[Tommy]

Wikipedia has good info on how various gauges work. But the water analogy helps a lot of people better understand electrical matters. Electricity involves the flow of elctrons along wires sort of like water flowing through pipes. The amount of water flowing through a pipe (watts for electricity) is determined by the pressure (voltage) and the size of the pipe (amperage). In a car, the battery is like a reservoir of pressure that can be used to push electrons through the electrical system. When the engine is started, the alternator acts like an electron pump to push electrons into the system. When the alternator produces more pressure (voltage) than the battery, it forces electrons back into the battery, recharging it.

An ammeter measures the flow of electrons along a pathway. If it is wired between the battery and alternator, it will indicate whether electrons are flowing into or out of the battery. The amount of electricity flowing into or out of the battery can indicate the state of charge of the battery and the effectiveness of the charging system.

A voltmeter indicates the voltage (electrical pressure) on a pathway. Since most car batteries charge to around 13 volts, a voltmeter on a fully charged car battery should read near that level. When the engine is running, the charging system should produce more than 13 volts to power the car and recharge the battery. By monitoring the voltmeter before and after the engine is started, you can judge the condition of both your battery and your charging system.

I personally perfer the voltmeter over the ammeter.