The batteries used in cars contain six separate cells. If a cell goes dead, the battery may appear to fully charge but won’t function properly. Once a cell goes dead, the battery is bad and must be replaced. The best method to test a car battery for a dead cell is to check the specific gravity of the electrolyte fluid. Specific gravity is the density of the electrolyte fluid compared to water. The specific gravity for the electrolyte is ideally 1.265. A low specific gravity in one cell compared to other cells means it is dead.
Put safety first. Car batteries contain sulphuric acid, which can harm the eyes and skin. Wear rubber gloves and safety glasses and work in a well-ventilated area. Keep open flames away from the battery.
Disconnect the battery cables using a crescent wrench, starting with the negative battery terminal (marked with a minus sign). Connect the battery to a battery charger and allow it to charge to capacity. For best results use a three-phase battery charger designed for lead acid batteries. When charging is complete, disconnect the leads to the battery charger.
Use a screwdriver to pry the plastic caps off the top of the battery. Take your time doing this so you don’t damage the caps.
Insert the rubber tube of a battery testing hydrometer into a battery cell. Hydrometers are used to test specific gravity and may be purchased at auto parts stores. Hold the hydrometer vertically and squeeze the rubber bulb several times to draw up the battery fluid from the cell. Make certain the hydrometer is full.
Read the specific gravity from the hydrometer indicator. Squeeze the bulb afterward to return the battery fluid to the cell and write the reading down.
Repeat the specific gravity test for each cell of the battery. Compare the readings. If any cell shows a specific gravity more than 0.05 less than the others, the cell is dead and the battery will have to be replaced. For example, if five cells read 1.260 and one reads 1.254 (a difference of 0.06) the cell is dead.
Temperature affects specific gravity readings. To maximise the accuracy of your readings, use a temperature conversion chart if the temperature is much greater or less than 26.7 degrees Celsius. To avoid having to reset your car radio and other accessories, purchase a memory saver that plugs into a car cigarette lighter (available at auto parts stores). The memory saver supplies enough power to maintain the settings while the car battery is disconnected.