There are a few ways to know it's time to change the battery in your vehicle. One is if your car stereo and clock keep getting reset, which means the battery is no longer holding a charge.Another way to tell is if your auto is slow to start or has trouble turning over; this could mean that the alternator is not charging the battery properly.It's also a good idea to periodically check the voltage of your battery; if it falls below 12 volts, that's a sign that it might be time for a new one.