Haven't bought anything heavy from Autozone in a couple of years.

Had the alternator fail in my 89' F-250. Actually was pulling 10 amps with the key off.

Decided to get er' done, and bought the remanufactured Duralast 75 Amp.

After the install voltage at battery was low while running at 13.7VDC.

Came out next morning, voltage at start-up was 14.7, and rose steadily over the next few minutes to 15.5 volts.

Everything I've ever seen lists this as overcharging.

Autozone brought in a Duralast "Gold" alternator, which is new.

Both alternators test out on their equipment at 15.5VDC which is a "Pass" by their equipment's standard.

Needless to say I left with my original alternator.

Would appreciate your insightful comments.

 

Sounds too high... but .. charging voltage also depends on the state of charge of the battery, and the temperature. "Normally" want to see somewhere between 13.8 and 14.2 volts.. at 80F; as it gets colder in temperature or the battery is partly discharged the voltage will be higher or rise and then come down as it reaches a full charge. It takes approximately two volts above the resting voltage of any battery to charge it completely, and batteries "like" to cook for a little while at this voltage too.

The last battery I bought took over 4 hours to charge up with an AC charger at a low rate. It's good practice to charge up a new battery before installation, and buy fresh stock.

You can determine the actual state of charge of a battery by first removing any surface charge. Turn the headlights on HI beam for five minutes (engine off) and then wait at least ten minutes before measuring the voltage at the terminals. Maintenance Free and AGM/VRLM batteries are fully charged at 12.80 volts, older lead-acid batteries with filler caps at 12.65 volts at 80 degrees.

Run the RPM up to around 2000 or so while all the headlights and accessories are on to load it up and measure that too. It will sag some but should maintain around 14 volts. Also make certain all the cables and ground connections at the block and firewall, frame etc are bright and clean and tight, just a few hundredths of an ohm resistance will cripple an alternator.

 

The display on my battery charger read 3% on my battery after the alternator that failed had a run at it.

I charged it to 99% before I did a start attempt. Took about 8 hrs.

After I had the 15.5VDC reading I shut down and the voltage at the battery terminals was 14.2VDC. Even an hour later the voltage at the battery terminals was 13.2VDC.

What concerns me is that if there were a bunch of Chinese voltage regulators out-of-spec, Autozone might reprogram it's testing equipment to show that as a pass.

Cannot help but wonder what this condition, if left unchecked, might cause to these older computer-managed trucks.

 

Probably won't hurt the computer but it will definitely dry out and fry a few batteries.

I once had a 2000ish Kenworth where the alternator got stuck on full. At idle it was north of 14 volts on the useless dash meter. Get up around 1500 RPM (about 3/4 of that engine's range) and it was close to 18 volts. I drove it to the mechanic like that. He fixed it and we were back in business the next day with no ill effects.

 

Right, that's the "surface charge" we talked about. That's normal, tho it has to be removed (or let the battery set overnight for 8 to 12 hours) to get an accurate state of charge reading.

15.5 charging volts just for a couple minutes is not a cause for concern on a battery that needs charging, unless it never comes back down - then there's a problem, either with the battery or regulator.

 

I actually had my alternator tested there last night because I bought my truck nonrunning and it sat for a year and a half before that and 6 months after I bought it. So when it came time to put the alternator and starter back in I decided to have them checked so I wouldn't have to take them back out if they were bad. Starter failed but the alternator passed and it read 15.5 volts unregulated and 14.4 regulated. Not sure why they are testing the unregulated voltage. Oh and my alternator was factory motorcraft. Could they be giving you the unregulated number?

 

When I was there, the screen had a designation labeled "actual", and that read 15.5VDC on both alternators.

But my battery was first fully charged by my battery charger, then start-up, and voltage reading, at the battery, rose to and stayed at 15.5VDC.

My other 89', a F-350, is at 14.2 with what appears to be the original alternator.

From what I've read, they should never read more than 2 volts over a battery's full charge, around 14.8VDC max,or less.

I've got a "Remy" brand (Ex Delco Remy?) new alternator coming from Rockauto Wed., so we'll see.

 

At 80F, yes. A battery won't even approach a full charge till somewhere around 14.x volts and moderate outgassing. They "like" to hang out for a little while there too. That's why a properly "sized" charger is important. Too much amperage and plates are warped, too little and the battery boils and loses electrolyte because it won't reach full charge in a reasonable time period, it takes too long to reach 14.x. Outgassing is good generally but has to be minimized.

Anyway as temperatures drop below freezing (and below zero) charging voltages must be compensated for upward above and beyond that to account for the increased internal resistance. The chart from batterfaq.org shows this. On the other hand, temperatures above 80F charging voltages must be reduced. If it gets cold enough, 14 volts is barely a float charge.

http://jgdarden.com/batteryfaq/carfaq9.htm

What are you using to measure voltage with, btw? I was a mite bewildered last winter, was reading around 16 volts on everything! Then I realized it wasn't possible, and remembered the last time that happened - Turns out the 9 volt battery in my Triplett was on its last legs. DVOMs get wacky in that case, keep fresh batteries in these for accuracy. It'll sneak up on ya.

 

Well, I use a Fluke 83, and there is a "battery icon" display visible at present, so your comment interests me, though I've never noticed erroneous readings during those periods.

 

ASSUMING that the 15.5 volts reading is accurate, and doesn't go down after a minute or two, battery is not defective, and it's not 32 degrees where you're at .. yes, it's too high.

But the correct charging voltage is definitely a moving target depending a few things. "Normally" 13.8 to 14.3 is what we want to see, at normal temps.

Here's some more info and tests:

http://www.aa1car.com/library/charging_checks.htm

 

Wanted to update the thread.

Ted, thanks for the input and the link, which has good information.

Installed the Remy brand alternator. Had to remove .007 from the outside dimension of the pivot bolt's portion of the alternator casting. It then would slide into the bracket.

Also had to run a rat-tail file through the pivot bolt's hole, as the pivot bolt would bind in the hole.

On start-up, the new alternator exhibited the same characteristic as the Autozone.

Voltage at the battery started at 14.7VDC, and this time rose as high as 16.1VDC.

Based on the information on the link I chose to overlook this, shut down and clamped a battery jumper cable from the negative post of the battery to the frame of the truck.

I didn't get a good reading on my meter for some reason with this arrangement, so I shut down and removed the cable.

Since then the reading has been consistently 14.7VDC or less always.

I did have the ground strap off my hood from last year when the engine was out, and that was reattached during the test, though with new hood hinges and being bolted to the firewall, it's hard to imagine that there wasn't a ground connection through there.

There is also the "clip" grounds from the core support on both sides to the frame, frame is painted, may have to improve these.

But the link in Ted's post revealed some interesting information that leads me to believe that the Autozone alternators are fine.

Wasn't aware that a voltage regulator was designed to go up that high, ever.

Though if it does it may indicate a problem.

Grounds on this truck are being given a second look.

 

Thanks for the update. Run a jumper cable from the negative post directly to a clean unpainted part of the alternator itself (if you want to try that) not to the frame. Poor or neglected grounds and connections means the battery won't get completely charged and cause failure. The grounds to the hood won't factor here. The others might, if the start and ground cables are more than 20 years old they should be replaced, clean up the block and frame connections. Very commonly overlooked.

Don't ever want to see 16 volts except at extreme temps, -25F below zero something like that and then not very long.