Starting and Charging

Basic Alternators

A basic alternator will comprise of a single generator winding (stator) which remains stationary, and a two pole magnetic field (rotor) that will move around in the stator.

Electrical Supply

A good supply of electrical power is needed for modern vehicles. Many components that were once mechanical are now driven by small electric motors and controlled by ECU's, hence the need for a good electrical supply. The engine requires large amounts of current to start and many other systems are now electrically powered.

Rechargable Batteries

A rechargeable battery is an electrochemical unit that converts an electric current into a modified chemical compound. This chemical compound in the battery stores energy, which is available as electricity, when it is connected to a circuit.

Starter Motor Function

A starter motor starts the combustion process. A high amount of mechanical power is required to rotate the engine and the starting current is usually in the region of 250-500 Amps. The starter motor connects into the ring gear on the flywheel in the gearbox to achieve this.

Number of Cells in a Battery

A vehicle's 12 volt battery is made up of six cells. Each lead acid cell has a nominal voltage of 2.1V, which gives an overall value of 12.6V for a fully charged battery. The cells are connected in series internally in the battery with lead bars.

Inertia Type

An inertia type motor is used on older vehicles. The main difference in design between an inertia type and a pre-engaged type is that the solenoid is not installed on the starter itself. The inertia type starter uses inertia to engage the pinion gear but is still operated by a remote solenoid installed in the engine bay.

Charging Batteries

Charging the batteries can be dangerous if not done correctly. The ignition must always be switched off. A technician should also always be aware of potential sparks when connecting cables; PPE such as safety glasses and gloves can provide protection when connecting cables. An ideal charge rate for a batter is 1/10th of the amp/hour capacity.

Starter Tests - Solenoid Feed

Checking voltage at the solenoid input (feed) terminal while cranking is another form of starter testing. The red positive pin should be attached to the feed wire of the starter terminal, and the black earth pin to the starter body or the battery negative post. The voltage can be less than the battery voltage, but ideally should be within ~0.5V of the battery voltage. So if a battery voltage was 12V, the minimum an acceptable solenoid feed voltage can be is 11.5V.

Alternator testing - Voltage

Connect the multimeter to the alternator. Turn the multimeter on and set it to check DC voltage. Place the positive lead to the output terminal on the battery positive and the negative lead to the negative terminal on the battery. Start the engine and observe the metre reading - this should be around 14.2-14.3V. Load the battery by switching on some high current drawing consumers i.e. media unit, air conditioning, heated rear windscreens etc, and rev the engine to ~1200RPM. Monitor the reading on the metre, which should remain above the nominal voltage of 12.6V.

Charging Batteries

Current flow reverses the chemical action. Sulphates are forced back into the electrolyte and combined with the hydrogen, while oxygen returns to the positive plate to form lead peroxide. The process continues while voltage is applied until all the lead sulphate has been converted.

Alternators

Early vehicles used DC generators. Electricity was generated by a rotor spinning within a magnetic field (the opposite of a DC motor). Modern day vehicles use alternators. Electricity is generated in the stationary stator windings by a spinning magnet: a rotor with an energised field winding and iron finger poles.

Discharging Batteries

Electrolyte hydrogen combines with the positive plate oxygen to form water. Electrolyte sulphates combine with lead on both of the plates to form a lead sulphate.

Preventing Damage on Solenoids

If the engine started under these conditions, it would drive the motor spindle to excessive speeds. To prevent damage, a unidirectional overrun clutch is fitted to the pinion. This allows the motor to drive the engine, but prevents the engine from driving the motor.

Low Voltage Reading

If the reading is low then the same test should be carried out at the alternator, rather than the battery. If the readout from the alternator is correct, it will indicate a broken wire from the alternator to the battery.

Maintenance Free Batteries

Maintenance free batteries are completely sealed, or have only a very small vent to allow for the expulsion of gases from the battery. The maintenance free battery does not lose water from the electrolyte in the same way as conventional lead acid batteries do. Liquids, gases and water vapour are also captured and returned to the battery cells.

Modern Alternators

Most alternators are constructed with three windings. The windings have been arranged so that a separate AC waveform is induced into each winding as they pass through the rotating magnetic field. This results in a three phase current, which gives a better utilisation of the alternator.

Conventional Batteries

Most vehicle batteries are of a conventional design, using lead plates in a dilute sulphuric acid electrolyte. This feature leads to the common description of Lead Acid batteries.

Pre-Engaged

On a pre-engaged motor, the drive pinion is brought into mesh by the action of the electromagnetic solenoid that is mounted on the starter motor casing.

Regulator

Output voltage from an alternator must be limited to prevent the battery from being overcharged and to protect the electrical equipment from an excessive voltage. The regulator sets the alternator voltage to ~14.8V. Since this voltage corresponds to a fully charged battery, the alternator must vary its charging current to suit the state of charge of the battery.

State of Charge

State of charge is best evaluated by measuring the density of the electrolyte in each cell using either a bulb type hydrometer or a refractometer. Specific gravity (SG) of the electrolyte indicates the state of cell charge.

State of Charge Testing

State of charge testing (SoC) is looking at how charged or discharged a battery is, not how much capacity it has. A fully charged battery should have an open circuit voltage of 12.65V. Voltage readings can identify defective cells.

Drive Belt

The alternator drive belt must be in good condition to ensure that the alternator operates correctly. A loose belt can affect charging and if the belt were to snap, then the vehicle will illuminate the battery light on the dashboard and the vehicle will stop charging.

Ampere Hour

The ampere hour (expressed as ampere hours or Ah) is the amount of current a fully charged battery can supply for 20 hours (industry standards) without having the terminal voltage fall below 10.5V. This test is made at a temperature of 80℉ or 26.7℃. If a battery can deliver 4A under these conditions then it is an 80Ah battery (4A x 20 hours = 80Ah).

Armature

The armature is made up of copper loops, the ends of which are connected to copper segments. The segments are fully insulated from each other and also from the shaft, to form a commutator through which the brushes can convey current to the armature windings. The core, which carries the windings, is mounted on a steel shaft and is made up of several soft iron laminations.

Battery Electrolyte

The battery electrolyte is a solution inside batteries. Depending on the type of battery it can be a liquid (the most common form) or a paste like substance. The electrolyte serves the same purpose despite the different types; it transports positively charged ions between the cathode (positive) and anode (negative) terminals.

Battery Cell Plates

The cell plates are formed in a lattice grid of lead-antimony or a lead calcium alloy. The grid carries the active material and acts as the electrical conductor inside the cell. The active materials are lead peroxide for the positive plate and spongy lead for the negative plate. A separator keeps these two plates apart.

Ring Gear

The drive from the starter motor is taken from a pinion gear to a large diameter starter ring on the engine. The can ring gear is fitted to the outside of the flywheel on a manual transmission or on the torque converter drive plate for automatic transmissions.

Testing Battery Voltage - Starter Tests

The first test on a starter should always be to confirm that the battery has sufficient voltage and is in a good state of charge. This will ensure that a battery fault isn't causing a mis-dagnosis.

Function of the Battery and Charging System

The main function of the battery and charging system is to provide a source of electrical power for all the starting and charging requirements on the vehicle. The electrical power needs to be provided under all conditions. The output from a battery is direct current. An alternator generates an AC current that is then converted to DC before it is fed into the battery in order to charge the battery.

Field Coils

The main magnetic field consists of four pole shoes around which are wound field coils. This produces a concentrated field with four magnetic poles. The field coils are wound to produce alternate north and south poles. The field coils are either conventional or wave wound depending on the type of starter.

Pinion

The pinion meshes with the ring gear only during starting and is made to slide axially on or with the spindle in order to engage the drive when operated.

Rotor

The rotor used in an alternator has several magnetic poles. This allows a greater total output per revolution due to the increased electrical pulses within the stator windings. The rotor is an electromagnet, not a permanent magnet as shown in a basic alternator. This allows control over the strength of the rotor magnetic field and therefore varies the output of the alternator depending on the battery state of charge.

Rotor Construction

The rotor winding is wound around an iron core, which is pressed onto a steel shaft. The two ends of the rotor field winding are connected to the slip rings mounted onto the shaft. Current is fed via a pair of brushes that contact to the slip rings, this then energises the winding and creates an electromagnet within the rotor.

Starter Tests - Voltage Drop across the Solenoid

The solenoid contacts can be checked for voltage drop by connecting a voltmeter or multimeter set to check DC voltage across the two main terminals of the solenoid. When the operating switch is open (the ignition key is turned), then the voltmeter/multimeter should register battery voltage.

Solenoids on Starter Motors

The solenoid has a soft iron plunger that is drawn into the magnetic field that is generated inside the solenoid when an electrical current from the battery is passed through the solenoid windings. On the plunger is a lever that is on a pivot so that when one end is pulled into the solenoid, the opposite end pushes the pinion into mesh with the starter ring gear. At the other end of the solenoid are electrical contacts that form the switch to pass the electrical current on to the motor.

Solenoid Windings

The solenoid on most pre-engaged motors has two windings: closing and holding. The closing winding engages when power is put through the solenoid. It allows the motor to rotate during the engagement phase. Once the switch contacts are fully engaged, the holding winding will hold them in place, and the closing winding will not conduct once the motor current has been switched on.

How Inertia Types Work

The speed of the motor rotating throws out the pinion on the spiralled part of the pinion shaft, forcing the pinion gear to engage with the ring gear. The gear is held in mesh by the force created and released once the starter motor is switched off.

Starter Motor Function

The starter motor is there to simply start the vehicle. The battery is key to both of these systems, and the battery needs to be in good condition in order to supply the starter and receive a charge from the alternator.

Stator

The stator is made up of laminated iron segments and three sets of copper windings. This unit is secured to the inside of the alternator casing. A larger gauge copper wire is used to form the windings. Three wires (one from each winding) extend out and connect to the rectifier, these carry the AC electro-motive force (voltage) induced by the rotation of the rotor inside the stator.

Diode Rectification

The three phase current generated in the stator windings must be rectified to DC in order to charge the battery. This is achieved with the aid of diodes in a pack, in this case silicon diodes. The diodes are arranged to block the negative waves of an alternating current and give a pulsating direct current.

Types of Starter Motor

There are two main types of starter motor: the pre-engaged and the inertia. The pre-engaged starter motor is the most commonly used these days.

Built in Hydrometers

These are indicators of battery health on maintenance free batteries. If the green dot is visible then the battery is sufficiently charged for further testing like a heavy load test. If the dot is a very dark green then the battery requires recharging before further testing is carried out. If a pale or yellow dot is visible then the battery needs replacing.

Starter Tests - Motor Voltage

This checks the voltage through the wiring and through the solenoid, and it will confirm the village that has dropped through the complete circuit. The positive pin from the multimeter should be attached to the starter motor itself. If the voltage is more than ~0.5V compared to the battery voltage then it indicates a high resistance through the solenoid. This would then indicate that either the solenoid or the entire starter motor would be in need of replacement.

Cranking Amps

This is another battery rating, and should not be confused with cold cranking amps. Cranking Amps are the batteries ability to deliver a cranking current at 32℉ or 0℃. The CA rating is tested in the same way as the CCA, except it is calculated at a higher temperature.

Starter Tests - Supply Voltage

This is the checking of the main feed into the starter motor from the supply wire. This should be as close to the battery voltage as possible, any drop in voltage here would indicate a high resistance. The red positive pin from the multimeter should be on the positive battery feed into the solenoid (the thicker of the two wires).

Cold Cranking Amps

This rating indicates the ability of a battery to deliver a specified current at low temperatures. The rating is determined by the amount of current a fully charged battery can supply for 30 seconds at 0℉ or -17.8℃ without having the battery terminal voltage fall below 7.2V.

Reserve Capacity

This refers to the time taken in minutes for a battery to discharge to a cell voltage of 1.75V (10.5V at the terminal) when supplying a constant current of 25 amps. This test reflects a typical current draw for a vehicle that would be needed if the charging system failed during night driving (when there is a heavier draw on the battery). The reserve capacity for a 40Ah battery will be approximately 60 minutes.

Starter Tests - Earth Circuit

To check the voltage drop on the earth line, the voltmeter/multimeter should be connected to the starter earth (commutator end bracket) by the positive pin and the battery earth terminal with the negative pin. When the starter operating switch is closed, the voltage reading should be practically zero.

Voltage Drop - Ground Side

To test voltage drop on the ground side, the red positive pin would be set on the ground point of the alternator, like the casing, while the earth pin would be set again on the positive battery terminal. The engine should then be started and the reading taken from the multimeter.

Voltage Drop - Positive Side

To test voltage drop on the positive side, the multimeter would be set to DC current, with the red positive pin set at the positive output of the alternator and the earth pin set on the positive terminal of the battery. The engine should then be started and the reading taken from the multimeter.

Battery Maintenance

When carrying out any battery maintenance, it is important to check the leads are in good condition. This can cause faults that may seem like a battery fault or can create heat around the batteries from high resistance.

Testing Batteries

When testing batteries, a battery tester should be used rather than a multimeter. A battery could read the correct voltage but still be faulty. You will need to read the data of the battery before using the tester, this could be the AH or CCA information from the battery.

Bridge Network Rectifier

When two field windings are used to produce two waveforms, an arrangement known as a bridge network rectifier is used. As there are now three field coils on most alternators, six diodes are required for rectification of all three phases of the alternator output.

Powering Starter Motors

While the starter motor is being operated, the battery is also required to supply current to the ignition system. If the battery is in a poor condition, the starter may drain the battery of sufficient current that the ignition system cannot function correctly, which would result in a non-start.

Vehicle Power Draw

With the engine off, the battery supplies electricity to the systems. With the engine running, the charging system supplies electricity to the systems. The alternator is driven from the crankshaft pulley, and its output voltage varies between 14V and 14.8V.

Alternator testing - Amps

Worn brushes limit field current, which can cause a low alternator output. To test, load the unit as in the voltage test and measure the field current with a current/amp clamp or use a multimeter. A reading close to the alternator's max output should be shown, but the manufacturer's data should be checked to confirm this whether via Autodata, a manufacturer specific data site or a sticker on the alternator.

Solenoids

he solenoid is mounted on the starter motor (for pre-engaged motors). The solenoid essentially acts as the on switch for the pre-engaged starter motor as it has a lower energy requirement than the motor itself. The solenoid has two purposes: it engages the pinion into the ring gear and it also switches the high current required by the motor.