My Alternator & Battery... a coming of age tale.

**Nola111** · 10-06-2007, 01:52 PM

Greetings folks,

So after all kinds of problems with battery drainage and carpc power obstacles, I decided to stop and remind myself that I need to see the forest between the trees to truly understand certain concepts. In other words, I've been doing weeks and weeks of troubleshooting, but I never backed up to look at the big picture long enough to realize that if I truly understand what's going on under my hood, I'll be way better equipped to solve my issues once and for all.

So right now, I admit with embarrassment, I really don't know fully what's going on in there.

What I do know is this: I have a Carnetix P2140 regulating my power, and with it's very slick software interface, PSUMoni, I'm able to see realtime voltage and amperage measurements of my battery, and all the power outputs of the unit itself. This is where things get hazy for me...

With my car running and the pc on and just showing the desktop, my readings in PSUMoni show that my battery is bouncing around the 12.1 to 12.4 voltage range for output. Then, if I start playing music, the voltage range will drop to about 11.3 - 11.8 v. Stop playing music, and it will go back up to the other range.

I have two questions...

1. How exactly does my alternator work... I mean, specifically, I was under the impression that my alternator should be putting out about 13.8v while the car is running. Why am I only showing 12.4v at best?

2. The 11.3 - 11.8v range while I'm playing music worries me. Does that seem low to anyone?

Please school me, for I am ignorant to the inner workings of my vehicle.

My PC specs and vehicle info are to your left...

Thanks!

**mt2man** · 10-06-2007, 06:08 PM

Hi it could be a voltage drop from the battery to altenator, loosing just 1 volt could affect your battery voltage alot. Im currently learning this at college at the moment and we were told to do a voltage drop test, you need to create a load in the circuit that's being tested. Then you use the multi meter to measure the voltage drop across the live connection while it is under the load. The voltage always follows the path of least resistance, so if the circuit or connection being tested has too much resistance some of the voltage will flow through the multi meter and create a voltage reading.
If you get a reading of over 0.15-0.2 volts then that could be abit of your problem, there could be a dodgy connection to the battery ternimal (well thats where i would start looking) or somewhere along the circuit. If you do find some corrosion or a dodgy connection it would be a good idea to clean the ternimal or connection and check to see if there is still a voltage drop. If you get a voltage reading of 0.1 or less volts than there should be a good circuit so there shouldnt be to much of a problem.

Well thats my opinion but im only 15 and am not an expert and some of what i said may have been in correct so i would read up a bit about it first. (but im a qualified mechanic, I got my NVQ level 1 a couple of weeks ago

)

Many thanks Jack

**Profozone** · 10-07-2007, 09:33 AM

I'm not sure how low it should drop once you start playing music, but if you turn everything off in the car and run the engine and you don't get somewhere between 13 and 15 volts, then I believe there is a problem. But you don't even have to do this yourself. If there are any auto parts stores still standing in your area (New Orleans, right?), you can go to one and they will hook up a battery tester and tell you the condition of your charging system. You don't even have to take it out of the car.

To me, if everything is off and your alternator doesn't put out 13 to 15 volts, especially when you give the engine a little gas, then there is a problem. But it could just be a bad connection, corrosion, etc.

Good luck.

**Nola111** · 10-07-2007, 06:52 PM

Thanks for the replies guys.

Yeah I'm actually just outside of New Orleans so there are definitely some places I can go to check.

Profo, you say "turn everything off in the car and run the engine..." So then, you're saying that it is possible that WITH many things running (i.e. AC, stereo, headlights, etc) it is possible that my voltage reading can drop to what I'm showing? I guess I didn't understand that part of it. I thought if the alternator outputs 13.8v, that means I should get a reading of 13.8v no matter what, unless something is wrong with it.

**DodgeCummins** · 10-07-2007, 11:52 PM

The first variable is in how the psumon measures voltage. If you have a bad connection from your computers powersupply to the battery, then all that resistance in between will lower the voltage. (assuming psumon is capable of accurate measurements).

Second variable is in how much load you are putting on your car's electrical system when looking at the computers voltage readout...A/C going...headlights on...1000watts of stereo cranking etc.

Third variable is what RPM you are running the engine at. Alternators have to reach a certain RPM to create the maximum power...unless you have a high end specialty alternator (listed elsewhere on mp3car.com) then yours is probably barely putting out any power with the engine at idle...and only puts out max power at highway speed engine RPM (3000?).

Finally...it also depends on how much power your alternator is even capable of putting out. The more power it puts out...the more drag on the engine, and you lose some power to the wheels in the process...with my big truck, the engine doesn't even notice the 120Amp alternator...but a small engine might be mated to a 50Aamp one...

**Nola111** · 10-08-2007, 06:12 PM

Thanks for the reply.

I have definitely given consideration to the possibility that the PSUMoni's readings may not be fully accurate, but Mike from Carnetix claims accuracy within a very small variable, so I think I'm comfortable with that. As to whether or not I have the best connection, that's another story. I think it's good though.

So RPM's really do matter? I have a buddy who seems to know a good deal about cars who says the RPM thing is a myth. He's not a mechanic though, so he could be talking out of his a$$.

I have no idea what my alternator is rated at. But if I had to guess, VW probably skimped on that (like most of the stuff they put in my car) to cut costs, and therefore it's probably a little crap one. I'll check the manual and see if it shows it in the specs.

**Crawdaddy** · 10-08-2007, 06:33 PM

you definitley should be reading between 13.6 and 14.5 volts, even at idle, with all your accesories off. Depending on what accessories you have on, how much they draw, and what your alternator is rated at, voltage may drop. If it does, this is not good, and you need a higher current alternator, second battery, or both. There is also always the possibility that your alternator and/or your battery may be failing. Both can be tested at your neighborhood AutoZone, Pep Boys, or the like that took 4 feet of water or more (j/k on the water part

). If you continue to have the issue by the time we do a gettogether down here, I can help you troubleshoot it.

**Kimota** · 10-08-2007, 10:26 PM

Funny I'm experiencing the exact same issue with the 2140 my voltage is even dipping below 9V on occasion while the car is running.

Although I do have 2 after market amps installed as well think I'll be taking a trip to my local PEP Boys.

**Nola111** · 10-09-2007, 07:23 PM

Thanks CDaddy. Hopefully I'll have it worked out before November. The next thing I'm going to do is measure the voltage coming from the factory wire harness with a multimeter. This is the direct factory connection to the battery that hasn't been touched, and it will help me determine if possibly the PSUMoni could be giving inaccurate readings.

Can I not test the output of my alternator by using a multimeter on the battery terminals?

**Bob T** · 10-11-2007, 01:46 PM

A frequent problem in automotive electrical systems is dirty battery terminals. The connection between the cables and the battery post should be clean. Also the ground (negative) cable needs to have a clean connection to whatever it is bolted to. Sounds to me like a vehicle electrical problem which would have to be diagnosed by measuring voltage drops point to point with an electrical load (like headlights) turned on. The reason the mfg. puts headlights on cars is to provide a load bank for troubleshooting.