onesickpantera

iTrader: (1)

I bought two new batteries this year. #1 went dead after running the stereo a few hours last weekend. Switched to all and fired right up, went for a cruise and #1 took a charge. But now it keeps dying quicker.

When on battery #1 the volt meter reads lower, about 10-11 volts, even when running. Is this a sign I have a bad battery or a bad alternator? Is there a way to check for sure?

Trippin

First I would suggest you attach your DMM to the positve post of suspect battery while the engine is running and your switch is set to that battery. Verify 12.7 - 13+ volts (within reason, excessive voltage confirms problem) on the meter while the engine is running. If voltage is confirmed, then perform a load test on the suspect battery and a specific gravity test on same. The answer should lie somewhere in between.

hoozeyurdaddy

what is the volt meter saying when on batt. 2 or on all.
what is the alt. how old, may consider installing new single wire alt, 100 amp or better.is the stereo battery a deep cycle battery? what size? 24,27,29,ect:

onesickpantera

iTrader: (1)

Both batteries are new "dual purpose" batteries. 1000 cranking amps, 115 amp hours, 180 reserve capacity.

With the switch on battery #2 volt meter reads just over 12 at idle. On #1 it was 10-11.

I charged the battery last night and it's now reading 12.6 volts. So, with the engine running my volt meter should read at least 12.7 volts? If not probably an alternator?

hoozeyurdaddy

when you check the volt ouput of the alt, have the throttle cracked open just a little, say 1500 rpm the get the altermator excited, you should have close to 14 volts with the engine running.

Edward R. Cozzi

A fully-charged, healthy battery should have 12.8 volts.
Alternator voltage should be more than that AFTER the alternator has been "excited" and is putting out. Don't trust your volt meter at the dash. Many times they lie. A scan tool is a good way to check for alternator output if you have an ECM or PCM.
If your batteries are two years old or older, you're on borrowed time anyway. Does your stereo system have big draw amps?

oldandtired

I am assuming that you have a single engine application. The problem will either be in the battery or more likely the cables. Use a voltmeter at the battery terminals to know. It's very unlikely that you have an alternator problem if one of the batteries is charging properly. While cranking the engine with the meter on the terminal, not the cable, check for voltage on #1. If that voltage is really dropping, then your battery is toast. If the voltage stays around 10, while cranking, then the battery is fine. If the voltage at the battery stays above 11 while cranking, then you definitely have a cable or ground problem. Do different checks of voltage at each cable end (chase the cable end to end) while cranking until you get to the starter to find where the voltage drop is to find which cable is the problem. Make sure that the ground cables are good with good terminations as well. Whenever I have had a similar problem, it usually turns out to be either the positive terminal at the battery needing cleaning/tightening or the ground lug at the engine.

Rene

onesickpantera

iTrader: (1)

Thought about this some more and it's definitely a charging problem. Even with a low battery the gauge should be reading 13 volts when running. The voltgage gauge is reading the voltage of the battery and the more I use the accesories the further it drops.

Now the interesting part. Went out today and left the stereo off all day. Voltage started at a little over 12 volts and dropped to about 11.5. Then went for a run and all the sudden voltage startet reading 13 at the gauge. 5 minutes later back to 12. So, could be a loose wire somewhere. Going to trace all the wiring tomorrow. If I don't find anything I will pull the alternator and have it checked.

hoozeyurdaddy

this is common for the exciter sys. to act like that, could be in wiring, but my bet is on the alt.

Turbojack

I have seen volt meters (not the one on the dash) read different voltages on the same battery, unless you have a high $ Volt meter. Fully charged battery is 12.7 volts. When checking with alt. running should be aprox 13.8 volts. I have seen on my EFI computer anything from 13.1 to 13.9