# Understanding 12V Car Battery Voltage Readings: Explained

• Automotive
• Hondaboi1729
In summary, the nominal voltage of a 12V battery is 12.6V when fully charged and composed of 6 cells, each producing 2.1V. However, when installed on a car engine, the voltage may read higher due to the alternator/generator producing a voltage of 14V or slightly higher. The voltage of the battery may also vary depending on its state of charge and other factors like temperature. The spark plugs require a much higher voltage of around 25,000V, which is generated by an electronic box of tricks. The frequency of the spark plugs firing may increase when the engine is revved, but the voltage at the plugs remains relatively constant. Overall, the voltage output of a battery is
Hondaboi1729
Why are these batteries stated as 12V batteries if often they have a reading of greater than 12V e.g 13 or 14 and when you rev the engine hard while in neutral and do a voltage test on some components in the car circuit you get a reading of much much greater like 40V! How is this working? Could someone please explain?

40 volts is a bit much. But you do need to provide significantly more than 12 volts to recharge a 12-volt battery.
If you allow the battery to discharge (engine off) into a moderate load, perhaps two headlights, you should get roughly 12 volts.
All batteries, especially rechargeable batteries, are like this.

A battery is considered dead when it can no longer perform in the appliance where it is being used.

Hondaboi1729
12V is the nominal rating. It distinguishes them from 6V or 24V lead-acid batteries.

DC-DC converters can change the voltage higher or lower to fit the needs of different components. If you check the USB charging port in your car, you should see 5V.

Hondaboi1729
So when you rev the engine hard the rpms get high- the frequency of the spark plug fire gets much higher which requires more work from the battery and given voltage output is on a circuit needs basis and not fixed the voltage output of the battery increases to the 20's, 30's ... Is this correct?
Is the nominal voltage of the battery simply when there is no load? when the ignition is turned off and nothing is drawing power it goes back to 12V?
does the battery just rapidly fall towards 12V when the the engine is not running and the alternator isn't charging it like at time 3:23 of this vid (skip to that time)

The battery slightly changes voltage as it charges and discharges. A charged car battery might get close to 13V, while if it drops below 12V you might have trouble starting the car.

You cannot quickly increase the battery voltage if you suddenly need it. You can only increase it during charging and it definitely won't go to 20V. When you put a heavy load on the battery the voltage might drop a little. It will almost always be in the 11-13V range.

How often the spark plugs fire won't change the battery voltage (the change should be negligible). If the engine is running, you are actually measuring the alternator voltage and it might change slightly depending on the rpms.

Hondaboi1729
Hondaboi1729 said:
So when you rev the engine hard the rpms get high- the frequency of the spark plug fire gets much higher which requires more work from the battery and given voltage output is on a circuit needs basis and not fixed the voltage output of the battery increases to the 20's, 30's ... Is this correct?
Didn't watch the vid, but whatever made you post the above you should completely forget. It quite simply wrong. An automotive charging system that is pushing system voltage to 20 volts or higher is seriously broken.

davenn and Merlin3189
I think I have been confused by some vids I've watched. I'll come back when I've experimented with some actual real life circuits and read up some more and still confused... thanks all.
Edit: I watched a video about charging systems- the 20+ volts I seemed to remember was the ac output voltage from the generator/alternator... my bad.

Last edited by a moderator:
Hondaboi1729 said:
So when you rev the engine hard the rpms get high- the frequency of the spark plug fire gets much higher which requires more work from the battery and given voltage output is on a circuit needs basis and not fixed the voltage output of the battery increases to the 20's, 30's ... Is this correct?

No.

The voltage needed by the spark plug is much higher than 12-14V, usually its nearer 25,000 volts. This is generated by an electronic box of tricks, including a coil, that acts like a step-up transformer to boost the voltage from 12-14V at the battery to 25,000 volts at the plugs. When you rev the engine the frequency of the firing increases but the voltage at the plugs doesn't vary much.

When the engine is running the battery is charged by the alternator. In order to "push" electricity back into the battery the alternator has to produce a voltage of at least 14V. The exact voltage it generates will depend on the rpm, however there is a regulator that restricts it to just over 14V so when the engine is running and the alternator and regulator are working the voltage on the battery should be around 14V. On my ride on mower I was measuring 14.4V yesterday.

Is the nominal voltage of the battery simply when there is no load? When the ignition is turned off and nothing is drawing power it goes back to 12V?

Yes. However the exact voltage will depend on it's state of charge. There is a table here which tells you the voltage vs charge but its not very reliable because the voltage will also vary depending on other factors like temperature.

https://image.slidesharecdn.com/dsc...stems-seminar-020311-44-728.jpg?cb=1297171312

Hondaboi1729
A lead-acid 12V battery is actually composed of 6 cells producing a nominal voltage of 2.1 volts. So if you measure the voltage of a fully charged battery it should read 12.6 V (= 6 X 2.1). Similarly, a 6V battery has 3 cells totaling a voltage of 6.3 V and a 24V battery has a 12 cells giving 25.2 V.

When you read a voltage higher than that when installed on an engine, you actually read the voltage produced by the generator/alternator. The voltage of the generator/alternator depends on its rpm. To make sure it stays constant, there is a voltage regulator installed on the generator/alternator output (today, the regulator is most likely incorporated within the alternator). The «true» voltage needed by a car electrical system is 14 V. All electrical components are designed to work at that voltage, because this is the voltage needed for recharging the battery as explained in previous posts. Therefore, the voltage regulator tries to maintain that voltage. When the voltage regulator exceeds 15 V, it is because it malfunctions.

Mechanical Voltage Regulator from a 60's GM car (was bolted on the firewall, apart from the generator/alternator)

A typical electrical gauge. Note the green zone (12-15V), the «overload» zone (11-12V) the «undercharging» zone (<11V) and «overcharging» (>15V).

Hondaboi1729
Hondaboi1729 said:
Why are these batteries stated as 12V batteries if often they have a reading of greater than 12V ...
May I introduce you to Battery University? From the Charge Methods section, Charging Lead Acid may help in your understanding.

Regards

CWatters and Hondaboi1729
Thank you so much Don! What an excellent resource that website is!

dlgoff

## 1. What is a 12V car battery?

A 12V car battery is a rechargeable battery that is used to power the electrical systems of a car. It provides the necessary energy for the engine to start and also powers the lights, radio, and other electronic components in the car.

## 2. Why is it important to understand the voltage readings of a car battery?

Understanding the voltage readings of a car battery is important because it can give you an indication of the health and condition of your battery. It can also help diagnose any potential issues with your car's electrical system.

## 3. What is the normal voltage reading for a 12V car battery?

The normal voltage reading for a 12V car battery is between 12.4-12.6 volts when the battery is fully charged. However, the voltage may decrease slightly when the battery is in use.

## 4. What does it mean if the voltage reading of my car battery is below 12V?

If the voltage reading of your car battery is below 12V, it could indicate that your battery is partially discharged or has a weak charge. This could be due to a faulty charging system or a malfunctioning battery.

## 5. How can I test the voltage of my car battery?

You can test the voltage of your car battery using a multimeter. Set the multimeter to the DC voltage setting, connect the positive (red) lead to the positive terminal of the battery and the negative (black) lead to the negative terminal. The voltage reading should be between 12.4-12.6 volts for a fully charged battery.

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