Car batteries can last three to five years on average, but when yours dies, it usually fails without a warning.

The truth is you might already have a dead car battery.

Your battery could be 75% charged or 100% charged and still be at death's door, the same way a cracked glass can be full of water. A fully charged battery could also be so aged, so sulfated and so weak that it can only hold a little bit of power for a brief period of time.

However, warm temperatures, a healthy engine and routine drives can mask that weakness. If the temps drop, a light gets left on or you dont drive a few days, that's when your car won't start.