A good car battery should read 12.4-12.9 volts when the car is off. Anything lower doesn't necessarily mean the battery is bad. Your car's electrical system may have drained it, or there may be an issue with your alternator. Recharge the battery and test it again later to see if it's holding a charge.
If your battery has a charge between 12.4V and 12.7V, your battery is fully charged and ready to run. If your voltage is below 12.2V, it needs to be recharged. Take a 30-minute drive on the highway or pick up a charger to bring the voltage back up. If your voltage is higher than 12.9V, the battery is overcharged.
If you test your car battery while it is actively running, a healthy unit will read between 13 volts and 15 volts. If your battery is fully charged after a long drive, the voltage output might read as ~12.4 volts – 12.9 volts.
A good alternator should produce about 13.5 to 14.5 volts with the engine idling. If the voltage reading is low (12.5 volts or less), it indicates a charging problem but does not tell you what's causing it.
Voltage is not the same as amperage
So, even though most car batteries are only 12 volts, they are capable of generating a current that's around 180 times as powerful as what's needed to power a 40-watt light bulb.
12V isn't a shock hazard, but it IS a burn hazard.
Even without a short circuit, if you make or break an electrical connection that has a lot of current going through it, the point at which the connection is made can get very hot very quickly and can burn your fingers.
Simply put, voltage can be thought of as “pressure,” so while a car battery may technically have enough amperage to kill you, the paltry 12 volts DC simply doesn't provide enough pressure to push any significant amount of amperage through the contact resistance of your skin.
A good alternator should maintain battery voltage between 13.9 and 14.8 volts (14.2 is optimum). Even worst-case, with all accessories turned on, there should be at least 13 volts at the battery.
For some alternators, it's normal to see as low as 13 volts. For some, a constant 13.6 volts is an indication of a problem. For others, it's normal to see as high as 16 volts for long stretches. Still, it's perfectly normal for others to see the alternator not charge at all intermittently.
12.6V volts or above - Your battery is healthy and fully charged. No further action is required. 12.5 volts - Your battery is at a healthy state of charge, but we'd recommend re-checking it within a few days to ensure the voltage hasn't dropped any further.
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.
It should be sitting between 12.6 and 12.8, though it can be a little bit lower depending on the weather. If the voltage is above 12.8, you should drain the battery a little bit by using the electrical components before turning it on. If the voltage reads below 12.6, you probably need to charge your battery.
A fully charged battery should register at 12.7 volts or more. If the voltage falls below this number, the battery should be recharged. A battery is only charged by one quarter at just 12 volts. Once it drops to 11.9 volts, your battery is considered dead.
if you are reading 12.8 then it is fully charged. My standby lead acid battery sits at 12.90 V after being disconnected from the charger for 48 hours. Most car batteries are a bit lower at about 12.6 V. Charging voltage is normally 13.6–14.8 V at full charge.
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.
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.
The 12.8v indicates the alternator had quit working. However in a small car if you had the airconditioning on, the electric radiator fan comes on and you have the headlights on that might happen. Keep an eye on the voltmeter. If the alternator is cutting out it will likely go bad soon.
Fully-charged, most car batteries will measure at least 12.6 volts (~13.0-13.2V for OPTIMA YELLOWTOPs), but low voltage doesn't necessarily equal a bad battery. Modern cars are very demanding from an electrical perspective, even when they're not being used.
When the vehicle is started, without the introduction of a load (the lights or air conditioner), the volts will usually rise to an average of 14 volts. If the multimeter reads at more than 14.8 volts, then this can be considered as overcharging.
It's The Current That Kills
The real measure of shock's intensity lies in the amount of current (amperes) forced though the body, and not the voltage. Any electrical device used on a house wiring circuit can, under certain conditions, transmit a fatal current.
While a car battery has enough amperage (electrical power) to kill you, it doesn't have enough voltage (electrical force – to push the electrons through your body). Your body is just not conductive enough to be fried by 12 volts.
To achieve this safe current level the voltage across the human body must not exceed 100 volts.