How the Starting and Charging System works

If you're having trouble starting your car, it could be the battery, starter, alternator, or another component of your starting and charging system that is responsible. It's important to inspect each one of them before you begin replacing parts. Whether you choose to diagnose your parts yourself or bring it to us, AutoZone is here to help you find out what's wrong, so you can do the job right the first time.

It's always a good idea to get a repair manual for your specific vehicle. Pop the hood and familiarize yourself with your starting and charging system. Learn about each component, what each component does and how to inspect for damages and other problems.

How Vehicle Starting and Charging Works

1. The battery supplies power to your vehicle

The starting battery converts chemical energy into electrical energy. The three essential parts for this conversion are the anode, the cathode, and the electrolyte.

  • The anode is the negatively-charged side of the battery
  • The cathode is the positively-charged side
  • The electrolyte is a buffer that separates the anode and cathode

Chemical reactions inside of the battery cause electrons to build up in the anode. Like with magnets, the similarly charged particles repel one another, but in a battery the electrolyte blocks these particles from merely switching sides by moving over to the opposite side.

If your battery is more than 4 years old, you should get it tested so you won’t be caught off-guard by a dead battery. You can do it yourself using a voltmeter or battery tester, or bring it to AutoZone to be tested for free.

2. Battery cables and terminals carry current from the battery

Battery cables connect the battery’s terminals to the vehicle. One end of each cable is connected to either the negative or positive terminal.

  • The other end of the negative cable is often connected to the engine or somewhere on the vehicle’s frame
  • The positive cable’s other end is typically connected to the starter or a fuse box

Because the electrolyte keeps the charged particles from crossing the battery, electrical current moves from the battery, through the cables, and into the vehicle.

Corrosion and loose connections can keep your cables from working the way they should. Be sure to inspect your cables for looseness or corrosion from time to time. If you have a problem, it’s easy to tighten the cables or to remove corrosion with a wire brush.

3. Relays, fuses, and the starter receive electrical current

The moment you turn the key to start in your car (or push the button) a signal is sent to a relay, or series of relays. The main relay in this group is referred to as the starter solenoid. The signal, coming from the ignition, is low amperage. When it hits these relays, and the starter solenoid, this engages a high-amperage current which directly engages the starter.

Your vehicle’s relays and fuses regulate current in the electrical systems.

  • A relay is basically a switch that allows small currents to control accessories that would require heavy switches and wiring. Relays reduce the amount of wiring needed by having larger contact areas that can handle large amounts of current. The starter solenoid is generally one of the largest relays in your car because it handles such a huge amperage amount.
  • Many vehicles have fuses in-line in their ignition system as well or a main battery or system fuse. These are often seen mounted either directly on the battery cable, or nearby in the engine’s fuse/relay compartment.
  • Fuses protect your vehicle’s electronic devices and wiring. If something like, say, your radio tries to draw too much current, the radio’s fuse will blow, cutting off the current’s path to the radio. This prevents the excess current from damaging the radio or melting wires in the system.

After reaching the relays and fuses, current continues to the vehicle’s various electrical systems.

4. The starter turns the engine

When the starter solenoid engages, the starter’s gear, or “bendix” moves out to engage the flywheel or flexplate of the engine, turning it over. The turning of the engine allows the fuel air-mixture to enter into the engine cylinders for combustion, starting the engine.

A clicking noise when you turn the key is a good indicator of low battery voltage that is sufficient enough to engage the relays, but not enough to turn the motor. Often times, you will hear a buzzing sound. This sound is your starter solenoid not fully engaging because of low or insufficient voltage. Or, in some cases, a click indicates that your starter has gone bad. In these cases, a visual inspection and/or removal and bench testing is needed, which AutoZone provides. If the starter is soaked with oil contamination and requires replacement, be sure the oil leak is repaired to prevent damage to the replacement starter.

5. The accessory belt or serpentine belt drives the alternator

Once the engine is turning, rotational force from the crankshaft turns the belt pulley. The belt transfers this rotational force to other systems, like the power steering pump, the water pump, and most importantly for this article: the alternator.

6. The alternator recharges the battery

The alternator is responsible for 2 things at the same time. It supplies voltage power to everything your car uses while it is running. While doing this, it also sends charge to the battery to maintain its charge as well, and in doing so, the battery acts as a capacitor. An alternator works no different than a windmill or turbine, generating AC power, which is then converted to DC power inside the alternator. The 3 items responsible for this inside the alternator are the rotor, stator, and rectifier. The alternator works similarly, except it is turned by the belt rather than a hand or the wind. The two components responsible for generating electricity are the rotor and the stator:

  • The rotor features magnetized finger poles that rotate surrounding the wire field winding. Each finger pole is a different length to produce alternating north and south poles for the winding.
  • The stator is just enough larger that the rotor can spin without touching the stator walls.
  • A rectifier is in a sense a valve. It converts AC power (Alternating Current) into DC power (direct current).

The spinning magnetized poles generate current in the winding, and this current provides the power that charges your battery. Inside your alternator, a voltage regulator makes sure that this voltage is maintained at a proper level for your car’s electrical system. This is usually somewhere between 14-16 volts.

Test your alternator using an alternator/battery tester or voltmeter (Learn how to test an alternator here). Ensure the drive belt is in good condition and does not slip on the alternator pulley. A dead battery, one that won’t stay charged, or headlamps and interior lights that appear dim can indicate that your alternator is possibly bad. Visually inspect your alternator for signs of the problem. If the alternator is replaced, be sure the battery is fully charged before starting the engine. This will prevent overload and potential damage to the replacement alternator. If the drive belt shows signs of wear, replace it to ensure the alternator will perform properly. If equipped with an automatic belt tensioner, inspect it for wear and binding/sticking. If it does not move smoothly it should be replaced along with the belt.

Remember that AutoZone can help with any of your Battery, Alternator, or charging system needs through replacement parts or testing. If you need additional help, browse through our Preferred Shops in your local area to help you diagnose and do the job!

Learn More About Starting and Charging Parts

Find out how individual components of this system function, and how to work on them.

Other Parts You Can Check:

Fuses and fusible links protect the circuits in the charging system. A fusible link is a fuse imbedded in a wire.

  • Inspect fuses for breaks or gaps in the wire filament
  • Test fusible links using a voltmeter

Dashboard warning lights, gauges and LED readouts can provide clues to the source of your problem

  • Make sure warning lights go off after your car has started
  • Make sure the voltage gauge shows a reading of 12 volts when the car is off and rises to 14 volts after it’s started
  • Pay attention to your Check Engine Light or Message Center Light; They can also indicate a problem

If you’re working on the system, stop by AutoZone for free battery, starter, or alternator testing.*

*Battery test and charge available only in-store in California. Starter and Alternator testing service not available in California.

Advice, how-to guides, and car care information featured on and AutoZone Advice & How-To’s are presented as helpful resources for general maintenance and automotive repairs from a general perspective only and should be used at your own risk. Information is accurate and true to the best of AutoZone’s knowledge, however, there may be omissions, errors or mistakes.

Be sure to consult your owner’s manual, a repair guide, an AutoZoner at a store near you, or a licensed, professional mechanic for vehicle-specific repair information. Refer to the service manual for specific diagnostic, repair and tool information for your particular vehicle. Always chock your wheels prior to lifting a vehicle. Always disconnect the negative battery cable before servicing an electrical application on the vehicle to protect its electrical circuits in the event that a wire is accidentally pierced or grounded. Use caution when working with automotive batteries. Sulfuric acid is caustic and can burn clothing and skin or cause blindness. Always wear gloves and safety glasses and other personal protection equipment, and work in a well-ventilated area. Should electrolyte get on your body or clothing, neutralize it immediately with a solution of baking soda and water. Do not wear ties or loose clothing when working on your vehicle.

FREE Loan-A-Tool® program requires returnable deposit. Please note that the tool that you receive after placing an online order may be in a used but operable condition due to the nature of the Loan-A-Tool® program.

Related Posts