A car battery can be bad even if it is still pushing 12 volts. Voltage is not the only factor that determines the condition of a battery. The capacity of a battery, also known as its "state of charge" (SOC), is an important factor that determines a battery's condition.
If the measured reading is less than 12.2 volts, the battery's resting voltage is weak, which means it most likely needs to be charged or replaced.
If a vehicle has a battery that keeps running down, but the battery and charging system both test OK, the problem may be a key-off current drain that's sapping the battery of its reserve power.
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 fully charged battery is 12.7V, a discharged battery is 12.1V so you are at approximately 1/2 charge. The more important reading is the battery voltage when you try to engage the starter. If that battery drops below 12.1 Volts, it just may not start your car.
A 12V battery with a voltage of 11.4V or less is fully discharged. Under 10.7V, a lead acid battery is dead. Lithium-ion and Lithium-polymer batteries also have a dead voltage – what this is depends on the battery configuration.
If the battery measures 12V with no load it is bad or badly in need of a charge. If it reads 10.5V or less when under starter load, you need a new battery.
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.
Next, you should only test a battery after it's been sitting unused for at least an hour. This is known as “resting voltage”. If you test it immediately after use, you can receive a false reading from a unit charged by your car's electrical system, also called a surface charge.
Even if your battery isn't old, corrosion on or around the battery can still be behind why your car battery won't hold a charge. Corrosion prevents the battery from reaching a full charge, so you'll be driving around with a poorly performing battery.
A dead battery can leave you stranded — if you're able to start your car at all — so watch out for these signs of a failing battery: Clicking sound when trying to start your engine. Slow cranking — may sound like “rurr, rurr, rurr,” when trying to start your vehicle. Illuminated check engine or check battery light.
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.
Classic sign of an old battery at the end of its useful life, it can have proper voltage but not be able to provide enough current.
It is generally true that a fully charged 12V lead-acid battery should have a resting voltage of around 12.6V to 12.8V, and a voltage of 12.3V is typically considered around 50% charged. However, there can be some variability in these numbers depending on the specific battery chemistry, temperature, and other factors.
For a car with a 12V engine, the normal value before starting is between 12.2V and 12.8V, and it should not exceed 14.8V after starting.
Answer: You would rather be safe than sorry when it comes to your car battery, as it is best to avoid getting stuck without a functional vehicle due to a dead battery. Therefore, most mechanics will recommend that you get your battery replaced when it reaches 50% of its full capacity charge.
If the engine starts but dies immediately, your alternator probably isn't keeping your battery charged. If a jump starts and keeps your car running, but the car can't start again off of its own power, a dead battery is probably your answer.
Car Battery Voltage
If it is in the 11-12 voltage range, you may need to have it professionally recharged by a mechanic or with a dedicated charging machine. Anything below that and you are probably best off replacing the battery.
Some cars will get up to five or six years out of their battery, while others will need a new one after only two years. In general, your car will usually need a new battery after three to four years.
The battery can be fully charged but still not be able to start the vehicle if it is failing or has a dead cell. Other likely issues are the fuel pump, ignition switch, battery cables, starter, or a clogged fuel filter.
2) Take a Voltage Reading
Cannot reach higher than 10.5 volts when being charged, then the battery has a dead cell. Fully charged (according to the battery charger) but the voltage is 12.4 or less, the battery is sulfated.