City-Data Forum What is the proper voltage for a fully charged car battery? (truck, engine)
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12-12-2013, 06:58 AM
 Location: SoCal 1,506 posts, read 3,168,004 times Reputation: 1120

Voltage doesn't matter as much as some think.

12-12-2013, 09:50 AM
 5,458 posts, read 6,121,819 times Reputation: 13941
I am guessing that your battery is fine, but that you have some other weakness in the system which gets the most stress when you do that first cold start in the morning.

Poor cable connection somewhere? Corrosion in a connection somewhere? A starter that is starting to die?

12-12-2013, 10:40 AM
 4,762 posts, read 10,224,033 times Reputation: 7792
A fully charged car battery is 12.75 volts or more.

12-12-2013, 10:45 AM
 Location: Prosper 6,268 posts, read 12,101,075 times Reputation: 9325
Quote:
 Originally Posted by vmaxnc That's alternator output, not battery voltage.
I know that. You don't stick your volt meter on the alternator, you put it on the battery terminals.

12-12-2013, 11:19 AM
 Location: Here 2,581 posts, read 5,471,896 times Reputation: 2568
Assuming the info the OP has given, the alternator is good.

Two things can drain a battery while it is sitting overnight:
- The battery is not holding a charge
- There is a parasitic drain in the wiring of the vehicle or with an electric component in the vehicle.

If you are saying it starts slower in the morning after 8+ hours than if you went to the store for 1-2 hours, this may be the case.

QUESTIONS
:
When you say it starts slow in the morning, is it consistent every morning after the same amount of hours? Is the voltage consistent every morning? What's the voltage at night before you go inside? And is it consistent?

TEST 1:
Test whether the slow crank/start and voltage changes if you let it sit 1 day versus let it sit 2 days.
Document.

TEST 2
[note that this test involves disconnecting the battery. if you don't have a memory saver device, you will clear any ECM (computer) settings and any radio settings]

Now do the same thing, but before you run each test, disconnect the battery before each time you let it sit. Again, do 1 day versus 2 days.
Document.

If test 1 and 2 both start the same (slow) speed and show the same lower voltage, then your battery is likely not holding a charge.

If test 2 shows that the car doesn't start as slow as test 1 (i.e. starts good) and the voltage hasn't dropped as much, then you may have a parasitic drain somewhere in the wiring or one of your electric components on the vehicle.

TEST 3: Tests the amperage a vehicle draws from the battery while the vehicle is off.
[note that this test involves disconnecting the battery. if you don't have a memory saver device, you will clear any ECM (computer) settings and any radio settings]

[also note, this test may be done at any time before or after test 1 and test 2]

- Make sure the car has cooled down enough so that the radiator fan doesn't come on when the car is off (this happens on some cars that have been running for a long time)
- If you have a multi-meter with mA or A (milliamp or amp) setting, locate the fuse inside and make sure it is good. Note that some meters have a 1A setting and a 10A setting. I would prefer to do it in the 10A setting to prevent the fuse from blowing.
- Make sure all lights are off and all doors are closed, and if there is a light under the hood, that is off too. Also disconnect any accessories from the Aux/Cig lighter
- Disconnect the negative battery terminal
- Hook up the positive (red) lead from the meter to the negative post on the battery.
- Hook up the negative/COM (black) lead from the meter to the negative cable
- You may have to wait a few minutes for any systems in the car to go into sleep mode.
- The range you will be looking for is 0-100mA or 0-0.100A. Ideal would be less than 30mA or 0.030A. For some vehicles, you may be ok with anything less than 60 A or 0.060A .
There should always be some small amount that is needed to keep memory to all your vehicle's computers and radio.
http://www.convertunits.com/from/mA/to/ampere

.

Last edited by NARFALICIOUS; 12-12-2013 at 11:35 AM..

12-12-2013, 11:24 AM
 1,205 posts, read 2,051,140 times Reputation: 1838
I have a gauge in my car that shows the voltage and it's always around 14 when I am driving

12-12-2013, 11:38 AM
 Location: Just East of the Southern Portion of the Western Part of PA 1,225 posts, read 3,123,626 times Reputation: 1435
Voltage on a battery that is not under load is not going to tell you much. You can have a bad battery still read good voltage and it drops immediately when a load is applied.

That is why most battery testers artificially load the battery. Take it to most any parts shop and they will test it for you.

12-12-2013, 11:54 AM
 Location: Here 2,581 posts, read 5,471,896 times Reputation: 2568
Quote:
 Originally Posted by Johnny C Voltage on a battery that is not under load is not going to tell you much. You can have a bad battery still read good voltage and it drops immediately when a load is applied. That is why most battery testers artificially load the battery. Take it to most any parts shop and they will test it for you.
OP already took it to a parts store.
Problem is, he took the car there after driving it, rather than the battery itself after it sat for a while.

12-12-2013, 12:12 PM
 Location: Just East of the Southern Portion of the Western Part of PA 1,225 posts, read 3,123,626 times Reputation: 1435
Voltage on a battery that is not under load is not going to tell you much. You can have a bad battery still read good voltage and it drops immediately when a load is applied.

That is why most battery testers artificially load the battery. Take it to most any parts shop and they will test it for you.

12-12-2013, 12:14 PM
 9,815 posts, read 13,883,984 times Reputation: 10707
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.
Ditto. For what I know, should be between 13 + and 14, 14 +. Anything that is below 12 is considered unacceptable.
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