If you're experiencing issues with your car's electrical system, such as a dead battery or dimming headlights, here's how to determine if it is caused by the battery or the alternator: 1. Check the battery voltage: Use a multimeter to check the voltage of the battery. A fully charged battery should read around 12.6 volts. If the battery voltage is low, it may be an indication that the battery is not holding a charge. 2. Check the alternator output: Start the engine and use a multimeter to check the voltage output of the alternator. The voltage output should be between 13.5 and 14.5 volts. If the voltage output is low, it may be an indication that the alternator is not charging the battery properly. 3. Check the battery connections: Make sure the battery connections are clean and tight. Loose or corroded connections can cause issues with the electrical system. 4. Load test the battery: Use a battery load tester to check the battery's ability to hold a charge. If the battery fails the load test, it may be an indication that the battery is bad. 5. Have the alternator tested: If the battery is holding a charge but the alternator output is low, it may be an indication that the alternator is bad. Have the alternator tested by a professional mechanic to determine if it needs to be replaced.