AC Voltage @ battery

[Jerryl]

Anyone measure ac voltage at the battery or alternator?
I read something about it but forgot what the max acceptable reading was.

Replaced the battery in my truck and decided to measure AC voltage. It was high AC, while DC reading was great. I realize AC voltage is the result of a bad alternator/diodes and started wondering . . . . . . if this question could generate an educating and intelligent discussion. lol

 

[Turbotre]

Did you measure the ac voltage with an oscilloscope?

 

[Pronto]

When my alternator started leakning AC volts it made my the lights pulsate. I forget what the AC volts were. I put a volt meter across the battery posts with it set on AC volts. AC volts through the system can mess up many electrical components.

 

[Jerryl]

Ac voltage was measured with a VOM. It was very high and I am not even sure if it is possible. The reading on the AC setting was 30. the lights do not flicker and everything appears to be stable which is why I am confused.

 

[Turbotre]

You won't be able to measure ac ripple on the battery with a regular vom,you would need a very high end meter,or best to use an oscilloscope.The ac present at a battery will be in millivolts,not volts.

 

[Jerryl]

Thought so on the mV, so i checked 4 times on 2 scales using a craftsman vom. i hear ya though.

 

[RmvBfrFlght]

Was out today so popped the hood and made some measurements. This isn't the GN but another car, uses the CS130 alternator (IIRC) at 105 amp capacity. DC volts was steady at 14.2 volts (at warm idle).

AC voltage was around 30 mV (milli-volts). It did bounce around a bit, say from 28 to 32 mV.

Used a Fluke 83 DVM for the measurements. I did need to wire brush the battery cable bolt heads to get a decent reading. Note that this car uses the same exact battery as the GN.

RemoveBeforeFlight

 

[achalmersman]

I'm not an expert, but I would say this is one of those situations where you need to be using a true RMS multimeter. The sine wave is not going to be clean which will cause inaccuracies with standard averaging meters . I use a fluke 87 v

 

[RmvBfrFlght]

The output of the alternator isn't a sine wave. It is a rectified 3-phase generator. They make great TIG welder power supplies, have one mounted and connected to a 5 HP 220V electric motor just for this.

Anyway, the ripple is minimal as the phases overlap each other.

RemoveBeforeFlight

 

[Jerryl]

I believe 30 mV acceptable.
Is there an easy way to lower that?

 

[achalmersman]

I know that it's not a sine wave. However if you are wanting to measure any kind of noise its going to have a ripple or some form of sinusoidal tendencies and averaging meters are intended to measure a clean sinewave they will not accurately work with any dirty waveform.

 

[RmvBfrFlght]

I believe 30 mV acceptable.
Is there an easy way to lower that?

Probably not, a big honkin' capacitor would likely help. But they can cause other issues. The 30mV doesn't bother me a bit. What is more important is the diode bridge assembly in the alternator. As GM moved to EFI they changed the diode bridge to use Zener diodes in the lower part of the bridge (the three diodes that form the negative side).

The Zener diodes at as transient voltage suppressors (TVS), and help keep spikes and noise out of the electrical system.

RemoveBeforeFlight