has a specific gravity of 1.280.

The specific gravity of an electrolyte is actually the measure of its density. The electrolyte becomes less dense as its temperature rises, and a low temperature means a high specific gravity. The hydrometer that you use is marked to read specific gravity at 80°F only. Under normal conditions, the temperature of your electrolyte will not vary much from this mark. However, large changes in temperature require a correction in your reading.

For every 10-degree change in temperature ABOVE 80°F, you must add 0.004 to your specific gravity reading. For every 10-degree change in temperature below 80°F, you must subtract 0.004 from your specific gravity reading. Suppose you have just taken the gravity reading of a cell. The hydrometer reads 1.280. A thermometer in the cell indicates an electrolyte temperature of 60°F. That is a normal difference of 20 degrees from the normal of 80°F. To get the true gravity reading, you must subtract 0.008 from 1.280. Thus the specific gravity of the cell is actually 1.272. A hydrometer conversion chart is usually found on the hydrometer. From it, you can obtain the specific gravity correction for temperature changes above or below 80°F.

Battery Capacity

The capacity of a battery is measured in ampere hours. The ampere-hour capacity is equal to the product of the current in amperes and the time in hours during which the battery is supplying current. The ampere-hour capacity varies inversely with the discharge current. The size of a cell is determined generally by its ampere-hour capacity. The capacity of a cell depends upon many factors, the most important of which are as follows:

Battery Ratings

Battery ratings were developed by the Society of Automotive Engineers (SAE) and the Battery Council International (BCI). They are set according to national test standards for battery performance. They let the mechanic compare the cranking power of one battery to another. The two methods of rating lead-acid storage batteries are the cold-cranking rating and the reserve capacity rating.

The cold cranking rating determines how much current in amperes the battery can deliver for thirty seconds at 0°F while maintaining terminal voltage of 7.2 volts or 1.2 volts per cell. This rating indicates the ability of the battery to crank a specific engine (based on starter current draw) at a specified temperature.

For example, one manufacturer recommends a battery with 305 cold-cranking amps for a small four-cylinder engine but a 450 cold-cranking amp battery for a larger V-8 engine. A more powerful battery is needed to handle the heavier starter current draw of the larger engine.

The reserve capacity rating is the time needed to lower battery terminal voltage below 10.2 V (1.7 V per cell) at a discharge rate of 25 amps. This is with the battery fully charged and at 80°F. Reserve capacity will appear on the battery as a time interval in minutes.

For example, if a battery is rated at 90 minutes and the charging system fails, the operator has approximately 90 minutes (1 1/2 hours) of driving time under minimum electrical load before the battery goes completely dead.

Battery Charging

Under normal conditions, a hydrometer reading below 1.265 specific gravity at 80°F is a warning signal that the battery needs charging or is defective.

When testing shows that a battery requires charging, a battery charger is required to re- energize it. The battery charger will restore the charge on the plates by forcing current back into the battery. The battery charger uses AC (Alternating Currnet) current from a wall outlet, usually 120 volts, and steps it down to a voltage slightly above that of a battery, usually 14-15 volts. There are basically two types of chargers, the slow charger and the fast (quick) charger.

The slow charger is also known as the trickle charger. It feeds a small amount current back into the battery over a long period of time. When using a trickle charger, it takes about 12 hours at 10 amps to fully charge a dead battery. However, the chemical action inside the battery is improved. During a slow charge, the active materials are put back onto the battery plates stronger than they are during a fast charge. It is always better for the battery to use a trickle charge when time allows.

The fast charger, or quick charger and sometimes called the boost charger, forces a high amount of current flow back into the battery. A fast charger is commonly used in shops to start an engine or get the vehicle out of the shop quickly because there is no time to wait for a slow charge. Fast charging is beneficial if you just need to start the engine; if time allows, use the slow charge.

When using a fast charger, do not exceed a charge rate in excess of 35 amps. Also, ensure the battery temperature does not exceed 125oF. Exceeding either one could cause damage to the battery.

If there is a possibility that the battery is frozen, do not charge the battery. Charging a frozen circuit can rupture the battery case and cause an explosion. Always allow the battery time to thaw before charging it.

It is easy to connect the battery to the charger, turn the charging current on, and, after a normal charging period, turn the charging current off and remove the battery. Certain precautions, however, are necessary both before and during the charging period. These practices are as follows:



Do not permit the baking soda and water solution to enter the cells. To do so would neutralize the acid within the electrolyte.

See that the vent holes are clear and open. Do NOT remove battery caps during charging. This prevents acid from spraying onto the top of the battery and keeps dirt out of the cells.

Check the electrolyte level before charging begins and during charging. Add distilled water if the level of electrolyte is below the top of the plate.


Keep the charging room well ventilated. Do NOT smoke near batteries being charged. Batteries on charge release hydrogen gas. A small spark may cause an explosion.

Take frequent hydrometer readings of each cell and record them. You can expect the specific gravity to rise during the charge. If it does not rise, remove the battery and dispose of it as per local hazardous material disposal instruction.

Keep close watch for excessive gassing, especially at the very beginning of the charge, when using the constant voltage method. Reduce the charging current if excessive gassing occurs. Some gassing is normal and aids in remixing the electrolyte.

Do not remove a battery until it has been completely charged.

Placing New Batteries in Service

New batteries may come to you full of electrolyte and fully charged. In this case, all that is necessary is to install the batteries properly in the piece of equipment. Most batteries are received charged and dry.

Charged and dry batteries will retain their state of full charge indefinitely so long as moisture is not allowed to enter the cells. Therefore, batteries should be stored in a dry place. Moisture and air entering the cells will allow the negative plates to oxidize. The oxidation causes the battery to lose its charge.

To activate a dry battery, remove the restrictors from the vents and remove the vent caps. Then fill all the cells to the proper level with electrolyte. The best results are obtained when the temperature of the battery and electrolyte is within the range of 60°F to 80°F.

Some gassing will occur while you are filling the battery due to the release of carbon dioxide that is a product of the drying process of the hydrogen sulfide produced by the presence of free sulfur. Therefore, the filling operations should be in a well-ventilated area. These gases and odors are normal and are no cause for alarm.

Approximately 5 minutes after adding electrolyte, check the battery for voltage and electrolyte strength. More than 6 volts or more than 12 volts, depending upon the rated voltage of the battery, indicates the battery is ready for service. From 5 to 6 volts or from 10 to 12 volts indicates oxidized negative plates, and the battery should be charged before use. Less than 5 or less than 10 volts, depending upon the rated voltage, indicates a bad battery, which should not be placed in service.

If, before the battery is placed in service, the specific gravity, when corrected to 80°F, is more than .030 points lower than it was at the time of initial filling or if one or more cells gas violently after adding the electrolyte, the battery should be fully charged before use. If the electrolyte reading fails to rise during charging, discard the battery.

Most shops receive ready-mixed electrolyte. Some units may still get concentrated sulfuric acid that must be mixed with distilled water to get the proper specific gravity for electrolyte.

Mixing electrolyte is a dangerous job. You have probably seen holes appear in a uniform for no apparent reason. Later you remembered replacing a storage battery and having carelessly brushed against the battery.

When mixing electrolyte, you are handling pure sulfuric acid, which can burn clothing quickly and severely bum your hands and face. Always wear rubber gloves, an apron, goggles, and a face shield for protection against splashes or accidental spilling.

When you are mixing electrolyte, NEVER pour water into the acid. Always pour acid into water. If water is added to concentrated sulfuric acid, the mixture may explode or splatter and cause severe burns. Pour the acid into the water slowly, stirring gently but thoroughly all the time. Large quantities of acid may require hours of safe dilution.

Let the mixed electrolyte cool down to room temperature before adding it to the battery cells. Hot electrolyte will eat up the cell plates rapidly. To be on the safe side, do not add the electrolyte if its temperature is above 90°F. After filling the battery cells, let the electrolyte cool again because more heat is generated by its contact with the battery plates. Next, take hydrometer readings. The specific gravity of the electrolyte will correspond quite closely to the values on the mixing chart if the parts of water and acid are mixed correctly.

Battery Maintenance

If a battery is not properly maintained, its service life will be drastically reduced. Battery maintenance should be done during every vehicle servicing. Complete battery maintenance includes the following:

If the electrolyte level in the battery is low, fill the cells to the correct level with distilled water (purified water). Distilled water should be used because it does not contain the impurities found in tap water. Tap water contains many chemicals that reduce battery life. The chemicals contaminate the electrolyte and collect in the bottom of the battery case. If enough contaminates collect in the bottom of the case, the cell plates short out, ruining the battery.

If water must be added at frequent intervals, the charging system may be overcharging the battery. A faulty charging system can force excessive current into the battery. Battery gassing can then remove water from the battery.

Maintenance-free batteries do NOT need periodic electrolyte service under normal conditions. They are designed to operate for long periods without loss of electrolyte.



Do NOT use a scraper or knife to clean battery terminals. This action removes too much metal and can ruin the terminal connection.

To clean the terminals, remove the cables and inspect the terminal posts to see if they are deformed or broken. Clean the terminal posts and the inside surfaces of the cable clamps with a cleaning tool before replacing them on the terminal posts.

When reinstalling the cables, tighten the terminals just enough to secure the connection, over-tightening will strip the cable bolt threads. Coat the terminals with petroleum or white grease. This will keep acid fumes off the connections and keep them from corroding again.

Non maintenance-free batteries can have the state of charge checked with a hydrometer. The hydrometer tests specific gravity of the electrolyte. It is fast and simple to use.

A fully charged battery should have a hydrometer reading of at least 1.265 or higher. If below 1.265, the battery needs to be recharged, or it may be defective.

A defective battery can be discovered by using a hydrometer to check each cell. If the specific gravity in any cell varies excessively from other cells (25 to 50 points), the battery is bad. Cells with low readings may be shorted. When all of the cells have equal specific gravity, even if they are low, the battery can usually be recharged. On maintenance-free batteries a charge indicator eye shows the battery charge. The charge indicator changes color with levels of battery charge. For example, the indicator may be green with the battery fully charged. It may turn black when discharged or yellow when the battery needs to be replaced. If there is no charge indicator eye or when in doubt of its reliability, you can use a voltmeter and ammeter or a load tester to determine battery condition quickly.

Battery Test

As a mechanic you will be expected to test batteries for proper operation and condition. These tests are as follows:

Figure 7-3 — Battery leak test.

To perform a battery terminal test, connect the negative voltmeter lead to the battery cable end (Figure 7-4). Touch the positive lead to the battery terminal. With the ignition or injection system disabled so that the engine will not start, crank the engine while watching the voltmeter reading.

Figure 7-4 — Battery terminal leak test.

If the voltmeter reading is .5 volts or above, there is high resistance at the battery cable connection. This indicates that the battery connections need to be cleaned. A good, clean battery will have less than a .5 volt drop.

Figure 7-5 — Battery voltage test.

The battery voltage test is used on maintenance-free batteries because these batteries do not have caps that can be removed for testing with a hydrometer. To perform this test, connect the voltmeter or battery tester across the battery terminals. Turn on the vehicle headlights or heater blower to provide a light load. Now read the meter or tester. A well-charged battery should have over 12 volts. If the meter reads approximately 11.5 volts, the battery is not charged adequately, or it may be defective.

To perform a cell voltage test, use a low voltage reading voltmeter with special cadmium (acid-resistant metal) tips (Figure 7-6). Insert the tips into each cell, starting at one end of the battery and working your way to the other. Test each cell carefully. If the cells are low but equal, recharging usually will restore the battery. If cell voltage readings vary more than .2 volts, the battery is BAD.

Figure 7-6 — Cell voltage test.

A battery drain test checks for abnormal current draw with the ignition off. If a battery goes dead without being used, you need to check for a current drain.

To perform a battery drain test, set up an ammeter, as shown in Figure 7-7. Pull the fuse if the vehicle has a dash clock. Close all doors and the trunk (if applicable). Then read the ammeter. If everything is off, there should be a zero reading. Any reading indicates a problem. To help pinpoint the problem, pull fuses one at a time until there is a zero reading on the ammeter. This action isolates the circuit that has the problem.

Figure 7-7 — Battery drain test.

Before load testing a battery, you must calculate how much current draw should be applied to the battery. If the ampere-hour rating of the battery is given, load the battery to three times its amp-hour rating. For example, if the battery is rated at 60 amp hours, test the battery at 180 amps (60 x 3 = 180). The majority of the batteries are now rated in SAE cold cranking amps, instead of amp-hours. To determine the load test for these batteries, divide the cold-crank rating by two. For example, a battery with 400 cold cranking amps rating should be loaded to 200 amps (400 ÷ 2 = 200). Connect the battery load tester, as shown in Figure 7-8. Turn the control knob until the ammeter reads the correct load for your battery.

Figure 7-8 — Battery load test.

After checking the battery charge and finding the amp load value, you are ready to test battery output. Make sure that the tester is connected properly. Turn the load control knob until the ammeter reads the correct load for your battery. Hold the load for 15 seconds. Next, read the voltmeter while the load is applied. Then turn the load control completely off so the battery will not be discharged. If the voltmeter reads 9.5 volts or more at room temperature, the battery is good. If the battery reads below 9.5 volts at room temperature, battery performance is poor. This condition indicates that the battery is not producing enough current to run the starting motor properly.

Familiarize yourself with proper operating procedures for the type of tester you have available. Improper operation of electrical test equipment may result in serious damage to the test equipment or the unit being tested.


The alternator has replaced the DC (Direct Current) generator because of its improved efficiency (Figure 7-9. It is smaller, lighter, and more dependable than the DC generator. The alternator also produces more output during idle, which makes it ideal for late model vehicles.

Figure 7-9 — Alternator.

The alternator has a spinning magnetic field. The output windings (stator) are stationary. As the magnetic field rotates, it induces current in the output stator windings.

Alternator Construction

Knowledge of the construction of an alternator is required before you can understand the proper operation, testing procedures, and repair procedures applicable to an alternator.

The primary components of an alternator are as follows:

Figure 7-10 — Rotor assembly.

The fingers on one of the claw-shaped pole pieces produce south (S) poles and the other produces north (N) poles. As the rotor rotates inside the alternator, alternating N-S-N-S polarity and AC current are produced. An external source of electricity (DC) is required to excite the magnetic field of the alternator.

Slip rings are mounted on the rotor shaft to provide current to the rotor windings. Each end of the field coil connects to the slip rings.

The stator assembly produces the electrical output of the alternator (Figure 7-11). The stator, which is part of the alternator frame when assembled, consists of three groups of windings or coils which produce three separate AC currents. This is known as three-phase output. One end of the windings is connected to the stator assembly and the other is connected to a rectifier assembly. The windings are wrapped around a soft laminated iron core that concentrates and strengthen the magnetic field around the stator windings. There are two types of stators—Y- type stator and delta-type stator.

Figure 7-11 — Stator assembly.

The Y-type stator has the wire ends from the stator windings connected to a neutral junction (Figure 7-12, View A). The circuit looks like the letter Y. The Y- type stator provides good current output at low engine speeds.

The delta-type stator (Figure 7-12, View B) has the stator wires connected end-to-end. With no neutral junction, two circuit paths are formed between the diodes. A delta-type stator is used in high output alternators.

Figure 7-12 — Stator assembly types.

The rectifier diodes are mounted in a heat sink or diode bridge. Three positive diodes are press fit in an insulated frame. Three negative diodes are mounted into an uninsulated or grounded frame.

When an alternator is producing current, the insulated diodes pass only out- flowing current to the battery. The diodes provide a block, preventing reverse current flow from the alternator.

Alternator Operation

The operation of an alternator is somewhat different than the DC generator. An alternator has a rotating magnet (rotor) which causes the magnetic lines of force to rotate with it. These lines of force are cut by the stationary (stator) windings in the alternator frame as the rotor turns with the magnet rotating the N and S poles to keep changing positions. When S is up and N is down, current flows in one direction, but when N is up and S is down, current flows in the opposite direction. This is called alternating current as it changes direction twice for each complete revolution. If the rotor speed were increased to 60 revolutions per second, it would produce 60-cycle AC.

Since the engine speed varies in a vehicle, the frequency also varies with the change of speed. Likewise, increasing the number of pairs of magnetic north and south poles will increase the frequency by the number pair of poles. A four-pole generator can generate twice the frequency per revolution of a two-pole rotor.

Alternator Output Control

A voltage regulator controls alternator output by changing the amount of current flow through the rotor windings. Any change in rotor winding current changes the strength of the magnetic field acting on the stator windings. In this way, the voltage regulator can maintain a preset charging voltage. The three basic types of voltage regulators are as follows:

The contact point voltage regulator uses a coil, set of points, and resistors that limit system voltage. The electronic or solid-state regulators have replaced this older type. For operation, refer to the "Regulation of Generator Output" section of this chapter.

The electronic voltage regulators use an electronic circuit to control rotor field strength and alternator output. It is a sealed unit and is not repairable. The electronic circuit must be sealed to prevent damage from moisture, excessive heat, and vibration. A rubberlike gel surrounds the circuit for protection.

An integral voltage regulator is mounted inside or on the rear of the alternator. This is the most common type used on modern vehicles. It is small, efficient, dependable, and composed of integrated circuits.

To reduce alternator output, the voltage regulator increases the resistance between the battery and the rotor windings. The magnetic field decreases, and less current is induced into the stator windings.

Alternator speed and load determines whether the regulator increases or decreases charging output. If the load is high or rotor speed is low (engine at idle), the regulator senses a drop in system voltage. The regulator then increases the rotor’s magnetic field current until a preset output voltage is obtained. If the load drops or rotor speed increases, the opposite occurs.

Alternator Maintenance

Alternator testing and service call for special precautions since the alternator output terminal is connected to the battery at all times. Use care to avoid reversing polarity when performing battery service of any kind. A surge of current in the opposite direction could burn the alternator diodes.

Do not purposely or accidentally "short" or "ground" the system when disconnecting wires or connecting test leads to terminals of the alternator or regulator. For example, grounding of the field terminal at either alternator or regulator will damage the regulator. Grounding of the alternator output terminal will damage the alternator and possibly other portions of the charging system.

Never operate an alternator on an open circuit. With no battery or electrical load in the circuit, alternators are capable of building high voltage (50 to over 110 volts) which may damage diodes and endanger anyone who touches the alternator output terminal.

Alternator maintenance is minimized by the use of prelubricated bearings and longer- lasting brushes. If a problem exists in the charging circuit, check for a complete field circuit by placing a large screwdriver on the alternator rear-bearing surface. If the field circuit is complete, there will be a strong magnetic pull on the blade of the screwdriver, which indicates that the field is energized. If there is no field current, the alternator will not charge because it is excited by battery voltage.

Should you suspect troubles within the charging system after checking the wiring connections and battery, connect a voltmeter across the battery terminals. If the voltage reading, with the engine speed increased, is within the manufacturer’s recommended specification, the charging system is functioning properly. Should the alternator tests fail, the alternator should be removed for repairs or replacement. Do NOT forget, you must ALWAYS disconnect the cables from the battery first.

Alternator Testing

To determine what component or components have caused the problem, you will be required to disassemble and test the alternator.

To test the rotor for grounds, shorts, and opens, perform the following:

Figure 7-13 — Testing for grounds.

Figure 7-14 — Testing for shorts.

The stator winding can be tested for opens and grounds after it has been disconnected from the alternator end frame and voltage regulator.

If the ohmmeter reading is low when connected between each pair of stator leads, the stator winding is electrically good (Figure 7-15).

Figure 7-15 — Testing stator for opens.

A high ohmmeter reading or failure of the test lamp to light when connected from any one of the leads to the stator frame indicates the windings are not grounded (Figure 7-16). It is not practical to test the stator for shorts due to the very low resistance of the winding.

Figure 7-16 — Testing stator for grounds.

To test for correct diode operation, disconnect the stator windings and perform the test with an ohmmeter as follows:

After completing the required test and making any necessary repairs or replacement of parts, reassemble the alternator and install it on the vehicle. After installation, start the engine and check that the charging system is functioning properly. Never attempt to polarize an alternator. Attempts to do so serve no purpose and may damage the diodes, wiring, and other charging circuit components.

Charging System Test

Charging system tests should be performed when problems point to low alternator voltage and current. These tests will quickly determine the operating condition of the charging system. Common charging system tests are as follows:

Charging system tests are performed in two ways—by using a load tester or by using a volt-ohm millimeter (VOM/multimeter). The load tester provides the accurate method for testing a charging system by measuring both system current and voltage.

Charging System Output Test

The charging system output test measures system voltage and current under maximum load. To check output with a load tester, connect tester leads as described by the manufacturer, as you may have either an inductive (clip-on) amp pickup type or a non-inductive type tester. Testing procedures for an inductive type tester are as follows:

With the load tester controls set as prescribed by the manufacturer, turn the ignition switch to the run position. Note the ammeter reading.

Start the engine and adjust the idle speed to test specifications or IAW manufacturer’s specifications.

Adjust the load control on the tester until the ammeter reads specified current output.

Do not let voltage drop below specifications (about 12 volts). Note the ammeter reading. Rotate the control knob to the off position. Evaluate the readings.

To calculate charging system output, add the two ammeter readings. This will give you total charging system output in amps. Compare this figure to the specifications within the manufacturer’s manual.

Current output specifications will depend on the size (rating) of the alternator. A vehicle with few electrical accessories may have an alternator rated at 35 amps, whereas a larger vehicle with more electrical requirements could have an alternator rated from 40 to 80 amps. Always check the manufacturer’s service manual for exact values.

If the charging system output current tested low, perform a regulator voltage test and a regulator bypass test to determine whether the alternator, regulator, or circuit wiring is at fault.

Regulator Voltage Test

A regulator voltage test checks the calibration of the voltage regulator and detects a low or high setting. Most voltage regulators are designed to operate between 13.5 to 14.5 volts. This range is stated for normal temperatures with the battery fully charged.

Set the load tester selector to the correct position using the manufacturer’s manual. With the load control off, run the engine at 2,000 rpm or specified test speed. Note the voltmeter reading and compare it to the manufacturer’s specifications.

If the voltmeter reading is steady and within manufacturer’s specifications, then the regulator setting is okay. However, if the volt reading is steady but too high or too low, then the regulator needs adjustment or replacement. If the reading were not steady, this would indicate a bad wiring connection, an alternator problem, or a defective regulator, and further testing is required.

Regulator Bypass Test

A regulator bypass test is an easy and quick way of determining if the alternator, regulator, or circuit is faulty. Procedures for the regulator bypass test are similar to the charging system output test, except that the regulator is taken out of the circuit. Direct battery voltage (unregulated voltage) is used to excite the rotor field. This should allow the alternator to produce maximum voltage output.

Depending upon the system, there are several ways to bypass the voltage regulator. The most common ways are as follows:



Follow the manufacturer’s directions to avoid damaging the circuit. You must NOT short or connect voltage to the wrong wires, or the diodes or voltage regulator may be ruined.

When the regulator bypass test is being performed, charging voltage and current will increase to normal levels. This indicates a bad regulator. If the charging voltage and current remain the same, then you have a bad alternator.

Circuit Resistance Test

A circuit resistance test is used to locate faulty wiring, loose connections, partially burnt wire, corroded terminals, or other similar types of problems.

There are two common circuit resistance tests: insulated resistance test and ground circuit resistance test.

To perform an insulated resistance test, connect the load tester as described by the manufacturer. A typical connection setup is shown in Figure 7-17, View A. Notice how the voltmeter is connected across the alternator output terminal and positive battery terminal.

Figure 7-17 — Circuit resistance test.

With the vehicle running at a fast idle, rotate the load control knob to obtain a 20-amp current flow at 15 volts or less. All accessories and lights are to be turned off. Read the voltmeter. The voltmeter should NOT read over 0.7-volt drop (0.1 volt per electrical connection) for the circuit to be considered in good condition. However, if the voltage drop is over 0.7 volt, circuit resistance is high and a poor electrical connection exists.

To perform a ground circuit test, place the voltmeter leads across the negative battery terminal and alternator housing (Figure 7-17, View B).

The voltmeter should NOT read over 0.1 volt per electrical connection. If the reading is higher, this indicates such problems as loose or faulty connections, burnt plug sockets, or other similar malfunctions.

Test your Knowledge

1. What substance is contained in a positive plate of a fully charged battery?

A. Calcium antimony
B. Lead peroxide
C. Micro-porous rubber
D. Sulfur dioxide

2. What type of gas collects at the top of a battery?

A. Antimony
B. Sulfuric
C. Sulfur
D. Hydrogen

3. What assembly in the alternator contains the heat sink, the diodes, the diode plate, and the electrical terminals?

A. Rectifier
B. Rotor
C. Stator
D. Grounding

4. What type of alternator stator is used in high output alternators?

A. Delta-type
B. Omega-type
C. Y-type
D. K-type