The most common causes of battery failure in cars are:

1. Age: Most car batteries last between 3 to 5 years. After that, they start to lose their capacity to hold a charge.

2. Corrosion: Corrosion on the battery terminals can prevent the flow of electricity.

3. Overuse: If you frequently leave your car lights or other devices on when the engine is off, it can drain the battery.

4. Extreme temperatures: High temperatures can cause the battery to evaporate faster, while freezing temperatures can reduce the battery's ability to hold a charge.

5. Lack of maintenance: Not keeping the battery clean and the terminals tight can lead to reduced performance and eventual failure.