Does quickly fluctuating system voltage indicate a weak battery?

[buffat1]

I bought my 99 PA new. I put at least 3 new batteries in it since. The last one that I installed one year ago is an Interstate. Each time prior to my replacing the battery, I noticed rapidly fluctuating system voltage as indicated by the dashboard instrument package where I can see one at a time, the parameter that I want to watch, such as coolant temperature, oil pressure, system voltage, etc. Each time, after I installed a new battery, the voltage has been pretty steady while cruising down the highway around 70. And each time, before I installed a new battery, the voltage has quickly bounced from 12 to 14, and everywhere in between, in what seems to be a random fashion with no pattern to it. I checked the acid specific gravity last night, and it was in the 1.225 to 1.235 range. The load test for an 880 CCA battery was right at 800 and held steady for 10 seconds. I charged it last night, and this AM the voltage was 13.0 and the specific gravity was temperature corrected to 1.265. I checked the battery in my wife's 1 year old Subaru sitting next to the PA in the garage and it was close to 1.300.

I am suspicious that the PA battery is defective although it is only one year old. On the other hand I am wondering about the alternator since it is origianal and the car has about 150 k miles on it. I don't understand why the system voltage, as incicated by the instrument display, bounces around so much when the battery is weak. I would think that the electrical control system would just the tell alternator to put out a little more voltage, but not to make it bounce around. I am also worried that the voltage bouncing around could burn out some of the electronic components. My shop manual shows a voltage regulator inside the alternator and another alternator control system inside the air box. The plug with the two wires going to the airbox has tiny conductors so I am assuming that they are carring just a low voltage computer signal. Has anyone seen the system voltage bouncing around a lot prior to a battery failure like mine has done in the past, and does anyone know just how this alternator control system works and exactly why the voltage fluctuates. The car runs fine, and without the dash electrical system instrument display, I would never know that I might have a problem.
Thanks for any personal expeerience and insight that you may have

 

[HotZ28]

I can feel you're pain, since I had an new Interstate 880CCA battery fail in my Grand Marquis within the first 1-1/2 years. Warranty took care of that, but I expected better/longer performance from Interstate. I suppose they are all made south of the border nowadays, so I assume all batteries are about equal. An alternator can not fully charge with a sulfated battery. Do you have a build date on the battery? Some sit for 2-yrs before being sold and sulfate while sitting. When buying a battery, always look at the build date and get the most recent production date!

A lead acid battery cell is fully charged with a specific gravity of 1.265 at 80° F. For temperature adjustments, get a specific gravity reading and adjust to temperature by adding .004 for every 10° F above 80° F and subtracting .004 for every 10° F below 80° F.

 

[buffat1]

I am going to see if I can see the build date without taking it out of the car, and I am definitely going to remember this the next time a buy a new battery. I bought this one at COSTCO and it has a full replacement warrantee for at least 2 and maybe three years. After that there is no warrantee. In the course of trying to get the lifting starp and the copper straps from the side terminals to the top connections out of the way, so Icould remove the cell covers I notices a little looseness in the negative side connection. This is held by an 8 mm bolt. I remember doing a very thorough job of cleanning all of the connections and tightening the 8mm bolt last year when I installed it. Maybe I didn't tighten it enough. I have been a back yard mechanic for over 50 years and I try not to break things, like I might have done when I was a lot younger and a lot stronger.

I started the car and put the voltage in the display. I had close to 14 volts at 2000 RPM. I put on the headlights, the blower at high speed, the rear defroster, the signal lights and the brake lights. The voltage slowly dropped to about 13.1 but held steady with no bouncing around. So I am hoping that maybe it was just the loose connection. But I will watch it for the next couple of weeks and if it acts up again I will be going back to COSTCO. Thanks for info.

 

[brockala]

Sorry you are having these issues. When you replaced the battery in past has it tested as weak? Over how many years have you had your PA. If you have only replaced the battery 3 times in 16 years then I don't think the issue is the battery.

If you are seeing voltage changes while driving the car, there are many issues that can lead to this. This time of year when we are less likely to use A/C, the secondary fan is at rest unlike in the summer when the A/C makes it run nonstop. When the secondary fan first kicks in you will see a drop in the volts as shown on the DIC.

Another thing is to consider that is you are not able to duplicate the volt changes when in the driveway, there could be a connective issue. Road vibrations unfelt in the car could cause the alternator plug to not full contact, or other areas that require strong connections. As you mentioned these alternators have an built in voltage regulator. I could be it is going to hell on you. Another thing to consider is your drive belt. If you have not had to replace the tensioner the spring in may be weak allowing less than optimal spin of the alternator. A slightly loose belt will likely leave a blueish tinge to the smoother pullies on the system.

I replace my battery every 4 years in the fall, no matter how good it seems.

 

[buffat1]

Thanks for your reply. I replaced the serpentine belt about 2 years ago. This was a bear of a job because the belt is inside of the front motor mount which had to be unbolted. Anyway the belt is good and the tensioner pulley is strong too. I have the tool to relieve the tension and it takes some good heft to unload the belt. This AM I used the car locally and the voltage was pretty steady. Maybe the little bit loose, 8mm, negative terminal bolt that I tightened, was causing a poor connection. But I can't believe I had the same voltage problem prior to the first three battery replacements because of a loose battery connection. I will just keep an eye on this and see what happens.