[artillerystevens]

Just out of curiosity and also because I am grasping at straws here, I pulled out my redtop optima battery and put in a standard 700 cca 12v battery out of one of my other pickups. Changing nothing else, voltage off of the alternator never dropped below 13.8 with or without load. Without load it went as high as 14.9 again this was all measured at the alternator. Voltage drop across the ground circuit was the same as before. HMmmmmm. The optima still reads 12.25 across the posts as did the standard battery. What gives?

[artillerystevens]

Any ideas at all guys? Maybe the second battery wasn't fully charged? Or is it some sort of resistance issue with the redtop? Any other ideas or input would be very appreciated.

[DanielC]

There is a voltage drop in all wires carrying a current. Are you measuring the voltage at the battery posts, or the alternator?
AGM batteries also have slightly different voltages than wet lead acid batteries.

[artillerystevens]

I measured the voltage both at the alternator and at the battery. I did see about a 1 volt less when measuring at the battery as opposed to the alternator. The thing that is odd is that with the optima battery installed the alt would put out a lower voltage than with the standard lead acid battery installed. 2-3 Volts lower actually. Why would that be? Either way I think I have found the source of my problems, I just don't understand it.

[DanielC]

I have always had good luck with wet lead acid batteries, and have not had much experience with AGM (optimal) batteries. I used to work at a boat dealership, for a high end boat line, the type of boat that has lots of speakers on towers, with multi hundred watt amplifiers. These boats usually had AGM batteries in them, and if they tried to run one AGM battery, and one conventional lead acid wet battery, it would cause problems. The two types of batteries would "not play nice with each other"
I believe the charging systems on even the newest cars, if they are equipped with wet lead acid type batteries are "tuned" to provide the charging rates that work best for lead acid batteries. A lead acid battery likes to be recharged as soon as it can, after a discharge. It can tolerate a fairly initial charge rate, but it needs to be cut back fairly quickly. The charging currents, and voltages for a lead acid wet battery are not the same as what is best for an AGM battery

You stated you are losing about 1 volt from your alternator to your battery. I would consider a .500 volt loss as barely acceptable, in each cable. Is there more loss in the positive cable, or the negative?
If the loss is mainly in one cable, that cable needs attention.

[T-bucket23]

What is the voltage at the sense terminal on the alternator. It appears either the alternator cant keep up, but I think you changed it already, or the alternator is not seeing the load on the system. As a quick check you could put a jumper from the large terminal to the two small terminals to see if it stabilizes. This is not a long term solution but is a good test.
I think you have eliminated a alternator ground issue so the only oter issue would be an exciter or sense issue.
The jumper will prove the alternator works.

[artillerystevens]

The voltage drop across the ground was as about as close to zero as I could hope to get, however the 1 V drop was from the alternator to the battery. When I tested the positive terminals I didn't do the actual "voltage drop" test I just took a voltage reading at the alt, then again at the battery. I will do an actual "voltage drop" test across the positive and also do as Chet recommended and just run a loop to the #1 and #2 terminals off of the batt terminal on the alt just to do a test.
Thanks again for all of the help.

[mud.man.rj]

One other thing to check that most don't think of is the battery itself. The car actually runs off of the battery not the carging system it only refills (charges) the battery with what is being used, so an old b attery or one that could have a broken cell in it will make the car run only on the charging system and will fluctuate as stated, if not sure just swap in a battery from something else you know charges.

[artillerystevens]

Ok, so I just went out to the vehicle and did some checks just to confirm and clean up the numbers I'm reporting here.
Voltage drop from alt case to battery ground is 31mV (.03 V) and voltage drop from alternator post to positive battery post is 175mV (.18 V) The voltage being put out by the alternator at idle is 13.17 V with no load on the system (i.e. cooling fan, headlights, air compressor) With a load applied to the system (all of those things turned on) Voltage at the alternator positive post drops to 12.2ish(it fluctuated slightly).
I also did a loop as chet recommended to the #1 and 2 terminals and the alternator output was 12.98 V. With the same load applied alternator output dropped to around 12.1 V.

So the bottom line is the alternator is putting out what it should, both with and without a load. I have had the exact same results with 2 different alternators, both of which I ran in my 77 Chevrolet pickup last week with no issues, and I also took them into O'Reillys and had them bench tested and they both checked out. The only thing that seemed to make a difference is swapping between the Optima battery (currently installed) and a lead acid battery (out of the 77)

Hope this helps clear everything up a bit!


I apologize for the 1 V drop reported earlier, I was mixing #s in my head and that 1 V difference was what my Voltmeter on my dash reads as opposed to the actual Voltage on the system read by a DMM.

[artillerystevens]

Originally Posted by mud.man.rj

One other thing to check that most don't think of is the battery itself. The car actually runs off of the battery not the carging system it only refills (charges) the battery with what is being used, so an old b attery or one that could have a broken cell in it will make the car run only on the charging system and will fluctuate as stated, if not sure just swap in a battery from something else you know charges.

I think that must be what is going on. As I stated in my post a moment ago, that did SEEM to solve the entire issue. I think I may just be going through denial that my over priced fancy Optima is failing me, AGAIN I might add.

The battery shows just over 12 Volts when measured across the posts and disconnected from the vehicle. I always thought that wouldn't be the case with a bad cell?

[DanielC]

What RPM are you running the engine at? Your battery charging voltage should be 13.5 to 14.5 volts, engine 1500 to 2000 RPM.

It is possible for any battery to have a cell with high resistance. Measure the voltage on it, without a load, and the voltage will check good.

Measure the voltage of the fully charged battery cranking the engine. It should stay above 10.5 volts.

[artillerystevens]

Originally Posted by DanielC

What RPM are you running the engine at? Your battery charging voltage should be 13.5 to 14.5 volts, engine 1500 to 2000 RPM.

It is possible for any battery to have a cell with high resistance. Measure the voltage on it, without a load, and the voltage will check good.

Measure the voltage of the fully charged battery cranking the engine. It should stay above 10.5 volts.

I'll give that a shot in the morning. I have been measuring everything at idle btw, approximately 800 rpm on my setup. That is the other part of this mystery that is stumping me. I haven't said anything about it since the beginning of the thread, but the voltage drops off slightly (.3-.5V) as rpm increases while driving.
I love charging systems!
I don't know what I'd do with out all of the help!

[artillerystevens]

Problem finally solved! I ponied up for a good lead acid battery and all of the problems vanished. Tested the optima at oreillys and it showed over 12v but only held a 30% charge. The battery tested "good" on the machine but never got to 100% charge and quickly dropped to around 50% after a couple minutes. Thanks again for all of the help!

Daniel, as far as checking voltage when cranking, you called it. 10.1 V