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Sounds normal Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Mon, 11 Jun 2007 12:18:25
In Reply to: Alternator voltage fluctuation question (see also below, aaron, Mon, 11 Jun 2007 10:40:54 Members see ZERO ads on the bulletin boards and get more feature. - Help Keep This Site Online - Signup |
I would believe the voltmeter. The EDU is not a precision voltage measurement. I regularly see 0.2 to 0.4 volt differences between the EDU and a measurement taken right off the battery. I also expect the voltage error to vary. The EDU measure the voltages at some point inside the dash - depending on voltage loads, there can be a voltage drop on the line where the EDU picks it up, AND a little voltage difference on the grounds.
Another thing to know about voltage regulators - they ARE designed to drop the charging voltage with temperature. Charging a battery generates heat, and too much heat will boil off the electrolyte. So the voltage regulator (built into the alternator) measures its own temperature, assuming it is in the ballpark of the battery. As the temperature increases, the VR reduces the normal output, so it doesn't charge the battery as fast, and won't boil the electrolyte. On a very warm day, in traffic, you can get some pretty amazing underhood temps.
Also, idle is a difficult operating point for the alternator to work. It's not spinning fast, so it doesn't have a lot of power to work with. Add in that at idle, chances are the brake lights are on, and with no airflow, a radiator fan is running, then throw in the AC, and yes, it is possible that the current drain would be so much that the alternator may not be producing all the current the car needs. In that case, the battery would kick in. But that's OK - the battery can supplement the alternator for a few minutes every few minutes without trouble - it's not like the battery is powering the whole car - it's just acting as a little supplement. And when the battery is warm, it has a lot more power available than when it's 20 degrees F.
posted by 192.249....
, Mon, 11 Jun 2007 12:18:25 <-- Viewing This Message, Thu, 14 Jun 2007 14:48:01
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