The battery is the heart of a car's electrical system and proper care is essential in order to ensure that the battery is in good shape. A car's battery has to perform two functions namely: supply the starter motor with enough electricity, so that it can start the car's engine and supply current to electric accessories of the car such as the music system and lights, when the car's engine isn't running.
A car's battery is inactive when its engine is running, since the alternator supplies the current required by the car's electrical system. The alternator also replenishes the current lost by the battery in starting the car's engine.
The main constituent of the insides of a car's battery is the electrolyte of the battery, which is generally Sulphuric Acid. The electrolyte of the battery helps the battery to hold its charge. If a car's engine fails to crank or cranks too sluggishly for the engine to be started, checking the strength of the battery's electrolyte can help to ascertain whether the car's battery has a problem or not.
All the cars these days are equipped with maintenance free batteries which enable one to know the specific gravity rating of the battery by just looking into the sight glass of the battery. The specific gravity is the measure of weight of an acid, when compared with weight of an equal volume of water. The ideal specific gravity range for a car battery's electrolyte is 1.230 to 1.300. If a battery has a specific gravity rating which is below this range, it would not be able to store the current required for starting the car's engine.
The strength of a battery's electrolyte reduces as the battery gets old. When a battery shows a specific gravity reading of between 1.200 and 1.230, it may still have enough energy to help the starter motor crank the engine except during cold weather, when the starter motor would require more power than the battery would be able to supply. But, if a battery's specific gravity rating falls to a value below 1.200, it will not have sufficient energy in it to start the car under any condition.
The sight glass of a maintenance free battery shows three different indications namely: a green dot inside a black border, black and clear or light yellow.
1. Green dot inside black border - Specific gravity of the battery is between 1.230 and 1.300 and the battery is in good shape.
2. Black - Specific gravity of the battery is between 1.200 and 1.230 and the battery could cause starting troubles in cold weather.
3. Clear or light yellow - Time to replace the battery with a new one.
If you are replacing your car's old battery with a new one, replace it with a maintenance free battery as it eliminates the need to constantly keep inspecting the battery to check whether it is in proper shape or not.
To keep your battery terminals in good shape and to avoid corrosion of the terminals, it is a good idea to spread a light coating of petroleum jelly onto the battery terminals once in a while.
Source - Motor Ward