One day I bought a new car battery to replace the old one, because the old one starts losing its charge, like would normally do after so many years of well exploitation. The next day I was so curious to measure voltage of a new car battery that I bought. So I grabbed my multimeter, cranked the car and opened the hood. I probed the battery positive and negative terminals and what I saw scared me a lot. Reading on the multimeter was 14.3V. But how when it should be 12V or so? Remember I started the car engine, alternator kicks in and start to charge car battery. Because of that, readings was different then the readings of a car battery while engine is not running. Multimeter was actually showing Volts from the alternator. So, you might wondering what should my car battery voltage be while driving?
But, what if you just took a measure with a multimeter and you battery readings are below 12V. You are now thinking that you need to buy a new one and throw this one away. But wait a second and read on. You can actually recondition an old car battery even if it looks like it’s time to buy a new one. I was able to recondition many car batteries to this day, and I will recondition them into the future as well. My point is that you can do that too, go to this page where I share my experience and how I does it! Remember the old battery from the beginning of this article that I mention? That was my first battery that I managed to recondition and bring it back to life again.
Now, get back to the answer.
Alternator does not charging car batteries all the time. Voltage regulators regulates amount of voltage that battery needs. For example, if battery voltage drops below 13V then regulator communicate with alternator and tells him to start charging the car battery. When car battery reach 14.2V to 14.3V alternator stops charging the car battery, and so on. The actual answer on that question is anywhere from 13V to 14.3V.
Car battery voltage numbers
When you buy a new car battery and when it’s full, it should be reading 12.7V at 21°C. Please bare in mind that if you take this measure in warmer weather it should be higher and if you take this measure in colder weather it should be lower. Before you taking a measure, give your car battery time to release surface charge, if you don’t do that it will give you false reading. And of course don’t attach any load to the battery.
I created this car battery voltage chart to help you understand the state of car battery charge and its capacity:
|Less than 12 volts||Dead|
What is alternator and how it works?
Car batteries operate at 12V range, so the alternator must produce higher Voltage outputs in order to have charging effect on the car battery.
When you take a reading of a regular car battery it should have about 12V to 14V without any load attached on.
Voltage will start to drop to what it can chemically produce on an uninterrupted basis. Not what it can store and quick release to a lighter amount of the load.
If the alternator is bad, in case when it’s working but not producing enough Voltage, then car battery will never fully charge, rather battery will discharging to the voltage of what alternator can output.
You can actually see this while driving car at night. When you speed up you will see higher light output at headlights, and when you slow down the headlights light will decrease light output.
If you replace the car battery and you start car engine and take a Voltage measurement and it showing below 12 Volts, that indicates that some of the alternator parts failing and it need to be replaced or repaired.
When it comes to older vehicles they had two different parts which are called regulator and generator. Newer vehicles have just one part which is called alternator, those are regulator circuit and generator in one.
All of the car electronics are made to work with a tolerance of a 10% or so. For example your USB port into your car can give you an output anywhere from 4.5 Volts to 5.5 Volts. And the majority of electronics in your car are made in such a way to have max tolerance of 20 percents. This is exactly because of those spikes and drops of Voltages in alternators.
Lets see an example of an older power adapters. They have a heavy transformer inside them. They are wound with a copper wire around the magnetic core in a distinct ration.
Those power adapters have no built in tolerance, so the are not recommended at all to use. With those power adapters you can get an output of 12 Volts DC from an 120 Volt outlet.
This 12 Volts are actually AC and it’s converted into DC with diodes and capacitors to filter out the wave output. In that way 12 Volt output is constant and stable.
Putting heavy load on it it’s never recommended because it will not work, but putting a lighter load will work fine. If the AC electric power drops to lets say 100 Volts AC then the output of the power adapter will also drop to an 10 Volts.
And if you are using that power adapter to power some electronic device it will not work perfectly. So let’s say that your alternator makes output of an 14.3 Volts, this is perfectly normal for the maximum tolerance adequate for the car equipment.
How to test alternator
Alternator voltage need to be at minimum 13.8 Volts in order to start charging the car battery. Any lead-acid car battery have 6 cells and a single cell need to be over 2.25 Volts to start charging. 13.8 Volts will be enough to charge car battery to the top and to maintain charge as well, but with this Voltage charging time will be to slow and will take a long time.
Alternator Voltage output must be at least 14.2 Volts to 14.5 Volts in order to fully charge car battery in acceptable time.
If the alternator charging above 14.5 Volts, car battery will faster release acid vapor, aka hydrogen gas and will start to corrode all over the top. To prevent enormous gas leakage car battery terminals at charging must never be greater than 14.7 Volts.
If alternator charging battery over 14.7 Volts, that can dry out the battery electrolytes. And can lead to battery explosion by the high battery hydrogen gas.
If you start seeing corrosion on the top of the car battery your alternator might charging at 14.8 Volts or greater, The 14.6 Volts will be just fine. For example if it’s a 14.3 Volts that would be sign of a weak alternator.
If it’s less than 14.2 Volts that would be a sign of a completely bad alternator. If you do a car battery voltage test across the battery terminals at regular rpm the Voltage should always be 14.3 Volts with or without full load of headlights, radio and everything else working.
If this is in some very old car you should consider to change the regulator to lower the alternator voltage. This will stop degradation of metal across the car battery from enormous charging vapors.
With a readings of an car battery voltage while running over 14.2V and less than 14.8V with the engine running at normal rpm speed and max load you are most likely have a larger alternator than you need. If you consider to buy a new larger alternator when your readings of a car battery while engine running at normal rpm are 14.2 Volts also running all electronics inside of a car is waist of money.
Take a readings of a car battery while engine is off with no load. You should get anywhere from 13.2V to 13.8V. If you measure different batteries you should get different reading because none of the batteries are the same and they are probably not at the same state of charge either, so the exact voltage might varies.
This measured voltages just before car engine turned off will slowly decrease and it will settle at a state of fully charged car battery. But if you are take reading just after the car engine is turned off it is good evidence that alternator is charging the car battery. Car battery was just charged if the voltage is over 13.2 Volts.