City-Data Forum What is the proper voltage for a fully charged car battery? (auto, idle)
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12-11-2013, 09:00 PM
 2,746 posts, read 3,421,309 times Reputation: 4253

It's not 12 volts it's 12.6 because each cell is 2.1 volts.

If the battery is

12.5 = 85% charged
12.4 = 65%
12.3 = 50%
12.2 = 35%
12.1 = drained

Reason I ask is I just bought a battery from Advance Auto parts about a month ago and noticed it has been slightly slower cranking over the first start of the day. So I checked it with my meter and it reads 12.4 - 12.5

The truck starts right up no problem after I ran the truck and shut it off during the day. So to me it sounds like the battery is not holding a charge overnight.

I went back to autozone and they used a machine that puts a load on the system and prints out a receipt that says it's good. The battery is rated at 775 CCAs but on the test came out 830 CCAs and 12.9volts.

The idiot who did the test told me "well the battery is at 12.9 volts" I said that's because I drove the truck here and the alternator charged it up its a surface charge. He says something nooo

Anyway these guys know just enough to be dangerous.

Those machines don't tell the voltage or how charged the battery is. Sure the machine will say good if it has enough power to do the load test even if it's drained some.

So is it true I should have a 12.6V reading on a fully charged battery anything less it needs to be charged? And it shouldn't dip under 12.6 in less then 24 hours?

12-11-2013, 09:07 PM
 Location: Here 2,581 posts, read 5,471,896 times Reputation: 2568
Those testers they use measure CCA and voltage.

There's no way to tell how it tests in the morning after sitting for 12 hours if you drive the car in.

12-11-2013, 09:23 PM
 Location: Prosper 6,268 posts, read 12,101,075 times Reputation: 9325
Hmm. I could swear I've seen my volt meter read 13.5 or so on a battery before (not connected to a car) but that was an SLA battery. I could be wrong though, been a while. I know at idle my batteries range from about 13.8 in the Jeep to 14.4 in the Cayenne.

12-11-2013, 09:57 PM
 Location: Volcano 12,971 posts, read 22,613,697 times Reputation: 10524
Quote:
 Originally Posted by eddie1278 So is it true I should have a 12.6V reading on a fully charged battery anything less it needs to be charged? And it shouldn't dip under 12.6 in less then 24 hours?
You misunderstand the basics... it isn't the voltage that matters as much as the available amperage. To use a classic analogy of water to electricity, voltage is like the water pressure in a pipe, while amperage is like the size of the pipe. All other things being equal, the work that a stream of water can do has more to do with the size of the pipe.

That's why a simple voltage check of a multicell wet battery doesn't tell you much, unless one of the cells is bad, which a voltage check can turn up. What really matters, in terms of cranking power in a car, is the total current flow available, aka the amperage, which can only be checked under load.

Here's a quick thought experiment that may help you to understand... if you stacked up 8 AAA batteries at 1 1/2 volts each, that would total 12 volts. Add one more and you'd have 13 1/2 volts, way above what you think you need... but try to start your car with it and you'd get nothing but a stack of warm batteries as they discharge in vain, because they aren't anywhere close to being able to produce enough current for the job.

In contrast, if you had a couple of old auto batteries lying around that only measured 11.5 volts each and couldn't quite crank a car by themselves, wiring them together in parallel to double the available current could probably do the trick.

12-11-2013, 10:13 PM
 Location: Not far from Fairbanks, AK 16,166 posts, read 27,421,913 times Reputation: 11834
It should be 12.6 volts, but for a battery that is fully charged and both battery cables disconnected.

12-11-2013, 10:59 PM
 Location: Brawndo-Thirst-Mutilator-Nation 15,140 posts, read 15,198,298 times Reputation: 10871
You will see more and more systems using a 42 volt battery.

Basically (12.6v * 3 = 37.8) + charging voltage = 42 volts. //high voltage system

12-11-2013, 11:48 PM
 8,402 posts, read 19,527,792 times Reputation: 6764
Quote:
 Originally Posted by MckinneyOwnr Hmm. I could swear I've seen my volt meter read 13.5 or so on a battery before (not connected to a car) but that was an SLA battery. I could be wrong though, been a while. I know at idle my batteries range from about 13.8 in the Jeep to 14.4 in the Cayenne.
That's alternator output, not battery voltage.

12-11-2013, 11:50 PM
 Location: Here 2,581 posts, read 5,471,896 times Reputation: 2568
Quote:
 Originally Posted by tickyul You will see more and more systems using a 42 volt battery. Basically (12.6v * 3 = 37.8) + charging voltage = 42 volts. //high voltage system
Which cars are these in now? Oh, right, none. No need to confuse the thread-maker.

12-12-2013, 01:47 AM
 760 posts, read 2,101,026 times Reputation: 825
I learned a lot from this interesting video. Great youtube channel.

How to Tell if Your Car Battery is Really Bad? Live Results Using 4 Different Testers - YouTube

12-12-2013, 05:37 AM
 Location: The Sand Hills of NC 8,776 posts, read 14,134,753 times Reputation: 11850
Quote:
 Originally Posted by eddie1278 It's not 12 volts it's 12.6 because each cell is 2.1 volts. If the battery is 12.5 = 85% charged 12.4 = 65% 12.3 = 50% 12.2 = 35% 12.1 = drained Is this true? I read conflicting answers all over the internet. Reason I ask is I just bought a battery from Advance Auto parts about a month ago and noticed it has been slightly slower cranking over the first start of the day. So I checked it with my meter and it reads 12.4 - 12.5 The truck starts right up no problem after I ran the truck and shut it off during the day. So to me it sounds like the battery is not holding a charge overnight. I went back to autozone and they used a machine that puts a load on the system and prints out a receipt that says it's good. The battery is rated at 775 CCAs but on the test came out 830 CCAs and 12.9volts. The idiot who did the test told me "well the battery is at 12.9 volts" I said that's because I drove the truck here and the alternator charged it up its a surface charge. He says something nooo Anyway these guys know just enough to be dangerous. Those machines don't tell the voltage or how charged the battery is. Sure the machine will say good if it has enough power to do the load test even if it's drained some. So is it true I should have a 12.6V reading on a fully charged battery anything less it needs to be charged? And it shouldn't dip under 12.6 in less then 24 hours?
I would say the guy who tested your battery knows more than you do because as far as I can tell he is not here on the internet asking for free advise.
When you come to the internet you get opinions, when you go to a quality shop, not Pep Boys, Sears or any of the other "budget" repair shops.
Take your car to a quality repair shop or the dealer if you are not a mechanic.
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