12 volts enough?
Thread Starter
Melting Slicks
Joined: Nov 2000
Posts: 2,663
Likes: 3
From: Palm Harbor FL
12 volts enough?
OK, this is non-Vette related but....
I have a friend that has a Nissan Sentra. Her battery went dead, and she was sold a new battery. I told her to ask the mechanic how many volts the alternator was putting out, and he said "12 volts". I thought there should be 13 to 13.5 volts to run a car and recharge the battery. The thing is, the car's still on warranty, but the battery was not, so I'm wondering if he wasn't trying to stall putting in a new alternator until the warranty ran out, and was taking her for the price of the battery.
What do you guys think?
I have a friend that has a Nissan Sentra. Her battery went dead, and she was sold a new battery. I told her to ask the mechanic how many volts the alternator was putting out, and he said "12 volts". I thought there should be 13 to 13.5 volts to run a car and recharge the battery. The thing is, the car's still on warranty, but the battery was not, so I'm wondering if he wasn't trying to stall putting in a new alternator until the warranty ran out, and was taking her for the price of the battery.
What do you guys think?
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
Voltage is just one component of the charging system. Amperage and the actual signal are just as important and are not visible by just reading voltage.
Testing alternator output just for voltage doesn't tell you much, but, yes, if the DVOM is showing 12V or below spec, then the alternator/regulator is suspect. Once you have a faulty reading, it's prudent to have a qualified tech load and signal test the charging system. It's quite common to have satisfactory voltage showing up in the alternator's output but to have insufficient amperage due to a bad diode or to have ac riding on the dc current.
This is where having a charge system diagnostics can be cheaper than throwing money at the problem. The battery may have been bad at the moment but may have had nothing to do with the problem and was needlessly replaced.
Testing alternator output just for voltage doesn't tell you much, but, yes, if the DVOM is showing 12V or below spec, then the alternator/regulator is suspect. Once you have a faulty reading, it's prudent to have a qualified tech load and signal test the charging system. It's quite common to have satisfactory voltage showing up in the alternator's output but to have insufficient amperage due to a bad diode or to have ac riding on the dc current.
This is where having a charge system diagnostics can be cheaper than throwing money at the problem. The battery may have been bad at the moment but may have had nothing to do with the problem and was needlessly replaced.
Thread Starter
Melting Slicks
Joined: Nov 2000
Posts: 2,663
Likes: 3
From: Palm Harbor FL
The situation is that the car is still under warranty. From what I've read, the car should be putting out 13.5 volts at idle. My suspicion is, that the "qualified" Nissan mechanic is trying to stall putting the alternator in that would have to be done under warranty, and selling her a new battery, in hopes that by the time they have to install the alternator, it will be off warranty.
Tech Contributor
Joined: Aug 1999
Posts: 14,370
Likes: 6,364
From: At my Bar drinking and wrenching in Lafayette Colorado
The charging system needs to be putting out around 13.5 to 14.7 volts. If it's putting out 12 volts, it's dead. If this car is under warranty and the alternator is putting out 12 volts, you have a warranty claim. But the guy who checked it and said it was OK with 12 volts obviously has no idea what he's talking about, so he also may not have known how to check it correctly... I wouldn't trust his "analysis." Better check it yourself.
Thread Starter
Melting Slicks
Joined: Nov 2000
Posts: 2,663
Likes: 3
From: Palm Harbor FL
Yeah, I have my suspicion that he's avoiding warranty work. I figured at 12 volts, it would probably just slowly discharge the battery. I don't think he ever tested it. I think he said "12 volts" because it was a 12 volt battery and he was talking to a woman.
Thanks guys.
Thanks guys.
Melting Slicks
Joined: Jan 2004
Posts: 2,672
Likes: 1
From: New Cumberland PA
I got to agree with the above posts. A charged battery should be 12.6 volts. The alternator has to put out more than that or it will not charge. Normally it around 13.5 to 14.5. If she is driving at night, it won't take long to discarge the battery. Easiest thing to do is put a voltmeter on the battery. If it is not in the desired range, take the car back to the garage and insist on talking to the manager if the mechanic is trying to giver her the run around. There is no sense going back with some indication there is a problem. It could have been that the mechanic didn't want to confuse her by saying the alternator was putting out 14v for a 12v battery.
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
Originally Posted by gerry72
Voltage is just one component of the charging system. Amperage and the actual signal are just as important and are not visible by just reading voltage.
Testing alternator output just for voltage doesn't tell you much, but, yes, if the DVOM is showing 12V or below spec, then the alternator/regulator is suspect. Once you have a faulty reading, it's prudent to have a qualified tech load and signal test the charging system. It's quite common to have satisfactory voltage showing up in the alternator's output but to have insufficient amperage due to a bad diode or to have ac riding on the dc current.
This is where having a charge system diagnostics can be cheaper than throwing money at the problem. The battery may have been bad at the moment but may have had nothing to do with the problem and was needlessly replaced.
Testing alternator output just for voltage doesn't tell you much, but, yes, if the DVOM is showing 12V or below spec, then the alternator/regulator is suspect. Once you have a faulty reading, it's prudent to have a qualified tech load and signal test the charging system. It's quite common to have satisfactory voltage showing up in the alternator's output but to have insufficient amperage due to a bad diode or to have ac riding on the dc current.
This is where having a charge system diagnostics can be cheaper than throwing money at the problem. The battery may have been bad at the moment but may have had nothing to do with the problem and was needlessly replaced.
but the above,....I have some issues with...namely that a bad/open diode can make some ripple on the line not ordinarily there, but that is will be apparent there is not enough alternator output, and a DVM will show that as lo output....certain an O' scope will show the story...but it's not needed....
as for ac riding on the dc current....I suggest you go back to school, as it's not an ALTERNATING current but a 1/2 phase rectified missing pulse of one of the three phases....so that means maybe 5 of 6 diodes are working....
IF indeed any diode actually shorted...and did't give up the ghost and burn out completely....the alternator windings would burn up quite rapidly....
GENE
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Drifting
Joined: Nov 2001
Posts: 1,469
Likes: 6
From: 68 427 4.11s Roadster
It depends on the type but alternators have far more than 3 phases. I think its closer to 12. This is to provide a less rippled dc output when rectified.
I don't think 1 diode dies but all of them. If the thing is shorted or wired incorrectly the magic smoke is released from all of them.
I don't think 1 diode dies but all of them. If the thing is shorted or wired incorrectly the magic smoke is released from all of them.
Thread Starter
Melting Slicks
Joined: Nov 2000
Posts: 2,663
Likes: 3
From: Palm Harbor FL
OK. Although I find this all EXTREMELY interesting and educational, and I say that seriously and without sarcasm, this is way more info than I need. I guess the point here is, if my friends output is TRULY showing 12 volts, there's probably something wrong. Whether it's diodes or regulator or alternator. Something probably has to be replaced that is probably under warranty. I just didn't want to see her get sold a battery, and then a few months from now have it go again, and find out, "Well, it's your alternator, but your warranty ran out 200 miles ago". 
If it's something that needs to be replaced, I'd rather see her get it done under warranty.
As always, thank you for the help and the education!!
If it's something that needs to be replaced, I'd rather see her get it done under warranty.
As always, thank you for the help and the education!!
