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 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.

 

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!

 

hay ngthing thats perfectly normal, my lil brothers stealth has a stock voltmeter , his reads the same as what u have described.

 

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.

 

perhaps your battery is going dead. but the voltmeter how you described it, is how it should work , on my brothers car, when he turns the turn signal on. his voltmeter drops when the light is on, and goes back up when its off. so like the voltmeter goes up and down then the turn signal on, thats how it is on his stealth,

then again i dont know how ur voltmeter was wired up

 

Your battery voltage while running should bein the 14s. Recheck the gauge.

 

I have it the volt meter wired in parallel off of the light bulb wiring for the gauges, would this affect anything? The wire doesnt come stragight from the battery either, it goes through my neons and PA too.

 

They are drawing current, and throwing off the numbers. If I had a voltmeter in my car, I would want to know what the BATTERY was reading directly.

 

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.

 

How, exactly, do you measure RMS voltage of a DC current?

 

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$$.

 

mzmtg

Stop being a donkey, mzmtg! We have too many donkeys running around the .org already!

 

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

 

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.

 

Where there are no cycles you can pick any arbitrary time period you want because you are not limited by the cycles.

 

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.

 

No, you don't have to integrate to and from infinity.

"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

 

Ok, technically, you can calculate the RMS value of a DC signal.

But, you wouldnt want to or need to.

 

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...

 

As the amps and watts are being used from the other components, the voltage will drop. When my subwoofer hits the deep bass sounds on a CD, my voltage drops bigtime making my lights dim. I need to grab a capacitor fast.

 

Good battery, I have one too.

 

OMG its Engineering class's all over again!! ahhhhhhhhhhhhhhhhhhhhh

haha

 

This will make you cringe:























THREE PHASE POWER!

 

I knew that I forgot something important. Thanks for the info .

 

That's nothing compared to Communications Theory. That class confused me so much. 3 Phase power is a walk in the park when compared to some of the stuff in Comm Theory. AM and all the other types of multiplexing and the formulas to go along with them are crazy.

Stereodude

 

It's been awhile for me, so I almost don't even know wha I am talking about anymore.

 

lol