Studiogeek

iTrader: (6)

Hi,
I am trying to understand the info from my voltmeter. Of particular interest to me is monitoring the condition of my alternator and the condition of my battery, particularly when I have the car off and the ACC/Radio/Amp on and playing.
When waiting for parking etc, I usually run the car but I want to get better mileage so sitting idling for 30 min has to stop. I turned my car off today, set it to ACC, turned on the lights for 20 seconds then off. When it settled it was 11.95 - 12.5Volts, battery meter all the way down with the red light on.
This is what happens every time I turn the motor off for the most part.
The car starts every time easily and the voltage looks normal (green) while driving no matter what.
Occasionally the meter will go red when I sit at stoplights but if i shift into neutral, the idle goes up and the light goes green. Is this normal, or should my car remain in the green even stopped at lights?
What voltage should it read while driving, stopped and idling in drive, idling in park.
When is it OK for it to be off but in the red below 12v, and when is it a problem that may lead to me getting stuck and needing a jump?

My alternator is new by the way.

Thank You.

jadu

iTrader: (42)

moved to the maintenance section.

sjc2467

That sounds normal. Nominal battery voltage is usually a bit over 12.6V (although not all batteries are exactly the same), however, it doesn't take much of a load to show a voltage in the low 12s or even high 11s with the car off (having the key in the accessory position does put a bit of a load on the battery). If the battery is disconnected, the voltage should return to about 12.6, although it may take several minutes (if an unloaded battery stays below 12 volts, it's either failing/dead or almost completely run down.

While driving, voltage should be higher, but with accessories on (headlights, etc) the alternator cannot always keep up at idle (lower speed=reduced output) so the voltage may drop a bit. As long as the voltage is normal above idle, you have nothing to worry about.

I'm not exactly sure what these alternators are regulated to, but generally the voltage should be 13.5-14.7 when running with minimal load. Even with all accessories on, the alternator should be able to maintain this voltage, with the rpm raised a bit (unless you have a significant aftermarket stereo, or something).

Studiogeek

iTrader: (6)

Thank You for that very helpful answer.

zeus87gn

If you have an sc400, i wouldnt trust a voltmeter unless its wired directly to the battery - the under hood wiring harness has the potential to cause vastly incorrect voltage signals due to the design - add in age and engine heat to degrade the insulation and those wires arent trustworthy