12.0 volts or below - At 12.0 volts your battery is considered to be fully discharged or 'flat' and should be recharged as soon as possible. The lifespan of your battery will be severely affected if it remains within this voltage range for extended periods of time.
A typical 12-volt auto battery will have around 12.6 volts when fully charged. It only needs to drop down to around 10.5 volts to be considered fully discharged. If it drops down below that, the battery will become damaged due to excessive sulfation.
At 12.4 volts the battery is considered 75% charged and can still ignite your engine. When the battery voltage lowers to 12.2 volts without the engine running, it is considered a bad battery that may start your engine but is no longer capable of holding a charge.
Connect the multimeter to the positive and negative battery terminals. If you don't have a voltage of around 12.6 volts, you may have a bad battery. Now start the car, and look for a revised voltage over 10. If your voltage drops below 5 when the car is running, it is bad and should be replaced right away.
A healthy 12 volt battery should maintain a voltage range from 9.6 - 10.5 volts under the load for a good 30 seconds straight.
Load Testing
As long as battery voltage remains above 9.6 volts, the battery is considered “good.” But if it drops below 9.6 volts by the end of the test, the battery may be “bad,” or the battery may have to be recharged and retested if it was not fully charged prior to the test.
To pass a load test, the battery must maintain 9.6 volts at 15 seconds when tested at one-half the CCA rating and 70°F (or above).
THREE TO FIVE, WITH GOOD BEHAVIOR
Battery manufacturers suggest the average lifespan of an auto battery falls anywhere between three and five years — and for good reason.
What Voltage Should My 12v Battery Show? A fully charged 12 volt battery should read at 12.6 volts on the multimeter. If the reading is lower than this, you'll want to charge your battery with either a battery charger or by taking your car for a drive.
11.9v indicates the battery is completely discharged. Anything under 12.2v means the battery is in a state of discharge, and a healthy battery should show 12.6v in a static state. Yours needs to be replaced. 11.9v indicates the battery is completely discharged.
An ideal battery voltage range for any car battery should range from 11.8-12.8v when starting up, increasing to 13.2-14.8v upon ignition of the vehicle. Regularly checking your battery's voltage could save unexpected breakdowns and prevent expensive repairs down the line.
Measuring your car battery's voltage can be a great way to determine how charged your battery might be. The ideal voltage with the engine running is between 13.7 and 14.7V. With the engine off, you should get a reading of 12.6 volts.
Typically, the voltage of a fully charged, 12-volt deep cycle battery is between 12.8V and 13V. But a dead battery's voltage is below 10 volts.
For a car with a 12V engine, the normal value before starting is between 11.9V and 12.8V, and it should not exceed 14.8V after starting.
If the voltmeter reads between 14-15 volts then the battery is normal. But if the battery reads over 15 volts or less than 13 volts, then there might be a problem with the alternator. The alternator is over charging the battery or not giving enough charge to the battery.
Often, a “dead car battery” simply means that it's fully discharged and the voltage is below functional 12V. You can jump-start the dead vehicle and drive it to let the alternator replenish the battery charge. Alternatively, you can attach the dead battery to a battery charger.
Most manufacturers recommend storing a battery for six to nine months maximum before use, and delaying use isn't going to extend the battery's lifespan of three to five years.
If the battery reads below 12 volts, it's considered “discharged.” Driving around can't revive a battery below 12 volts, and attempting to do so might damage the alternator. Instead, connect the battery to a jump starter or battery charger either before or right after doing a jump-start.
If you suspect your battery might have gone bad, measuring voltage is a good first step. If the battery is below 12.6 volts, you should attempt to charge it with a battery charger.
Therefore, most mechanics will recommend that you get your battery replaced when it reaches 50% of its full capacity charge.