# Battery question with volt readings - Car struggles when cranking



## nap (Dec 4, 2007)

a fully charged battery will read 12.6 volts. Believe it or not, a battery reading 12.5 volts is only 75-80% charged. Given the low voltage when cranking, I would say there is an issue somewhere. The voltage reading when running is also low. It should be between 13.8 and 14.2 typically BUT that does not mean the charging system is an issue not does it mean the battery is at issue. It could be either, or both for that matter. But to start with, charging at idle is not proper. It needs to be revved, typically 1500 rpm or greater when testing for charging voltage. 

so, to start with, check voltage with it revved a bit. If it is still low, take the batter out of the car, have it charged and tested. You cannot properly test a battery if it is not fully charged.


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## Bob Sanders (Nov 10, 2013)

13.6 volts when the car is running doesn't necessarily mean there is something wrong. Your alternator will produce *UP TO* 14.5 volts (sometimes 14.7) when the car is running, but it can be anywhere between 12.5 and 14.5 depending on what and how many accessories you have running at the time.

I would be more concerned about the voltage dropping to 10.5V on start up. This means the battery is getting old and is no longer producing the rated amperage output. You need to bring it in and have a cranking test done on it becaues it is entirely possible for the battery to APPEAR normal (normal voltage at rest) but fail in amperage output.


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## nap (Dec 4, 2007)

Bob Sanders said:


> 13.6 volts when the car is running doesn't necessarily mean there is something wrong. Your alternator will produce *UP TO* 14.5 volts (sometimes 14.7) when the car is running, but it can be anywhere between 12.5 and 14.5 depending on what and how many accessories you have running at the time.


well, when you are testing you should turn off all accessories. I suppose I should have said that. 

but even with that, 13.6 is low if the engine is revved so the alternator puts out full voltage. 

it should never be as low as 12.5 though. Even at idle the alternator should produce enough such that the battery is not discharging (at least with only OEM equipment installed) and at 12.5, it is.

but with that said, a bad battery can cause the voltage readings to be low so, you pull the battery, have it fully charged (if possible) using a battery charger and then a load test is performed. You cannot properly test a battery that is not fully charged and if it will not reach full charge, it is a bad battery.


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## cjm94 (Sep 25, 2011)

10.5 volts is not out of line while the starter is cranking if it returns to normal voltage quickly it's probably not too bad. What is the age mileage of vehicle? Starters get weak and could be drawing too many amps.


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## ddawg16 (Aug 15, 2011)

Start checking for bad grounds. Next, I'd think tired starter before thinking bad battery


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## amodoko (May 29, 2011)

Ok, well one thing that is good to hear is that it could POSSIBLY just be a bad battery based on what you guys had mentioned (I would not like to have to deal with a bad alternator since I've never messed with one). I assumed with my voltage readings, it was indicating a bad alternator. But I'm relieved to know it could just be the battery also now.

Someone had asked what the age of the vehicle is. It is a 2009 Volkswagen Jetta. I believe it has around 70,000 miles. The owner said that they think the battery was only changed once, and that was in the first year they owned it due to a faulty alternator. The car was new the so the dealer apparently fixed the alternator and put in a new battery. If she is correct about the history, then the battery would be about 5 years old. But she may also be wrong. 

I realized when I checked the 13.6 volt reading when the car was running at idle, I just told her to turn on and turn off her headlights and the reading of 13.6 stayed the same (it dipped for a slight second but when back up to 13.6). But I forgot to pay attention to the the other accessories and they may have been on, so I'll have to do that test again and make sure all other accessories are off. 

I'm doing this more for my understanding than anything else.


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## Oso954 (Jun 23, 2012)

Clean the battery terminals.

Dirty/oxidized terminal connections cause a lot of "battery/alternator issues".


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## ukrkoz (Dec 31, 2010)

Oso954 said:


> Clean the battery terminals.
> 
> Dirty/oxidized terminal connections cause a lot of "battery/alternator issues".



:thumbup::thumbup::thumbup::thumbup::thumbup::thumbup::thumbup::thumbup::thumbup:

As this is high resistance/voltage drop area right here, both for charging and for use of the battery.


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## amodoko (May 29, 2011)

Thanks guys, I had checked the terminals when I first did the voltage readings and they appeared clean to me. So unless the corrosion is minor and I'm not seeing it, I believe the terminals are sufficiently clean. Anyways, the owner took her car out of town before I could mess with it more, so I'll have to wait till she brings it back. She had said that for some reason it started working fine and now it doesn't have to crank hard and it starts right up. We never even did anything to it, so maybe it is the starter acting up.


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## amodoko (May 29, 2011)

I just wanted to give an update to this thread for anyone that was curious about the results/solutions with the Jetta. Keep in mind that I don't know much about cars so the way I will write about the situation will seem very amateurish.

The owner of the Jetta (my close friend) came back in town and said that it had been working okay but would sometimes crank long to start but it would be okay and start. They parked their car at my place when they came in town, and then went back to try to start it when they wanted to go drive to get dinner. However, it wouldn't start but it would make kind of a high pitched starting sound (like the starter was just spinning and not catching anything, however it did it with what I thought to be a decent amount of force... meaning it didn't sound like it was straining to spin the starter just making a weird noise while spinning) and the interior would beep 3 times and the "brake" light in the dash would flash too. 

She thought it just needed to be jump started because her husband had actually left the lights on and had drained the battery. So I thought, okay, just a drained battery. I went to jump start it and it would not work. However, I was getting a good connection from my car battery to her car I believe because all the lights inside and outside her car were bright when connected with the jumpers (they were dim without my car attached). So I thought maybe the starter was actually bad since her car was getting enough juice from my car at least to start. But then I read online that sometimes weak batteries can do some kind of funny things... 

So I took my multimeter and measured her car battery after I had charged her battery with mine for about 15 minutes and it read something like 9.6 volts! So I was like okay, the battery is done. I just went and got another new battery and put it in and now the car works fine. I let it idle for a while and the voltage on the battery seemed to be stable at around 14.0 (with maybe a few accessories on... I forgot to check). 

So hopefully that will fix it in the long run, if not, I would assume it would be a bad alternator or there is power being leaked/drawn away through some bad wires/connections. 

I just find it a bit weird that the car wouldn't start even though my jumper cables were attached from my car's battery (while my car was running) to the Jetta's battery. *You would think it would just draw the power needed from my battery and start the Jetta, but I guess it doesn't work that way when the dead battery is really dead? Can anyone enlighten me with regards to why that is? *I would think if the donor car's battery is good that it should be able to start a car with a dead battery since the dead car would just draw power from the good car's battery with the jumper cables.


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## cjm94 (Sep 25, 2011)

I have run into that a few times. If the battery has a high enough internal drain/short it will consume to much of the voltage to allow a jump start. Voltage with the new battery seems ok.


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## amodoko (May 29, 2011)

Ahh, ok so a very drained/shorted battery will not allow a car to be started even with a good jumper cable connection. Now I know


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## ChuckF. (Aug 25, 2013)

I always record the dates that I put a new battery in my vehicles on a sticky on the bulletin board. When the battery gets five years old I figure you are on borrowed time.


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## RocketGal (Mar 29, 2012)

Measure cold cranking amps (CCA) to ensure a battery (even at nominal voltage approx. of 12.5 DC) will be able to start your car.


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## Brainbucket (Mar 30, 2015)

A bad battery will cause all kinds of problems. An alternator needs good 12 volts to work properly. It's not a generator. Good operating voltage is 13.5 to 14.5 volts engine running. I condem batteries when cranking voltage hits 10.5 volts or under. Computer systems on these cars don't like low voltage.:thumbsup:


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