Everything from the car ignition system, to the on-board computer in modern cars, is dependent on it.
Automotive batteries are usually referred to as SLI (Starting, Lighting, Ignition) batteries, as they supply electrical energy and enable working of ignition system, lights, and starter motor of a car. They are (electrochemical) lead-acid type batteries. Most of them are a combination of six galvanic cells, connected in series.
Every cell is made up of lead and lead oxide plates, dipped in sulfuric acid (with a concentration of 35%). A chemical reaction between the plates and the electrochemical fluid enables the storage of charge and discharge of a battery.
The reversible reaction of lead, lead oxide, and sulfuric acid, reacting to produce lead sulfate, is responsible for the discharge and recharge of the battery.
Each of the six electrochemical cells provides a voltage output of 2.1 V, providing a total voltage output of 12.6 Volts. It is essential that the sulfuric acid be diluted and a certain minimum water level be maintained inside, for efficient charging and discharging in the battery.
The battery is charged by the car's own dynamo, called an alternator. When the car runs, the dynamo is driven to produce charge, which is supplied and stored in the battery.
If the alternator is not working properly, then the battery will get discharged faster. A low battery voltage level may indicate that something's wrong with the battery or the alternator. So, when checking an alternator, the battery output voltage needs to be tested.
Do not smoke when you are checking, as a safety measure. Wear safety glasses and gloves while testing. Check the physical condition of the battery. Check for any cracks in the outer casing. Make sure that all the battery connections are in order and aren't suffering from damage.
The first stage requires that you shut down the car and test output voltage. Put the digital multimeter dial on DC volt. Connect the positive test lead to positive terminal of the battery and negative test lead is connected with negative battery terminal.
If the voltage output is anywhere in the range of 12V to 12.4V, then the battery is in excellent condition. If it's substantially below these values, you need to get it checked.
Next, start the ignition and put the car in neutral gear. Let it stay in idling mode for a while. Then, check the output voltage again. If voltage is in the range of 13.8 V to 14.3 V, then the battery is working fine. If the voltage is lower than these values, you need to check the alternator and the battery.
Majority of car electrical problems arise from malfunctioning of the alternator and a dip in battery charge. Make sure that you make checking of battery voltage a regular exercise, or else you may be left high and dry, with a dead battery.