I got a new battery last November to replace the OEM one. It was an AGM battery from Oreilly's since they are one of the few on the market that aren't made by Johnson Controls, and supposedly of higher quality.

Everything is great until a few weeks ago. I noticed that my truck was slow to crank if it sat with the hazards on even for a few minutes. (i use my hazards at work).

I didn't drive the truck at all the past weekend, we took my wife's car everywhere. I went to start it this morning and it was dead, just a 'click' when you turned the key. I got it jumped and tested it in the driveway and got ~14.35v with the truck running, and somewhere north of 12 when off. I took it to Oreilly's since I suspected the battery, obviously. The tester showed the battery was good, but the alternator was only putting out 48 amps, which is about half of what it should be. So now I can acknowledge that the alternator is bad, but how would that make the battery mysteriously drain over the weekend? The tech said it could be that the battery isn't getting fully charged, but does voltage just drop for no reason?

Thanks for any advice.

 

14.35 volts running doesn't sound like a bad alternator to me, and an alternator doesn't push out 48 or 100 amps, it responds to draw.

To me, it sounds like a bad battery .... or maybe residual drain.

Yes, if you use the battery a lot and not driving it enough to recharge, that will result in a low state of charge ...
... and doing it a lot over and over results in shorter battery life.

 

ALL parts houses, dealers and auto repair shops now use "conductance" testers to test their batteries. I have a $550. OTC conductance testor that I use frequently but more for CCA's and HR's than load. When push comes to shove I go to the carbon pile load tester which is still the Gold Standard as far as I'm concerned. Find someone to give it a TRUE load test with a carbon pile. It should be charged up fully. I have actually seen once or twice, strange situations where a battery would test good with a load tester (true load) and the next day fail. After 3 times, I called it quits. The Bear is right, 14.35V is in an excellent range and the purpose of the regulator is to regulate the amperage, which is what it's doing. No alternator is gonna put out 96 amps for long (48x2) unless there is one heck of a demand on the electrical systrm, and I do mean a HIGH demand. Even if the battery is dead, that high a charge for very long would overheat and cook it.

 

Ok thanks. I didn't think about the load on the alternator and the specific output. The guy just hooked up his handheld tester and saw 48 amps. There was some test he could run that would increase the load on the alternator and the bearing would whistle when load increased.

Residual drain is just some accessory drawing on the battery when the truck is off, right? I thought it was very strange that the truck was dead after sitting two days.

 

You can test your alternator easily yourself. Start up your engine and while its idling disconnect the negative lead from your battery. (You could disconnect the positive side instead and will produce the same results but that's kinda dangerous). Don't want to touch anything. LOL

This way the engine is relying completely on the alternator.

If the idle dips or the engine dies, your alternator is weak or bad and your truck is relying too much on the battery.

Good Luck

 

good to know. I'll do that once it stops raining. thanks!

 

You could do that in the old days and not risk much, but today with the vast electronics which are sensitive to electrical spikes and such, you run a very real risk of frying any one or several components.

Best way remains with a simple volt meter with engine running, checking voltage across the + pos and - neg terminals of the battery, with and without loads on. Anything above 13.8 or so up to a max of about 15 vdc is good.
Before testing though, before starting the engine at all, check battery voltage .... should be at least 12.5 vdc and 12.8 is better.

The tester we use most at AAP will test battery and system under start and run conditions, loaded and unloaded, and it looks at ripple. We also have an "amp clamp" that we can hook around the negative cables and looks for current draw when engine off besides amp load under start.

 

Listen to The Bear! DON'T do the remove the battery cable trick while it's running. You used to be able to do that without danger. Not any more. What Bear said is true. Want to risk wiping out some expensive electronic components? Be my guest, give it a try, just don't send me the bill.

 

OK, thanks. Won't pull the cable, but I will verify voltage with engine off. I'll report back soon!

 

i have a ford f-150 2004 lariat 2wd. and i am having a problem when i shut the truck off and remove the key, the truck turns on by itself. it does not start but all lights and radio turn off and on repeadely. Anyone have a guess as to why? would appreciate it , kinda annoying lol

 

If it has slowly gotten worse my money is on the alternator. Batteries usually just crap out quick and when its the alternator it slowly happens because battery holds on for awhile. And 14.3 is
low if his battery was low it should've been aking for a full charge. But batteries can do weird things make sure you test all cold and hot as that makes a difference on both too.

 

This is why I recommended the negative lead. "No harm there"
I did mention the positive lead is dangerous.

I've always checked my alternators that way and have never damaged anything.
Guess I've been lucky

 

Don't overlook the possibility of a bad (corroded) cable at the starter or even the starter itself.

 

It would really be best if you started your own thread instead of jumping into someone else's. Thank you.

 

Yeah, lucky for sure.

Not all forms of damage are going to be immediately evident either.

Removing the negative cable from a battery first is safer than removing the positive cable first only on a negative ground system when disconnecting the battery to either remove / replace or eliminate drain for long periods of storage ... because while undoing it and with it removed, you are not likely to accidentally short the battery with a wrench touching the metal body.

Leave the negative cable in place and start wrenching on the positive cable and let a wrench hit metal and you'll know it was a mistake fast enough.

Likewise, on a positive ground system it is safer to pull the positive cable first.

Electrically speaking, either way when the first cable is pulled loose from the battery terminal, no matter which one it is .... the battery is out of the system and that's when you get spikes if the alternator is running.