Lead-acid automotive batteries convert chemical reactions into electrical energy using an electrochemical process. Over time, the lead plates in a battery will combine with the sulphuric acid electrolyte to create lead sulphate crystals. As these crystals cover the lead plates, the electrochemical process slows to a point that the battery is no longer a useful source of electrical energy.
Unfortunately, a fully charged battery looks much like a discharged battery. To tell whether a battery can be used or not, the battery must be tested with a device called a digital multimeter.
Turn on the multimeter. Set the multimeter scale to "Volts DC."
Attach the positive (red) probe to the positive battery terminal. Attach the black (negative) multimeter probe to the negative battery terminal.
Read the multimeter display; the voltage reading should be between 12.2 and 12.6 volts DC.
A battery with a voltage reading above 12.2 volts but below 12.6 volts may require charging before use.