There are several ways to tell if your car's 12V battery is bad. Usually, a faulty or weak battery would immediately impact your car's electrical and electronics accessories. So, take a look and see if the headlights are running dim, or if the engine is slow to crank. Some cars also have dedicated battery warning lights on the dash, which would light up if it notices something wrong with the car's battery. Otherwise, you can inspect the battery visually. Should you notice a rotten egg smell, have your battery replaced as soon as possible. The latter can sometimes be at risk of overheating and catching on fire. You may also take a close look at the battery itself. If you spot any swelling on the battery housing, it would be a good idea to have it replaced, as well.