The charging system is what provides the electrical current for your vehicle. Without a charging system, your battery will be depleted and your vehicle will shut down. The charging system gauge or warning lamp monitors the health of this system so that you have a warning of a problem before you get stuck.
There are two types of gauges used to monitor charging systems:
It measures system voltage. A modern automobile has a 12-volt electrical system. A fully charged battery will read about 12.5 volts when the engine is not running. When the engine is running, the charging system takes over so that the voltmeter will read 14 to 14.5 volts and should stay there unless there is a heavy load on the electrical system such as wipers, lights, heater and rear defogger all operating together while the engine is idling at which time the voltage may drop. If the voltage drops below 12.5, it means that the battery is providing some of the current. You may notice that your dash lights dim at this point. If this happens for an extended period, the battery will run down and may not have enough of a charge to start the car after shutting it off. This should never happen with a healthy charging system because as soon as you step on the gas, the charging system will recharge the battery. If the voltage is constantly below 14 volts, you should have the system checked. If the voltage ever goes above 15 volts, there is a problem with the voltage regulator. Have the system checked as soon as possible as this "overcharging" condition can cause damage to your electrical system.
It measures amperage. If the battery is fully charged and there is minimal electrical demand, then the ammeter should read close to zero, but should always be on the positive side of zero. It is normal for the ammeter to read high positive amperage in order to recharge the battery after starting, but it should taper off in a few minutes. If it continues to read more than 10 or 20 amps even though the lights, wipers and other electrical devices are turned off, you may have a weak battery and should have it checked.