From: SFPhysics@aol.com
Date: Fri, 7 May 1999 21:12:14 EDT
Subject: Auto Generator/Battery
To: pinhole@exploratorium.edu
The automobile alternator output is regulated by a solid state regulator that
varies the field coil strength to provide appropriate output current and
voltage under changing loads. The alternator supplies all of the current to
run the automobile and the battery curent is used only when starting the
motor or operating an electrical device with the motor not running (accessory
position for radio, overhead light, etc.). When the battery, lead - lead
sulfide - sulfuric acid, is discharged, the battery voltage goes lower. As
the battery potential is less than the usual 13.8 volts of a fully charged
battery, the alternator regulator keeps the voltage up until the battery
potential rises to meet the alternator potential and the current into the
battery ceases to flow in large quantities. This whole process of the
charging the battery back for the next start usually occurs within a few
minutes. The currents are awesome. A starter draw for the typical gasoline
engine may be in the order of 200-300 Amperes for a few seconds. The
recharge may be up to 30 Amperes for several minutes and lesser currents for
more extended periods until the potential of the alternator and the battery
reach approximate parity.
A word of warning, the lead acid battery will produce huge currents under
shorted conditions, so all precautions should be taken never to accidentally
short out the contact posts. The hydrogen produced when the battery is being
recharged has also been known to detonate with a spark on rare occasion.
While not at high voltage, batteries carry their own dangers. As with any
form of potential energy, be safe!
Regards Pinholers,
Al Sefl
SFPhysics@aol.com