How to Tell If an Automotive Battery Is Bad?
- 1). Charge the battery fully before testing it. You can take a long daytime drive or use a battery charger. Let the car sit for 12 hours to dissipate any surface charge the battery may have.
- 2). Find the specific gravity of the electrolytes in your battery with a hydrometer--if you have an unsealed battery. (If you have a sealed battery, skip this step and go to Step 3.)
Pry the caps off the battery with a screw driver. Squeeze the bulb of the hydrometer and release it while the tip is in the electrolyte. Read the floating ball in relation to the printed scale on the hydrometer and see if it meets manufacturer specifications. If not, the battery should be replaced.
- 3). Check the electrolyte in sealed batteries by looking at the "eye" in the battery case, a round plastic peep hole in the top or side of the battery. A solid green color means the electrolyte is in good condition, while black means that must be replaced.
- 4). Test the battery's voltage with a digital voltmeter or multimeter. Attach the positive lead of the voltmeter to the positive battery terminal and the negative lead to the negative terminal. Attach them directly to the terminals and not to the top of the battery cable clamps. Compare the voltage reading to the manufacturer specifications. A good battery will have a voltage of 12 to 12.9 volts. If it is lower than this, replace it.
- 5). Get another person to start the vehicle while the voltmeter is still attached. Watch the voltmeter reading very carefully. It should not show less than 10 volts at any point while the car is being started.