Cars · Lesson 6 of 15Intermediate7 min read

The Electrical System

Every modern car runs on electricity, even one that burns gasoline. The battery starts the engine, the alternator keeps everything powered once it’s running, and a web of wires, fuses and grounds carries current to every light, sensor and computer.

Volts, amps and ohms

Voltage is electrical pressure: the push that makes charge move. Current, measured in amperes (amps), is how much charge flows. Resistance, measured in ohms, is how hard the path is to push through. Ohm’s law ties them together. For example, a 12-volt battery pushing through 6 ohms drives 2 amps; through 8 ohms, only 1.5 amps.

voltage = current × resistance (E = I × R, so I = E ÷ R)

The battery

A car battery is a lead-acid battery: plates of lead and lead dioxide in sulfuric acid. Each cell makes about 2 volts, so six cells in series make a 12-volt battery. It’s rechargeable: running current backward through it reverses the chemical reaction.

A well-charged 12-volt battery reads a bit over 12 volts with the engine off. The Navy’s mechanic course says a reading around 11.5 volts means it’s undercharged or may be bad. Charging a battery gives off hydrogen gas, which can explode if a spark or flame reaches it, so batteries are charged in a ventilated space, away from sparks and smoking.

Starting

When you turn the key or press the start button, a small current flows through the ignition switch to the starter solenoid. The solenoid is a heavy-duty switch: it connects the battery to the starter motor with a big current, and the starter spins the engine’s flywheel until the engine runs on its own. On automatic transmissions, a neutral safety switch lets the starter work only in Park or Neutral. Don’t crank the engine for more than 30 seconds at a time, or the starter can be damaged.

Charging: the alternator

Once the engine runs, a belt spins the alternator, a generator. Spinning a magnetic field past coils of wire makes alternating current. Six diodes in a rectifier turn that into direct current for the battery and the car. A voltage regulator controls the output; most hold about 13.5 to 14.5 volts. That’s why a healthy charging system reads higher with the engine running than the battery does with it off.

Source: U.S. Navy Construction Mechanic Basic, Chapter 8 (most regulators hold about 13.5 to 14.5 volts).

Circuits, grounds and fuses

Electricity needs a complete loop. Cars save a wire in nearly every circuit by using the steel body and frame as the return path: one battery terminal, the negative on most cars, is connected to the body, and each part is grounded to the metal near it. A loose or corroded ground breaks the loop just as surely as a cut wire.

Open circuit: a break in the path, such as a loose wire, a corroded terminal or a blown fuse. Nothing works.
Short circuit: current takes a shortcut, often where insulation has worn through. Too much current flows and can overheat the wiring and start a fire.
Fuses protect against shorts: a thin metal strip melts and opens the circuit before the wiring overheats.
Relays let a small current control a big one. Current through a small coil makes a magnetic field that pulls a switch closed, the same idea as the starter solenoid.

Parts can be wired in series, one after another, so a single break stops everything, or in parallel, side by side, so one failure doesn’t affect the others. That’s why one burned-out headlight doesn’t turn off the other.

Reading a simple wiring diagram

Trace a circuit the way current flows: from the battery’s positive side, through the fuse, through the switch, to the load (a lamp, motor or other device), and then to ground. When a load doesn’t work, a technician checks each link in that chain: power at the fuse, power through the switch, power at the load, and a good ground.

Read a wiring diagram from the battery’s positive side, through the fuse and switch to the load, then to ground. Ground symbols mean “connected to the car’s metal body,” which carries current back to the battery. Source: U.S. Navy Construction Mechanic Basic, Chapter 7.

Safe multimeter checks

A digital multimeter set to DC volts is a safe first tool. Touch the red probe to the battery’s positive terminal and the black probe to the negative terminal.

Engine off: a well-charged battery reads a bit over 12 volts. Around 11.5 volts means it’s low or failing.
Engine running: most charging systems hold about 13.5 to 14.5 volts. A reading close to the engine-off number suggests the alternator isn’t charging.

Check voltage with the meter across two points. Leave current measurements on car circuits to experienced hands: the starter draws far more current than a hand meter can carry. Check a fuse by eye (a broken strip) or by testing continuity with the fuse pulled out of its socket, and replace it only with one of the same amp rating.

Parts and what they do
Part	What it does	Signs of trouble
Battery	Stores energy to start the engine and run accessories.	Slow cranking, clicking, a reading near 11.5 volts.
Starter and solenoid	Spin the engine to start it.	Clicking or no cranking even with a well-charged battery.
Alternator	Recharges the battery and powers the car while running.	Battery light, dim lights, low running voltage.
Voltage regulator	Holds charging voltage in range.	Running voltage too low or too high.
Fuses	Melt to protect wiring from too much current.	One circuit dead; a fuse that blows again points to a short.
Relays	Let a small current switch a big one.	A dead circuit with a good fuse; a technician tests it.
Grounds	Return current to the battery through the body.	Loose or corroded connections break the circuit.
Wiring harness	Bundles of wires carrying power and signals.	Burned smell (like burned toast) from overheated insulation.

You canWhat you can safely do

Measure battery voltage, engine off and running, as above.
Find the fuse boxes and fuse chart in your owner’s manual, and replace a blown fuse with one of the same rating.
Keep the battery terminals clean and tight. Wear eye protection: battery acid is caustic.
Learn to jump-start a car safely: Jump-Start a Car.

StopWhen to call a professional

A fuse that blows again right away means a short somewhere; find the cause rather than fitting a bigger fuse. A smell like burned toast often signals an electrical short and burning insulation. Wiring repairs, computer problems and anything near the airbag system belong with a technician. In hybrids and electric cars, anything orange or marked high-voltage is off-limits (Lesson 11).

Check yourself

Tap an answer to check it, or open “Show the answer.”

A 12-volt battery drives current through a 6-ohm circuit. How much current flows?
Show the answer
2 amps. I = E ÷ R = 12 ÷ 6 = 2 amps.
Why does a car battery read more voltage with the engine running?
Show the answer
The alternator is charging it. The alternator and regulator hold the system at about 13.5 to 14.5 volts while the engine runs.
What does a fuse do?
Show the answer
Melts to open the circuit before the wiring overheats. A fuse is a deliberate weak link that protects the wiring.
In most cars, what carries current back to the battery?
Show the answer
The steel body and frame. The body and chassis are the return path, so each circuit needs only one wire.
A battery reads about 11.5 volts with the engine off. What does that suggest?
Show the answer
It’s undercharged or may be bad. A well-charged battery reads a bit over 12 volts. Around 11.5 means it’s low or failing.

Go deeper

U.S. Navy, Construction Mechanic Basic (NAVEDTRA 14264A), Chapter 7: Basic Automotive Electricity The Navy’s chapter on basic automotive electricity.
U.S. Navy, Construction Mechanic Basic (NAVEDTRA 14264A), Chapter 8: Automotive Electrical Circuits and Wiring Batteries, charging, starting and testing in depth.
OpenStax Chemistry 2e, 17.5 Batteries and Fuel Cells The chemistry inside a lead-acid battery.
OpenStax College Physics 2e, 23.5 Electric Generators How a spinning coil and magnet make electricity.
Being an Adult 101: Jump-Start a Car Step-by-step jump-starting.

Sources

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A free course from Teddy Rook Book. It explains how cars work in general and follows U.S. sources; your owner’s manual and your vehicle’s service information are the final word for your car, along with state and local rules.