What should my bat voltage read?

[Lhorn]

I'm fighting a knock problem and notice that voltage recorded by SM is around 12.2. I'm afraid low voltage is causing me to lean out.
Straight off of the alternator at idle it reads about 13.2 at idle. When the Ramchargers dual fans kick on, the voltage drops to about 12.5 (read with multimeter). Read at the battery, I get nearly identical readings. When cruising around town, with the engine all warmed up, the SM reads about 12.2. Are these pretty abnormal?


Car has Impala SS alternator and had 2 local parts stores put it on their machine and it passed. Battery is only a year old and also passed. Using ohm meter I've checked and my grounds seem good. PS head grounds moved to intake. Fuel pump has hotwire kit. On top of this, if I leave the battery connected, my battery is dead in a few days and have been unable to identify a short thus far. Not sure if I've got a short or a charging problem or both.

Any ideas are greatly appreciated. Thanks.

 

[turbo nasty]

Sounds like a substantial parasitic drain.

Do u have a aftermarket stereo and alarm on the car. if so disconnect and see if that is your drain.
The factory alarm in these cars can go bad and drain the battery.

AANy switches, crazy wiring that is suspicous if so trace it and verify

 

[sbbpwr]

i like 14.2 no load @2000 r.p.m. sounds like you have a weak alt. to me

 

[ES1]

Could bad battery cables be the problem?

 

[BetMinesFaster]

+1 on this one

i like 14.2 no load @2000 r.p.m. sounds like you have a weak alt. to me

I totally agree with this, 12.5 volts is pretty much a discharge, 14.2 is perfect, computer and ignition need voltage to operate at peak, sounds like a weak alternator to me also, make sure you have good battery connections and that the large wire from the back of the alternator to the positive cable is good and intact, and that the plug in to the side of the alternator that charges the field is intact and has battery voltage with the ignition switched on, if all that looks good Id suspect the alternator....hth, Ryan

 

[jasjamz]

Same alternator here and my stock fan runs all the time. Never see 12 anything while driving in the scanmaster. I also have a system with 3 amps. I would find that draw and go from there.

 

[Lhorn]

Basically a stock stereo system. The battery cables are the originals and look intact. The wire from the alt to the battery also visually looks good. Putting the ohm meter on it, it reads 0 resistance. I agree with the concerns about the alternator, but two stores say it looks OK. If it didn't cost $100 or so, I'd just try a new one. What is the best way to determine if it's a charging problem or a parasitic draw? I guess if there is a big draw/short, I should see the volts jump up when I pull the right fuse out? That make any sense?

Thanks

 

[turbo nasty]

Hows the neg cable to the turbo braket/block area ?

Notorious for getting oily and loose.

 

[IrvJr]

I dont think a parasitic draw will cause you to read the low voltage at idle. I would swap with a known good alt and maybe get the battery tested as a precaution.

 

[Lhorn]

I agree that the alternator sounds suspect. Seems like getting a reading straight off of the back of the alternator shouldn't read low and then drop even more when my fans kick on. I might try getting it checked at another parts store. Just a bit hard dropping money on a part they keep telling me is fine.
The neg battery cable does indeed go to the front of the block. I'll check it again.
Thanks. Any other advice is welcome.