Odd Battery Voltage Readings on Multimeter

[KidCoronet]

A fellow member asked me a question about an old post which got me out to run some checks on my charging system...and ran into something weird.

When I hook up my multi-meter (new battery in it) with correct polarity (black to neg - red to pos) I get wildy fluctuating readings from like 2 something volts to 15.4...just flashes up, down and all around?

When I hook them up in reverse (red to black - black to red) I get a steady -14.88 volts which would be pretty normal.

Other things to note.

The alternator is a new Powermaster and checks out fine. Battery is brand new. Ballast resistor looks good at 1.8ohms.

I did check the voltage from the high side of the ballast resistor to the positive side of the battery and get a 2 volt reading. Seems high. Must have a voltage drop somewhere?

All the grounds have been triple checked and all wiring brand new and nothing loose.

Anybody ever run into this issue with reversed, crazy voltage readings?

 

[65CopCar]

Check with another meter.

 

[pnora]

Check with another meter.

Or on another car.

 

[Don Frelier]

14.88 volts would be high in my book.
13.5V - 14.0V

 

[KidCoronet]

Ok just checked it on another car of mine and it's reading a steady 14.24 volts both ways. Looks like the meter checks out.

 

[65CopCar]

You checking it right at the battery? What is the ripple voltage?

 

[65CopCar]

And what is the meter you are using?

 

[KidCoronet]

Yessir, checking right at the battery.

The multimeter is a Southwire 100305

I've never checked for ripple voltage but I put my meter on 200v ac setting and it's saying 32

 

[69Bee]

Mopars are internally rectified so output is DC.

 

[KidCoronet]

Yessir, checking right at the battery.

The multimeter is a Southwire 100305

I've never checked for ripple voltage but I put my meter on 200v ac setting and it's saying 32

Mopars are internally rectified so output is DC.

I checked everything in DC in my original post but wasn't too familiar with ripple current mentioned by 65CopCar which I read was measured in AC....flying blind on that one

I also rechecked that my alternator, volt regulator, motor, etc were all well grounded and they are...mystified atp

 

[69Bee]

Ripple is a noise byproduct on an AC signal and usually measured in mV or smaller. Once the generated AC signal is rectified and filtered by the output capacitor, very little to no ripple should be present and would not expect there to be with your new alternator unless it is faulty. You need to put you meter in the DC Volt measurement 20v position and measure the voltage. There should be slight the no variation in the measurement. I believe the large swing you saw was due to measuring with the meter in the AC position. There should also be a "Peak Hold" selection on the meter, read the instructions. Selecting a much higher voltage setting (600v for example) can display your low voltage oddly due to the number of decimal places. You should use the next higher range number beyond the expected voltage you are looking for.

 

[khryslerkid]

Try setting the meter on 20v dc

 

[KidCoronet]

Thanks guys but that is exactly how I measured everything - 20v DC and the readings just spike up and down unless I reverse the probes which then gives me the -14.88v number. I only measured once in AC when I was experimenting with measuring ripple current.

 

[kiwigtx]

Check with another meter.

...and put fresh batteries in the multimeter.

 

[65CopCar]

So, if you measured 32 volts AC, if that was volts not millivolts, you might have a shorted diode in the alternator. If the diodes are accessible, you can check them with the meter.

If that car was in my shop, and I saw that ripple, I would pull out the oscilloscope and see what the waveform looked like.

You will get my electrical engineering dissertation. LOL! There are many dynamics going on. Reason I asked where you measured the voltage is because the battery itself is a big sink. You cannot vary the voltage much at the battery terminals. The battery is a very stiff voltage and current source (capable of maybe 1000 amps) so its virtually impossible to measure 2 volts on the battery terminals and then swing to 14 volts if the battery is good. The battery won't allow it. The meter could be getting confused on the actual voltage swing levels. The variable is internal resistance of the battery. If the battery is in poor condition, internal resistance will increase. You can make some calculations on internal battery voltage drop due to internal resistance. Been a long time since I did battery internal voltage drop calculations so I'm a little rusty on that. Good batteries publish internal resistance values when new. The good batteries have a low internal resistance.

Another variable is the resistance and impedance of the wires feeding the battery and alternator. So if you are measuring at a spot other than the battery, you cannot discount resistance and impedance losses and the inductance of the cables. At that point the voltage can swing a lot more than at the battery. Resistance is measured at DC (0 frequency) and impedance is resistance at a frequency other than DC.

If you have a failing or a shorted diode in the alternator, you can be shooting AC to the battery. I have seen it before but the voltage/current was suppressed.

The alternator is a 3 phase synchronous generator with a full wave rectifier on the output, so it has 6 diodes in it.

With so many dynamics going on, I still question the meter. A good quality meter like a higher end Fluke might give you a different result than the meter you are using. I was dead serious to see what a different meter would read.

 

[65CopCar]

To show a difference between a good and a low end meter, the meter you are using has an input impedance of 1 meg ohm. My old Fluke 87 (was considered a higher end meter at one point) has an input impedance of 10 meg ohms. There is a real difference between a cheap and expensive meter, but 99% of the time the cheap meter is adequate. I did look up the input impedance of both meters.

 

[65CopCar]

Check out the section on how to test the diodes.

Bad Alternator Diode Symptoms: (Causes And How To Test)

A car alternator isn't necessarily that complicated and doesn't have too many components. However, diagnosing an electrical or charging issue can often have

vehiclefreak.com

 

[RJRENTON]

14.88 volts would be high in my book.
13.5V - 14.0V

It all depends on the accuracy of the instrument. 0.08 volts is nothing to be concerned about. When was the accuracy (calibration) of your device checked against what standard? OR....is you life insurance paid up???...as the 0.08 volts could be world ending.....where did you get your voltage range information.....FSM or some other source???? Just curious......
BOB RENTON

 

[RJRENTON]

Mopars are internally rectified so output is DC.

That's the FUNDAMENTAL operation of an alternator. FYI....the Mopar alternator uses a THREE PHASE FULL WAVE BRIDGE RECTIFIER DESIGN.... changing the induced AC voltage in the stator windings to DC via the 3 phase full wave bridge diode array.....
BOB RENTON

 

[RJRENTON]

To show a difference between a good and a low end meter, the meter you are using has an input impedance of 1 meg ohm. My old Fluke 87 (was considered a higher end meter at one point) has an input impedance of 10 meg ohms. There is a real difference between a cheap and expensive meter, but 99% of the time the cheap meter is adequate. I did look up the input impedance of both meters.

Input IMPEDANCE of the measuring device is important.....the Higher the better...as the displayed accuracy will be better. The cheaper instruments are ok for information only....for more accuracy, use a Fluke or similar device....just my opinion.....
BOB RENTON