streckfu's

Is it the actual charge of the battery or output of the alternator?

GlenL

It's measuring the voltage across the meter. With bad grounds and connections this is different then the voltage across the battery or the output voltage from the alternator. The voltmeters themselves are also not that accurate.

The best way to use it is to see that there's more voltage when the car is running, like 1 to 2 volts, than when it is off. This shows that the alternator is working.

Voltages in a car are really not exact. A good "12v" battery will show 12.5v and a good alternator will put out 13.5 to 14v. Factor in the errors at the gauge itself and a single measurement doesn't really say what's working and what is not with any real specificity.

(TMI? Stuck at home sick this weekend.)

streckfu's

I've metered 12.3-12.5 static and 13.5 with the car running so I knew things were OK with the alternator.

I believe my battery is going bad as it doesn't hold charge for very long. I used to have to put it on the trickle charges once every 5-6 months. It been progressively getting worse and now will barely hold a charge for a day.

The car runs fine once started and will run all day. Until I stop and then it's slow to crank or will not start. Right now the car won't start (just the starter click) at 12.3VDC measured.

I've checked for a drain, by measuring for current accross all the fuses and the battery (measuring between the negative post and negative cable with it disconnected). Accross the batter cable, it shows 9mA dropping and stabilizing at 7.9mA. The is nothing measured accross the fuses.....

I'm having the battery load tested tomorrow (and replaced under warranty if bad). It is an Interstate.

bearone

driving, sitting overnite, the batt should be around 12.6 when tested.

87951

streckfu's

bump.

yoyoguy2

you can't really say "it's measuring the alternator" or "it's measuring the battery" because the two are connected to each other though some large copper wire. It measures the voltage in the whole system (above stated inaccuracies given, of course), so when the car is off, it's measuring just the battery, and when it's running, it's measuring the alternator AND the battery together.

when you crank it over and it doesn't start, how far down does the meter go?

IceShark

Yeah, there is a lot of drop from the alternator to the battery and then over to the dash voltage gauge so it is a bit hard to say exactly what you should be seeing on the dash gauge. Somewhere around 14.0 would be great. And the hotter the climate and engine bay the lower the voltage will be. Which is why the heat shield and vent hose to the front are so important.

But once you fire the car up and see voltage rise the alternator is working to some degree.

Even if the battery is only at 12.3v at rest that should be plenty to crank the starter over and start the car. So the battery is shot and can't quick discharge or some wiring issues = heavy resistance. Take the battery in for a load test. If you can't dump 400 amps at once that is likely your problem.