Battery low but quasi-stable--why?

[rocketman4321]

Car left my daughter sit the other day--she'd driven a few miles and sat with the engine off, running the blower, for about 15 min. That should be <10% of the battery capacity, even if she shouldn't've done it.

Car started about an hour later, got home, and battery read about 12.3 V. I took the battery tot he auto parts store last weekend, and it passed the load test (592/650 CCA) but read 12.2 volts ("is low on charge"). It's been running fine all week on the regular commute--8 miles, 20-30 min. But the battery voltage remains low.
I just tested it after today's run and the battery reads 12.45 volts. At idle, it's about 13.6 or 13.65, and maybe 0.1 V more rev'd up. Those numbers don't change much under load (lights, blower, seat heaters, rear defroster, radio). At idle under load, I read 60 mV from the neg terminal to the alternator housing, similar to the valve cover, and maybe 30 mV to chassis. Too hot in there to run a positive drop test, but I replaced that cable about 3 years ago. Battery connections are clean and the cell electrolyte levels normal. No warning lights through all this. Checked for parasitic loads and I read only 0.01 mA--almost too low. Glove box light is burned out. Belt seems OK and tension proper. I have to check the balancer rubber but no problems with the a/c or p/s.

What am I missing? Is it simply that the 3-year-old "gold" battery won't take a charge? Does the fact that the alternator steps up to the increased load mean it's OK? Thanks for any ideas.

[misha]

Alternator is NOT designed to recharge the battery.
It is designed to keep it charged and to provide enough voltage supply for ALL car's consumers while driving

If you want to properly recharge your battery,put a battery charger on it.

Voltage output on battery terminals should be 14.2-14.4v at idle with ALL consumers OFF.
With consumers ON it should read 13.5-14v but not below 13.5v.
Everything below 13.5v is low and not enough to keep the battery charged on short trips.

Blower is the highest power consumer,especially if it was at highest setting.Running it for a 15min with engine turned off,probably a radio was also ON...i'm surprised that it even started afterwards.

[precopster]

Regulator is the biggest suspect here.

Always measure voltage at alternator post first to see if cables have voltage drop due to resistance. If more than a 0.1V of voltage drop when compared to voltage at the battery the cables are going bad.

A good regulator will provide more than 14.0V at the alternator post. Check chassis connection on the subframe for corrosion and also for corrosion where it attaches to the transmission/engine block.

[misha]

Always measure voltage at alternator post first to see if cables have voltage drop due to resistance. If more than a 0.1V of voltage drop when compared to voltage at the battery the cables are going bad.

Precopster....Volvo user's manual says if more than 0.2V under load.

...A good regulator will provide more than 14.0V at the alternator post.

Not if it's under load like headlights,heater defroster,blower & radio ON.
13.8-14v in that situation would be more realistic on warm engine.

[rocketman4321]

I'll check the alternator terminal tomorrow when the engine's cool, and also the positive side harness drop. Won't a low battery pull down the alternator post though? Can a voltage regulator fail so as to just run a little low?
Blower is a 25 amp fuse x 0.25 hrs = ~ 6 amp-hr out of a 90+ amp-hr battery. Daughter duly admonished, but even twice that shouldn't discharge the battery if it was well charged to start.
I'd also like to check the cable drop to the fuse box, though I'm not sure why that would affect charging. Is there a good place in there to get at the 4-wire (red) cable? I took off the top and couldn't even see the large harness entering. Thanks!

[BEJinFbk]

FWIW, instead of just taking the battery to the test site,
take the whole car. Most auto parts store use newer test
equipment that will diagnose everything in the primary
charging system, not just the battery. And it's much easier.

And I humbly refute that the alternator won't charge a battery.
If the car will start, it may take a while but the alternator is
certainly capable of getting a battery back up to float. It may
take a while, but alternators produce sufficient voltage and
usually much more current than most line voltage chargers.
A long drive may be necessary, but voltage is voltage.
( I mean, how does the battery KNOW? Just sayin'...)

Again, the easiest place to start for most folks is down at the
parts store with a parking lot diagnostic. That will usually help
you eliminate most of the things that your neighbor is convinced
is the problem. ( Besides - Doesn't he still have your timing light
and the good hedge clippers? Just accept one of his beers, then
Smile, Nod and Agree! )

I'd suggest you take advantage of the cool tester at the store, eliminate
some stuff and then chase down your gremlin from there - We'll be here!

[wizechatmgr]

If you really think it is a voltage drop issue, put your positive terminal from the multi-meter on the positive terminal of the alternator and the negative on the positive at the battery. You should have <= 0.2V typical - preferably less. Then test from the chassis of the alternator to the negative, you should get the same. If either of these is out of spec you've got a bad primary/positive cable or a bad ground.

In my old VW I had to attach a ground strap from the alternator to the frame. For whatever reason the existing grounds were not sufficient.

Are we sure the battery is being charged sufficiently? I'd like to see that on a 5 amp charger for about 5 hours, then wait a half hour and see what you get.

[rocketman4321]

Negative-side voltage drop was OK as reported above, about 60 mV. Tried the positive side this morning--engine cold but otherwise per the book. Was about 170 mV, and the battery may have been charging at the time as well, so I think that's good. Also checked the harmonic balancer--there's a few minor cracks in the rubber but nothing close to failure and it doesn't budge. Alternator pulley spins smooth and quiet.

I've now pulled the battery and it's on the charger. I'll see how high I can get it and what it loses overnight. I'll report back.

[misha]

Voltage drop test must be performed when everything is warm....after 30min of driving and without turning the engine off.When everything is cold,results could be deceiving and could show a ZERO voltage drop.

[wizechatmgr]

Voltage drop test must be performed when everything is warm....after 30min of driving and without turning the engine off.When everything is cold,results could be deceiving and could show a ZERO voltage drop.

Good catch, that I did forget to mention...