How to Diagnose a Dead Battery Cell
- 1). Charge the battery on a battery charger. Any testing of the battery must be done on a fully charged unit. Testing a battery that is not fully charged, will give false readings during the testing process. Follow the battery manufacturer's instructions for charging that type of lead acid battery. If you are unsure as to the charging process, locate the manufacturer's service phone number. This toll-free number is located on top of all batteries sold in the United States.
- 2). Allow the battery to sit idle for at least three hours after charging. It is preferable to allow 12 hours to pass for the best results in testing. Remove the surface charge from the battery. Attach a car headlamp to the battery and allow the lamp to operate for a few minutes. Be sure to use the correct voltage lamp for the battery. In other words, connect only a 12-volt lamp to a 12-volt battery. Turn the lamp off. The battery is ready to be tested.
- 3). Use the DC voltmeter and check the voltage of the battery. Place the red lead from the voltmeter to the positive (+) terminal on the battery. Touch the black lead from the voltmeter to the negative (-) terminal on the battery. The voltmeter should read in the area of 12.4 volts or greater for a 12-volt battery. A 6-volt lead acid battery should read 6.2 volts or greater. Anything less than this voltage, for either type of battery, indicates a bad cell.
- 4). Use a hydrometer to check the specific gravity of the battery fluid in each individual cell. If the battery is a sealed lead acid (SLA) skip this step and proceed to step 5. You will not have access to the individual cells in an SLA battery. Remove the top caps from each cell. Pull a sample of the cell's fluid into the hydrometer. A specific gravity reading of 1.225 or higher indicates a good cell. Lower readings may indicate either a bad cell or sulfation to the lead plates. In either case, a low specific gravity reading will indicate a bad battery cell.
- 5). Attach the load tester to the battery following the manufacturer's directions. In most cases, the red clamp will be connected to the positive (+) terminal on the battery. The black clamp will be attached to the negative (-) terminal on the battery. Depress the load switch for the amount of time indicated on the face of the tester. Different battery types will have various testing load times. Read the meter on the face of the tester. The meter will have a description as to what the readings mean on the face of the load tester. In most cases, a "good" reading means a good battery. A "bad" reading means a bad battery.