The document discusses common reasons for car battery drain, including: 1. Faulty charging from an alternator that is not working properly can drain the battery even when driving. 2. Old batteries that are past their lifespan of 4-5 years may not hold a full charge, causing starting issues. 3. User errors such as leaving lights on overnight can drain the battery if not caught. 4. A defective alternator that does not properly charge the battery or has circuit issues can automatically drain the battery.