A good battery should be between 12.4 to 12.6v at rest, it will drop some if any accesories are on (lights, radio, phone or gps chargers, flashers, etc) if you have an automotive voltage/amperage tester available, you can do a load test on the battery (hook it up and load it by 1/2 the cca rating of the battery for 15 seconds) if it drops below 10v, it's time for a new battery. Most autoparts stores can do this test for you, but beware, if they don't know what they are doing, they could test it as needing replacement when it does not. Also, the test must be done fully charged. Also, after testing it will be mostly dead and unable to start your car, requiring a recharge. After starting the engine battery voltage goes very high. Thats normal.

Case Study 1: if your car doesn't start also no power at all (no dashboard lights/no other lights working) but after you check it then recognize the battery is also OK. Then this may be a car ground problem.You will need to check your car grounds.: