Charging system basics

How does my automobile's alternator and battery work?

Charging System Basics
There is a great deal of misunderstanding regarding the operation and upgrading of a vehicle charging system. In this section we will cover the basic operation of the charging system, as well as some recommendations for upgrading the charging system

 Alternators

An alternator is designed to accomplish a couple of tasks in a vehicle. One task is to keep the vehicle battery charged so that it will start the engine. The other task is to supply current to the vehicles accessories when the engine is running. This holds true unless the alternator’s voltage drops to a point of equal potential to the battery. At this point, the battery will also begin to deliver current. As long as the alternator is at a higher potential than the battery, the alternator is the vehicles main power source. This also means that if the alternator is at a higher potential, the battery is nothing but a load on the alternator. Each battery on average will consume about 5-10 amps of current from the alternator just to stay charged.

Think of the alternator as a pump that makes water that is at a higher level (potential ) than the water in the battery. At this point the alternator pumps the battery with water (current).

 Current Flow (Alternator Off)

Shown in the diagram below is the current flow with the engine off. Notice that current flows from the battery to the vehicle accessories. The alternator supplies nothing and is essentially out of the circuit.

 Current Flow (Alternator On)

Shown in the diagram below is the current flow with the engine running. Notice that current now flows from the alternator to the battery and the vehicle accessories. The battery supplies nothing as long as the alternator can supply enough current to keep its voltage higher than the battery. Whether you have 1 battery or 10, when the engine is running the battery is a load.

 Battery and Alternator at the Same Potential (Engine Running)

Shown in the diagram below is when the alternator and the battery are at the same potential or the same voltage when the engine is running. This is when the accessories and the addition of a high powered audio system draws more current than the alternator can supply. At this point, the current flows from both the alternator and the battery to supply the vehicles accessories and the audio system.