How can tell your battery need changing?

Your question needs clarification. Car battery, Cell phone battery, camera battery, etc.

Question was posted in car batteries category.

You can check it with a digital volt meter car not running. Set the meter to 20 volts DC. Here is what you will see.

12.6 volts = 100% charge

12.4 volts = 75% charge

12.2 volts = 50% charge

12.0 volts = 25% charge