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Car Battery Voltage: Know the Basics

Fernando the electronics guy is an electronics engineer. He earned his Bachelor of Science in Electronics Engineering from UC Riverside.

Car battery voltage doesn't look like this. However, it's a nice picture.

Car battery voltage doesn't look like this. However, it's a nice picture.

Car battery voltage may be confusing for people that have never dealt with it. They are especially confusing when the battery goes bad. Or, when it goes dead overnight. Most consumers are used to regular common household batteries such as the 9-volt battery and 1.5v battery. What does it mean for the battery to be a 1.5v battery?

Car batteries are rated at 12 volts. As above, ask yourself the same question. Why is the car battery rated at 12 volts?

The Voltage Drop

When electricity passes through a system, things happen. These things cause the voltage to be lower at the output than the input. When the car battery voltage passes through said system, it is said to be dropping voltage. It doesn’t drop all of it, however - just much as it needs to drop and moves on.

What does this have to do with car battery voltage? If you’ve ever noticed, a fully charged car battery is usually about 12.6 volts.

Most electrical and electronic systems in automotive applications today drop voltages all around. The overall electrical and electronic systems have been designed in a way where it needs at least 12 volts from the battery. If this voltage is exceeded, the system knows how to ensure it only extracts what it needs.

So, in essence, you need at least 12 volts. This is sufficient. However, this also means anything below 12 volts doesn’t allow the complete system to work and function properly.

How to test car battery with multimeter

Grab any multimeter. Find the voltmeter option. Make sure it’s DC and not AC. Note: AC looks like a squiggly, wavy symbol. DC is straight and looks reminiscent of the paint on the road for those middle lanes.

With positive probe on voltmeter option and negative probe on negative port, get to work. Place positive probe to positive terminal of battery. Place negative probe to negative terminal of battery.

It doesn’t matter if you do this backwards. You’ll still get a number. You can ignore the negative sign in this instance. But for good practice, put positive to positive and negative to negative. Build the wrong habits and you might find yourself in some trouble when you’re measuring other things that aren’t voltage.

There will be no sparks, due to how voltmeters operate. It’s measuring the potential difference between two points – not current.

Once you have that number, compare it to what it should be.

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Normal car battery voltage

Usually, 12 volts is considered normal. However, due to possible parasitic drain (common with old cars), some have at least 10.8 volts. Shoot for at least 11.8 volts to minimize damage.

Car battery voltage range

The car battery voltage may range anywhere from 0 volts to 12.8 volts when turned off, and up to 14.8 volts when the car is running. The extra voltage is due to the alternator connected to the battery probes. Note how I mentioned the alternator is connected to the probes. So why doesn’t checking the car battery voltage process get messed up because the alternator is there?

The alternator only pitches into voltage when the car is turned on because there is voltage coming from the alternator to the battery. When the car is turned off, there is no reverse current or reverse voltage effects. The reason for this is the diodes found on the alternator. They simply block the battery from seeing the alternator. However, these same diodes allow the alternator to see the battery.

Fully charged car battery voltage

When turned off, you’re looking at about 12.8 volts. After start, anywhere up to 14.8 volts (thanks to the alternator adding in voltage).

Car battery reading 10.8 volts

For most applications, that’s fine. Your battery discharged a bit. It’s still usable, it should start the car. If it doesn’t, charge the battery first before attempting again.

Whenever the battery falls below 12 volts and you know you weren’t using the stereo, look for parasitic drain. Usually, the headlights are left on, or the interior lights were left on for an extended period of time. Whatever the cause, you need to figure out why it's dipping down to 10.8 volts and fix the problem.

My concern is if you don’t find the reason the car battery voltage is 10.8 volts, there may be damage to the system.

Worst case scenario, the diodes on the alternator are allowing current to flow backwards while the car is resting and turned off. This usually warrants a rebuild of the alternator or a complete replacement altogether.

Minimum voltage to start a car

As stated above, the minimum voltage to start a car is regarded as being 10.8 volts. Anything below that and you may not start the car. This isn’t completely accurate, as it depends on the load your vehicle is receiving in attempting to start the car. This load is different for every vehicle – welcome to the engineering world.

All in all, keep your voltage as close to 12 volts as possible and you won’t have to worry about minimum voltages.

Conclusion

Car battery voltage is confusing for many. But with a little bit of knowledge and knowhow, it doesn’t have to be that way. Understand the voltage drop and why it happens. You’ll start to see why the car battery voltage sits a little above 12 volts (12.8 volts max). You’ll also start to understand why checking the car voltage while the car is on doesn’t give you an accurate picture of the battery’s health (up to 13.8 volts thanks to the alternator pitching in).

This content reflects the personal opinions of the author. It is accurate and true to the best of the author’s knowledge and should not be substituted for impartial fact or advice in legal, political, or personal matters.

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