Can an Alternator work the opposite way?

[Andrew_Pakula]

Recently I have been having problems with my battery draining over night even though nothing has been left on.<P>The battery was tested twice and it seems to be fine. Since the battery was from Canadian tire I let them take a look to see what might be causing the energy drain.<P>The guy there is trying to tell me that the alternator is bad and its working the opposite way it is supposed by draining the battery instead of charging it, Is that even possible? It just didn't sound right to me.<P>But here's another question if it is the alternator how is it draining the battery over night when the car is off?<P>If I unplug the battery over night then I can still start the car but if I leave the battery hooked up the battery will be dead in the morning.<P>Anyone know what might be going on?<P>This alternator I have is only 4 months old.... and has a 6 months warrenty I believe.<p>[ August 03, 2002: Message edited by: Andrew_Pakula ]

[Quazi5]

With all due respect to the tire guy, but he's smoking crack if he really thinks that the alternator can drain the battery. This is how it works... There is a coil inside the alternator right? Right! What else is there you ask? A magnet... A fairly big one. Now that magnet is connected to the pulley shaft which does what? That's right, it turns the magnet as the belt moves. When that magnet is spun, it creates an Alternating Current... (aka AC)which moves back and forth sorta like <- -> <- ->. Now the AC is then forced through what are called "diodes". Diodes are little resistors that only let current travel one way. this current that is traveling one way would be called what? Damn, your catching on. Direct Current (DC) and it looks like -> -> ->. Now.... this direct current that comes out of the alternator is then run through a voltage regulator. Think about the name and figure it out for yourself. Anyway... to make a long component lesson short. The DC that comes out of your alternator can in no way "drain" your battery because of the diodes. Try this. Take your car to an Auto Parts Store. You may have to remove the battery or the alternator, but have them check both. If your battery is good the your alternator isn't putting out enought voltage to charge the battery properly. If your Alternator works within specs, then it's your battery saying "Replace me cause I'm can't hold a decent charge!" ...... there is one other thing. You have excessive Key Off Drain, which is the drain on your battery to keep your CPU from resetting, or your preset radio stations from disappearing everytime you turn off your car.<P>Hope I could shed some light. If any of the statments said herein are untrue then e-mail me at Quazi5@bolt.com and let me know what was wrong.

[Andrew_Pakula]

Thanks for the info, I am going to bring it to my regular shop which will be able to give me the truth with what is going on with it. I'll let you know what it turns out to be.

[Mike 94PGT]

<P>Yes, an alternator can be a path through which the battery can drain. The rectifier bridge is a series of diodes internally that convert the AC waveform produced as the field cuts across the stator coils into DC the rest of the car can use. If one or more of those diodes becomes defective such that it conducts both ways (rectifier diodes are only supposed to conduct one way), there will be a path through which the battery will drain.<P>An alternator with this problem will be making "dirty" electrical power (because the rectifier isn't functioning properly) and might be heard as a whine in the stereo speakers (for instance) or even one audible under the hood.

[Quazi5]

*slaps my forehead* Dhot... Sorry... he's right. If your diodes are defective then the AC current would drain your battery. My bad. [img]shrug.gif"%20border="0[/img]<BR>Thanks Mike.

[mmonid]

Hey, just a thought, but there could be somthing that is not turning off and draining the battery ie. trunk light, stereo, or some other short in the car. Sometimes it is a little stupid thing that is causing all the problems.