The first and most likely indication of a low battery in your vehicle would be a hard starting problem caused by slow cranking, Yahoo Autos describes. If the battery seems weak or fails to crank your engine normally, it may be low. To find out, you need to check the battery's "state of charge."
As Yahoo Autos explains, "A battery is nothing more than a chemical storage device for holding electrons until they're needed to crank the engine or run the lights or other electrical accessories on your vehicle. Checking the battery's state of charge will tell you how much juice the battery has available for such purposes."
The charge level depends on the concentration of acid inside the battery. The stronger the concentration of acid in the water, the higher the specific gravity of the solution, and the higher the state of charge.
On batteries with removable caps, state of charge can be checked with a "hydrometer." Some hydrometers have a calibrated float to measure the specific gravity of the acid solution while others simply have a number of colored balls. On the kind with a calibrated float, a hydrometer reading of 1.265 (corrected for temperature) indicates a fully charged battery, 1.230 indicates a 75 percent charge, 1.200 indicates a 50 percent charge, 1.170 indicates a 25 percent charge and 1.140 or less indicates a discharged battery. On the kind that uses floating balls, the number of balls that float tells you the approximate level of charge. All balls floating would indicate a fully charged battery, no balls floating would indicate a dead or fully discharged battery.
Some sealed-top batteries have a built-in hydrometer to indicate charge. The charge indicator only reads one cell, but usually shows the average charge for all battery cells. A green dot means the battery is 75 percent or more charged and is okay for use or further testing. No dot (a dark indicator) means the battery is low and should be recharged before it is returned to service or tested further. A clear or yellow indicator means the level of electrolyte inside has dropped too low, and the battery should be replaced.
