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Okay I know the basics, the battery is around 12.5, give or take .3 or so. I know that when the alternator is running, the voltage is more like 13.8-14.4

But the other day I was driving in a friends car who had a voltmeter (voltameter, voltometer?) anyways he had a device that measured the voltage :) and while he was driving, with the rpm's at about 2000, the car's voltage was 12.8

Should your voltage ever be 12.8 while you are driving? Or is that abnormal? Thanks.

edit: sorry can a mod move this to electrical, thanks, hit post without thinking

could be a weak alt., could be a bad guage or bad wiring to it........but yea it sounds a little low to me

How clean are the battery terminals? Dirty terminals can create quite the voltage drop. Also, corroded cables could do it too. Start with cleaning the terminals.

Also, just try it while sitting in the driveway, with an DVOM on each terminal. There could be a voltage drop through his voltmeter somewhere if something is corroded.

ridr4lif wrote:How clean are the battery terminals? Dirty terminals can create quite the voltage drop. Also, corroded cables could do it too. Start with cleaning the terminals.

Also, just try it while sitting in the driveway, with an DVOM on each terminal. There could be a voltage drop through his voltmeter somewhere if something is corroded.

Yea, definately try just using a mulitmeter and try that on the batt terminals to see if there is something wrong with the meter itself first.

could be where ever he connected the voltmeter, is a regulated output

Sorry I don't think I explained it well enough....not sure if this will make any difference. When I saw it, it was reading 14.4v most of the time, and then when he would stop at a stop light it would go down. But there would be times when he was driving and it would read 12.8 or so.

So it wasn't 12.8 the whole time, it was reading correctly. That's why I'm puzzled.

alternator is dying

Alternators usually put out max voltage around 2000-3000 rpm. So at a stop light, the voltage may drop slightly. Is he running a large amp? If so he may need to upgrade his alternator. I would take it to a shop and have them load test it first.

depends on the car...for example i had a mustang with a pretty big cam in it that would barely idle and didnt really come alive untill 2500 rpm........at idle the voltage dropped way down because the alt. was not spinning as fast....i know this is an extreme example but its the same theory......