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The heart of a car's electrical system are the battery and alternator. Together, they supply the vehicle with the electricity needed to power the brain of a car; the engine control unit (ECU). The electrical system also consists of critical components, including the

• starter
• ignition system
• fuel injection system
• emissions system

And some modern luxury cars contain more than 100 individual computers. These are just a few.

• Air bag system
• Antilock brakes
• Antitheft system
• Automatic wipers
• Entertainment system
• Electronic-controlled transmission
• GPS system
• Power/heated seats
• Stability and traction control

At 12 volts, a fully charged battery stores the electricity necessary to not only power all the above components but, most important, to start the car. The battery consists of six stacked lead plates immersed in a water and sulfuric acid mixture. Explosive gases escape through the battery cover vents. Take care when charging or jump-starting batteries. “Maintenance-free” batteries replace sulfuric acid with a gel, which is safer.

A dead or dying car battery is one of the most common problems motorists face. In addition to possibly preventing the car from starting, a poorly performing battery can affect the operation of almost any vehicle system. Use only high-quality batteries, and replace your battery at the end of its useful life as part of proactive, preventive maintenance.

The alternator is a magnet rotating inside looped copper and an iron core. It produces electricity that is delivered to the battery. When the engine is running, a drive belt spins the alternator pulley. Depending on its size, an alternator can generate between 1.5 and 2.5 volts, resulting in a total electrical system output of 13.5 to 14.5 volts. A voltage regular keeps total system output below 14.5 volts to prevent overcharging and damaging the battery.

When you turn the ignition key, electrical current is sent from your battery directly to the starter, which cranks the engine’s flywheel and moves the pistons. At the same time, the battery sends electrical current to the ECU, which signals the fuel injection system to inject gasoline mixed with air into the engine. The ignition system then ignites a spark to begin combustion.

Even before the car is started, electric power is sent to the fuse box, which activates accessories including lights, power windows, and the radio. Accessories that draw heavy amperage, such as aftermarket stereos and entertainment systems, can overload batteries and alternators, wearing them out twice as fast as normal.

Some newer lighting systems contain light-emitting diodes (LEDs), tiny individual lights that can burn several times longer than traditional bulbs while requiring a fraction of the electrical energy—something the battery and alternator appreciate. LEDs are also much brighter, allowing automakers to improve the quality and safety of nighttime driving.