Stereo & Electronics - alternator or battery problem help

ok im a mechanic for trade but im kind of second guessing myself.

i have an 02 ss dumdumdumdum. here is what happened, i left my car to sit from monday to saturday with a radar detector plugged in. i did start it once ( barely) on wednesday to see if it had battery left . i should have unplugged the radar but forgot. i went to start it today (sat) and it was dead . so i jumped it and it fired right up , voltage was low so i figured it needed to charge more. so i drove home for a good 45 minutes when i noticed this and it was still at about 13.0 volts avg (12.7-13.2) so im thinking its the alt. so i take it home and let it set for a couple hours and it starts right up when i go back to it. i try starting it and pulling the cable from the battery and the car runs fine and when i checked it read (13.7-14.0) from being cold started . then proceed to take a drive as it read 13.8 and went down to about 13.5 on my aeroforce. i also load tested the battery( yellow top ) and it was fine

for electronics i have a viper alarm, aeroforce scan guage, and a a 500 watt sub amp.

i have these all in for some time with no probs until i let the car sit or i just never noticed.

i just dont want to replace the alt and it not be the prob.

any advice or experience?

bump TTT

doesnt sound liek thers a problem at all

sounds like you let the battery die from the radar being plugged in and let the battery charge back up and all is good

fyi most autoparts store will test alternators

I am having almost an exact same problem with my car. It drives and performs fine but my voltage reads damn near 12 volts:

http://img210.imageshack.us/img210/4417/tropicare002ht1.jpg

The car takes sits for a couple weeks at a time and I accidentally left the keys in the ignition overnight last week. Jumped the car and it fired just fine. Ever since the voltage has been pretty low.

Trust me , youll know when your alternator is going out ..My voltage guage read 9-10 volts and the check guages light will come on.. You guys are fine.

Trust me , youll know when your alternator is going out ..My voltage guage read 9-10 volts and the check guages light will come on.. You guys are fine.

Well what should I/we do to charge the battery back up? Mine has been driven quite a bit since this has started happening. Next time the battery will go on a trickle charge. Is it time to just replace the battery???

Furthermore, since I killed the battery, the gauges reset themselves when first cranked on two occasions. The needles just went to max and then back to norm

When the battery is very low, it is showing a large resistance load to the charging circuit. Basically the low battery is needing a large ammount of current to just charge it up. So while this is still happening the voltage will drop.

Basically you have to figure out what is the wattage needed to charge the battery. With that relationship established, you can see that inorder to increase the amprage out, the voltage will drop. (W = V x A) (Example 100 W = 10V x 10A or 100 W = 12v x 8.333A or 100 W = 13V x 7.69A) Current is what is really needed to recharge the battery so while it's in a low state, the battery is going to demand more current which will cause voltage to drop.

Never trust your dash gauge in an absolute sense. You can somewhat trust it in "it usually read 13~14 but now it's reading 11~12" (relative sense) but you can't trust it as in "My gauge says 12V so my battery is 12V." You need a multimeter for that.

Never trust your dash gauge in an absolute sense. You can somewhat trust it in "it usually read 13~14 but now it's reading 11~12" (relative sense) but you can't trust it as in "My gauge says 12V so my battery is 12V." You need a multimeter for that.

x2, they aren't very accurate

When the battery is very low, it is showing a large resistance load to the charging circuit. Basically the low battery is needing a large ammount of current to just charge it up. So while this is still happening the voltage will drop.

Basically you have to figure out what is the wattage needed to charge the battery. With that relationship established, you can see that inorder to increase the amprage out, the voltage will drop. (W = V x A) (Example 100 W = 10V x 10A or 100 W = 12v x 8.333A or 100 W = 13V x 7.69A) Current is what is really needed to recharge the battery so while it's in a low state, the battery is going to demand more current which will cause voltage to drop.

Good to know but what exactly do you suggest to do once I figure out the necessary wattage?

Sounds like a bad voltage regulator, and since it's attached to your alternator you will have to replace it. I just had the same thing happen to me this past weekend. My warenty covered it so the stealership replaced it and problem solved.