how to read a voltmeter?
11-13-2001, 05:36 PM
#1
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how to read a voltmeter?
I've got a voltmeter installed in my car, and I'm not quite sure if I am reading it correctly.
At idle with no power accessories on, volts are 12-13, when charging its 12.5-13.5
With headlamps on and stereo, volts drop to a little under 12.
With headlamps, stereo, and fan, it dips to 10-11
Should I be concerned whe nit drops below 12? I've been sitting at stoplights revving just to keep the volts up. How do I read this?
At idle with no power accessories on, volts are 12-13, when charging its 12.5-13.5
With headlamps on and stereo, volts drop to a little under 12.
With headlamps, stereo, and fan, it dips to 10-11
Should I be concerned whe nit drops below 12? I've been sitting at stoplights revving just to keep the volts up. How do I read this?
11-13-2001, 10:19 PM
#3
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Join Date: Oct 2000
Posts: 1,402
Originally posted by DaEnigma
I never trust a VOM unless they are RMS and calibrated... Use a RMS VOM directly on the battery then you will know how accurate it really is.
I never trust a VOM unless they are RMS and calibrated... Use a RMS VOM directly on the battery then you will know how accurate it really is.
When I power the car on (without starting the engine) I get 12 volts. With the engine on, regardless of what other electrical devices are on, it stays constant at 14 volts.
I have a BRAND NEW (3 weeks old) Sear Die Hard "Gold Series" battery!
11-13-2001, 11:01 PM
#4
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Re: how to read a voltmeter?
Originally posted by ngthing
I've got a voltmeter installed in my car, and I'm not quite sure if I am reading it correctly.
At idle with no power accessories on, volts are 12-13, when charging its 12.5-13.5
With headlamps on and stereo, volts drop to a little under 12.
With headlamps, stereo, and fan, it dips to 10-11
Should I be concerned whe nit drops below 12? I've been sitting at stoplights revving just to keep the volts up. How do I read this?
I've got a voltmeter installed in my car, and I'm not quite sure if I am reading it correctly.
At idle with no power accessories on, volts are 12-13, when charging its 12.5-13.5
With headlamps on and stereo, volts drop to a little under 12.
With headlamps, stereo, and fan, it dips to 10-11
Should I be concerned whe nit drops below 12? I've been sitting at stoplights revving just to keep the volts up. How do I read this?
11-14-2001, 01:25 AM
#6
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Join Date: Dec 2000
Posts: 1,030
Originally posted by ngthing
so, should I worry about this and keep revving, or is it no big deal? i get scared my battery is going to get drained. there's been ocasions where the car wont start up after i've seen the draw for a while.
so, should I worry about this and keep revving, or is it no big deal? i get scared my battery is going to get drained. there's been ocasions where the car wont start up after i've seen the draw for a while.
then again i dont know how ur voltmeter was wired up
11-14-2001, 02:53 AM
#8
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Originally posted by medicsonic
Your battery voltage while running should bein the 14s. Recheck the gauge.
Your battery voltage while running should bein the 14s. Recheck the gauge.
11-14-2001, 05:15 AM
#10
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I think you should have your charging system checked with numbers like that. You have a "weak link" in the system someplace. And I don't mean something is lose, I mean either the battery or alternator is weak. You should be running around 14 volts.
11-14-2001, 05:24 AM
#11
Originally posted by DaEnigma
I never trust a VOM unless they are RMS and calibrated... Use a RMS VOM directly on the battery then you will know how accurate it really is.
I never trust a VOM unless they are RMS and calibrated... Use a RMS VOM directly on the battery then you will know how accurate it really is.
11-14-2001, 10:23 AM
#12
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That is easy, you don't . IIRC RMS is a derived from an alternating current voltage. Since DC never alternates you cant get the Root Means Square value if its 0.
Originally posted by mzmtg
How, exactly, do you measure RMS voltage of a DC current?
How, exactly, do you measure RMS voltage of a DC current?
11-14-2001, 10:47 AM
#13
Originally posted by Nismo87SE
That is easy, you don't . IIRC RMS is a derived from an alternating current voltage. Since DC never alternates you cant get the Root Means Square value if its 0.
That is easy, you don't . IIRC RMS is a derived from an alternating current voltage. Since DC never alternates you cant get the Root Means Square value if its 0.
I knew that, I was just being an a$$.
11-14-2001, 06:33 PM
#15
Until you posted your second post
I was just about to say the same, your alternator or battery is weak. I would suggest hooking up the DVOM directly on the battery terminals and do your test the exact same way. Your volt reading should increase. You can always hook it back up to your present location after checking at the battery terminals
11-15-2001, 06:52 AM
#16
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Originally posted by Nismo87SE
That is easy, you don't . IIRC RMS is a derived from an alternating current voltage. Since DC never alternates you cant get the Root Means Square value if its 0.
That is easy, you don't . IIRC RMS is a derived from an alternating current voltage. Since DC never alternates you cant get the Root Means Square value if its 0.
For example if I have 12V at a constant. I square it and get 144. I then take the square root and get 12. Then integrate 12 between 0 and 1 (for a one second sample) and I get 12. I then divide by the period of one second and get 12V. As you can see the RMS of a DC voltage is not 0.
Stereodude
11-15-2001, 10:25 AM
#17
Originally posted by Stereodude
Actually that is incorrect. The RMS value of a DC voltage is the DC voltage. The whole idea of RMS is to take the time average of a signal that varies. You integrate the square root of the square across one period of the waveform and divide the result of that integral by the period. That will give you the RMS value of any repeating signal. If the signal does not repeat or have a cyclic nature finding the RMS is much more difficult.
For example if I have 12V at a constant. I square it and get 144. I then take the square root and get 12. Then integrate 12 between 0 and 1 (for a one second sample) and I get 12. I then divide by the period of one second and get 12V. As you can see the RMS of a DC voltage is not 0.
Stereodude
Actually that is incorrect. The RMS value of a DC voltage is the DC voltage. The whole idea of RMS is to take the time average of a signal that varies. You integrate the square root of the square across one period of the waveform and divide the result of that integral by the period. That will give you the RMS value of any repeating signal. If the signal does not repeat or have a cyclic nature finding the RMS is much more difficult.
For example if I have 12V at a constant. I square it and get 144. I then take the square root and get 12. Then integrate 12 between 0 and 1 (for a one second sample) and I get 12. I then divide by the period of one second and get 12V. As you can see the RMS of a DC voltage is not 0.
Stereodude
But you cant integrate over one cycle when there are no cycles.
11-15-2001, 10:33 AM
#18
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Originally posted by mzmtg
But you cant integrate over one cycle when there are no cycles.
But you cant integrate over one cycle when there are no cycles.
11-15-2001, 10:58 AM
#19
Originally posted by Stereodude
Where there are no cycles you can pick any arbitrary time period you want because you are not limited by the cycles.
Where there are no cycles you can pick any arbitrary time period you want because you are not limited by the cycles.
The voltage in an automotive DC system does fluctuate, but it doesnt cycle regularly. Therefore depending on the random bit of time you pick, you will get different RMS figures. So you cant really use RMS in any meaningful way on a DC circuit.
11-15-2001, 11:59 AM
#20
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Originally posted by mzmtg
When there are no cycles, you have to integrate to and from infinity...
The voltage in an automotive DC system does fluctuate, but it doesnt cycle regularly. Therefore depending on the random bit of time you pick, you will get different RMS figures. So you cant really use RMS in any meaningful way on a DC circuit.
When there are no cycles, you have to integrate to and from infinity...
The voltage in an automotive DC system does fluctuate, but it doesnt cycle regularly. Therefore depending on the random bit of time you pick, you will get different RMS figures. So you cant really use RMS in any meaningful way on a DC circuit.
"The square root of the mean of the square. RMS is (to engineers anyway) a meaningful way of calculating the average of values over a period of time. A signal is squared, averaged over a period of time, then the square root of the result is calculated. The result is a value, that when squared, is related (proportional) to the effective power of the signal. Unfortunately, calculating the RMS value of anything but a simple sine wave (.707 of peak) is very difficult."
Stereodude
11-15-2001, 12:49 PM
#22
Only reason I stated using a RMS VOM that is calibrated is due to the fact that RMS meters are usually better built and more accurate. Which is also why they are usually more expensive. I also agree you can get RMS from DC but it is pointless.
So what I was saying is get a quality calibrated true RMS digital AC/DC VOM and test the output directly from the battery...
Look here for a pretty good How2 for checking votage on a car.
http://www.inct.net/~autotips/batterm.htm
And this as always is IMHO...
So what I was saying is get a quality calibrated true RMS digital AC/DC VOM and test the output directly from the battery...
Look here for a pretty good How2 for checking votage on a car.
http://www.inct.net/~autotips/batterm.htm
And this as always is IMHO...
11-15-2001, 12:50 PM
#23
Re: how to read a voltmeter?
Originally posted by ngthing
I've got a voltmeter installed in my car, and I'm not quite sure if I am reading it correctly.
At idle with no power accessories on, volts are 12-13, when charging its 12.5-13.5
With headlamps on and stereo, volts drop to a little under 12.
With headlamps, stereo, and fan, it dips to 10-11
Should I be concerned whe nit drops below 12? I've been sitting at stoplights revving just to keep the volts up. How do I read this?
I've got a voltmeter installed in my car, and I'm not quite sure if I am reading it correctly.
At idle with no power accessories on, volts are 12-13, when charging its 12.5-13.5
With headlamps on and stereo, volts drop to a little under 12.
With headlamps, stereo, and fan, it dips to 10-11
Should I be concerned whe nit drops below 12? I've been sitting at stoplights revving just to keep the volts up. How do I read this?
11-15-2001, 12:51 PM
#24
Originally posted by dashingMax
Very True! Mine does the same thing as you ngthing.
When I power the car on (without starting the engine) I get 12 volts. With the engine on, regardless of what other electrical devices are on, it stays constant at 14 volts.
I have a BRAND NEW (3 weeks old) Sear Die Hard "Gold Series" battery!
Very True! Mine does the same thing as you ngthing.
When I power the car on (without starting the engine) I get 12 volts. With the engine on, regardless of what other electrical devices are on, it stays constant at 14 volts.
I have a BRAND NEW (3 weeks old) Sear Die Hard "Gold Series" battery!
11-15-2001, 01:02 PM
#27
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Join Date: Aug 2000
Posts: 2,807
I knew that I forgot something important. Thanks for the info 
.
.
Originally posted by Stereodude
Actually that is incorrect. The RMS value of a DC voltage is the DC voltage. The whole idea of RMS is to take the time average of a signal that varies. You integrate the square root of the square across one period of the waveform and divide the result of that integral by the period. That will give you the RMS value of any repeating signal. If the signal does not repeat or have a cyclic nature finding the RMS is much more difficult.
For example if I have 12V at a constant. I square it and get 144. I then take the square root and get 12. Then integrate 12 between 0 and 1 (for a one second sample) and I get 12. I then divide by the period of one second and get 12V. As you can see the RMS of a DC voltage is not 0.
Stereodude
Actually that is incorrect. The RMS value of a DC voltage is the DC voltage. The whole idea of RMS is to take the time average of a signal that varies. You integrate the square root of the square across one period of the waveform and divide the result of that integral by the period. That will give you the RMS value of any repeating signal. If the signal does not repeat or have a cyclic nature finding the RMS is much more difficult.
For example if I have 12V at a constant. I square it and get 144. I then take the square root and get 12. Then integrate 12 between 0 and 1 (for a one second sample) and I get 12. I then divide by the period of one second and get 12V. As you can see the RMS of a DC voltage is not 0.
Stereodude
11-15-2001, 01:08 PM
#28
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Location: Detroit Metro Area
Posts: 1,617
Originally posted by mzmtg
This will make you cringe:
THREE PHASE POWER!
This will make you cringe:
THREE PHASE POWER!
Stereodude
11-15-2001, 01:19 PM
#29
Originally posted by mykizism
OMG its Engineering class's all over again!! ahhhhhhhhhhhhhhhhhhhhh
haha
OMG its Engineering class's all over again!! ahhhhhhhhhhhhhhhhhhhhh
haha
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