Lead-acid car batteries have a voltage of 12 volts. But when you measure the voltage of your vehicle’s battery with a multimeter, the device often shows higher voltage readings. In many cases, the multimeter will show 14 volts instead of 12 volts. Is there something wrong with the battery or your vehicle? Or do you have a 14-volt car battery instead of a 12-volt one?
A typical lead-acid car battery is made up of half a dozen cells arranged in series. Each battery cell is rated for 2 volts. Thus, the combined voltage of a battery with six cells is 12 volts.
However, a fully charged battery will show more than 12V. Its battery cells will also display higher voltages.
There are several reasons for the discrepancy between manufacturer-claimed voltage and voltage readings. Let’s look at each factor that makes it seem like you have a 14-volt car battery.
First things first: Is the battery fully charged? A fully charged car battery will show higher voltage because of a phenomenon called surface charge. We’ll go over surface charge later, but essentially it’s excess voltage that the multimeter can detect.
Even if the car battery isn’t completely topped off, it will show higher-than-expected voltage if the engine has been running recently. A battery that has recently charged will have lower voltage than a fully charged one, but the reading will still exceed 12V
The voltage changes according to how much the car battery has charged. The closer the battery is to a full charge, the higher its voltage. As the state of charge goes up, the voltage reading rises more rapidly.
Most alternators generate at least 14 volts.
Likewise, on hybrid and electric vehicles, the 12-volt battery will receive 14.5 volts from the high voltage battery through the DC-to-DC converter. That’s more than the 12V stated for lead-acid car batteries.
However, this is normal. The alternator’s voltage must exceed the battery’s voltage to overcome the latter’s internal resistance. A 12-volt car battery is composed of six 2 volt cells, thus the “12 volt” designation.
The electrical current flows through the battery’s cells, charging the battery. However, the battery’s internals offer resistance that impedes the current’s flow. A current with insufficient voltage can’t pass through the battery, preventing it from charging the device.
A battery with sulfated plates (coated with sulfur) will have higher resistance and will require higher voltage to charge. When using a heavy duty shop battery charger on medium or high charge, if the voltage required to “push” a charge into the battery exceeds 15.6 volts, the battery is sulfated, and that’s a go-no-go test.
The alternator’s charging is directly tied to what the battery voltage reading is. The alternator’s target voltage is usually between 14 and 14.5 volts. The voltage regulation system built into an alternator’s field control circuit measures battery voltage and regulates the alternator’s field strength accordingly.
The alternator’s target voltage is usually between 14 and 14.5 volts.
– Richard McCuistian, ASE Certified Master Automobile Technician
So, the alternator must produce enough voltage to simultaneously overcome the car battery’s internal resistance while also powering myriad parts.
Additionally, the alternator isn’t limited to charging the car battery. It also powers various electronics and electrical parts while the engine is running. Some of these systems, such as the HVAC, are power intensive.
The alternator can cause a phenomenon called surface charge while charging the battery. Surface charge is one of the reasons for the higher voltage readings in the battery.
During the charging process, the lead-acid battery needs time to turn lead sulfate into lead and lead dioxide. Thus, most of the charging happens on the surfaces of the battery plate. The battery’s exterior ends up with a state of charge that’s higher than that of its interior.
Surface charge slightly increases the battery’s voltage. More importantly, it gives a false voltage reading on how much charge the battery contains. Thus, professional grade battery testers will tell you to turn on the headlights or some accessory to remove the surface charge before completing the test.
Fortunately, there are ways to minimize surface charge’s effect on voltage readings. One way is to switch on an electronic or electrical part, such as the headlights or tail lights. The resulting electrical load uses up some of the battery’s capacity and removes most of the surface charge.
Another way to remove surface charge is to let the car battery rest. Turn off the engine and leave your car alone for several hours.
Temperature is another factor that can affect the car battery’s voltage. If it gets hot, the battery will show a higher voltage. Conversely, cold weather will lower the voltage. A discharged battery runs the risk of freezing in extremely low temperatures.
And again, some professional battery testers will have an infrared measuring device built in for non-contact measurement of the battery temperature while the battery is being tested before the test can be completed accurately.
Instead of a multimeter, some mechanics recommended using a hydrometer to measure the battery’s charge state. The hydrometer measures a liquid’s density or specific gravity. It can determine the amount of sulphuric acid in battery fluid. The higher the sulphuric acid percentage, the higher the battery’s state of charge.
However, this approach only works with older car batteries that you can open to extract samples of their battery fluid for testing. Many modern batteries are sealed, preventing you from opening them.
If you start your vehicle when it’s cold, the alternator will initially send a high-voltage current to the car battery. The powerful current can overcome the high resistance of a cold battery, charging it faster. That’s why if the alternator belt is a little loose it will squeal for a few seconds when the engine is first started and then the squealing will stop.
Following the early surge, the alternator will reduce its current output based on measured battery voltage.
Whether it’s a part as simple as the headlights or a system as complex as the HVAC, your vehicle has many electronics and parts that run on electricity. When you switch them on, they impose an electrical load on the alternator.
Electrical load reduces the car battery’s voltage. It can be bad because the battery requires a certain voltage level to charge, and if for some reason the alternator can’t keep up with the electrical load, the battery will be drained even as the vehicle is being driven.
The next time you take a multimeter voltage reading of your car’s battery and wonder why it’s more than 12V, it doesn’t mean your vehicle has a 14-volt car battery. It’s normal for the battery to show a voltage reading that exceeds 12 volts, and you can lower the voltage to get more accurate readings with a multimeter.
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