I recently replaced the alternator with a remanufactured Denso unit from Rockauto. I was wondering if it is normal for the battery voltage to fluctuate like this at idle? This is a video I took testing:

 

All my cars tend to have a fluctuation of .1-.2, but typically hold steady, so say they'll be at a consistent 13.7 at speed w/ no real fluctuations.

HOWEVER, I wonder if they're just reading it (taking samples) slower than yours. It kind of was a large range and otherwise all over the place, so I can see the concern.

 

Just ran a test on my 2014 ISC. Original Alternator. At idle for 1 minute.. it held steady at 14.08v. FWIW.

I used a Fluke DMM with leads directly on the battery.

 

There are too many variables to compare one car to another, simple things like the battery state of charge, how warm the engine is - example: engine temp, fans, cabin temperature setting (AC/Heat fans-compressors, blower motors), kicking on/off that make changes to system voltage.

Most of the time, it's a moving target like the video and >13.5v is good. Constant voltages above 14.65/14.75 can boil the acid from a good battery.

 

Just tested on:

2001 BMW 740i Sport, 14.2, steady
2000 Toyota 4Runner 13.8 steady
2004 BMW 330CI 14.1 steady

All on idle, no accessories on (like lights, AC/heater, radio), warmed up to temp. I also pulled out an older cheaper DMM ($40), thinking maybe my FLUKE ($400) has a buffer... but same deal. It held steady and didn't fluctuate. So not sure, maybe both my DMMs have buffers in them? But I just realized I don't think I've ever seen my DMM fluctuate like that. Maybe others can chime in.

Also, ideally, your V reading should be at 13.8-14.2. Seeing it drop to even 13.3 is low. Could be an issue with the Voltage regulator on the Alt or yes, maybe your car battery needs to be replaced.

Also, looking at these vids of properly working Alternators... none of them jumped around. And this is just a small sample... I probably clicked on 40 vids. I found one vid where it did, but he said in that case it wasn't charging properly.

 

I have a voltmeter inside the car and it is showing the same thing. Before I changed the alternator the voltage did not fluctuate like that nor did it drop that low. I am thinking that i got a bad alternator.

 

Reman or rebuild?
Remanufactured meets OEM spec in all aspects.
Rebuilt may not.

 

https://www.rockauto.com/en/moreinfo...434544&jsn=802

 

Yep, either the Voltage Regulator is bad, your battery or the alternator itself. I'd say return and replace it.

 

That's what I said, and in other posts too.

I'd be cleaning and checking grounds, cables, and connections before swapping the alternator.

 

I had the battery and alternator tested at Autozone and supposedly the regulator is faulty. Funny this is that the car runs normal, no issues.