Battery light is on... barely

[seiferguy]

Hi m42club, first "real time" poster, long time lurker. I wanted to ask about some battery issues I've been having. Here's some backstory:

It started last Thanksgiving, I was driving home to visit my parents. The weather was very cold, there was a blizzard at the time. On my way home, my check engine light & anti-lock light went on. By the time I made it home, my lights were hardly dim and barely producing any electricity.

I had my alternator replaced. Sadly, I couldn't get a good one, so I settled for an Ultima-brand alternator from O'Reilley's. Turns out these things don't charge the battery while it's at idle, so I had to throttle the gas at stops to keep the battery from draining. Despite that, my battery still died, especially in the cold weather. I gave up, and after a few months on that one, ordered a Bosch one from BMA.

Get it installed, everything is good, until driving, and my anti-lock light comes on and my battery light was flashing. Turns out the bolt was loose and the alternator got disconnected. Oops. Unfortunately, that was the last straw for my current battery. I had it replaced for a battery from Les Schwab.

Driving back up to college, my anti-lock and check engine light come on, with the battery light flashing. Turns out the bolt got loose again. I had to get my battery jumped in order to have it start after the driving I did. I had it tightened very hard this time. So far no issues, until now.

As I've been driving around for the past few weeks, I've noticed that my battery light has been on, but very dim. It's almost unnoticeable in the day, but at night, it's a little noticeable. Tonight, I drove home, and the light was on with a much brighter dim. Not full brightness, but it's quite concerning. I just checked my alternator - the bolts are on tight, still. The battery connection seems fine. What else could be causing the problem? I'm sort of at a loss here.

[DesktopDave]

Most of the issues I've had with alternators ended up being the battery.  Do you have a digital multi-meter (DMM)?  If so, what's the voltage on your battery?  Should be slightly over 12vdc disconnected from the car, slightly more if it's a warm day.  Anything under 12vdc is troublesome.  With the car off, I'd also disconnect the negative cable & put the DMM in series with it (hold one lead on the battery post, the other on the cable lug).  Set it to DC amps, check if it stabilizes under 250mA.  The car's computers will pull more than that when you re-connect them, but the car should pull much less than 250mA at rest.  If you have more than a small current running, a short might be causing your problems.

I typically take questionable batteries to stores for a free load test.  O'Reilly's, Advance Auto, etc can give you a better idea of the battery's real condition (if you get a competent tech).

Once you can eliminate a few problems, you can track down the real culprit. Do you have the manuals for our car (Factory Repair Manual, ETM, Bentley, Haynes)?

Until you figure this out, do you have a trickle charger or anything like that?  If the car has a short that's slowly draining the battery it'll save some aggravation, and maybe even your alternator...

[seiferguy]

Thanks for the advice. I went and got my battery tested at Les Schwab, the reading while the car was off was at around 12.1 vdc. However, when the car started, rather than jumping up to about 14 volts, it stayed around 12 volts, so obviously something isn't charging it. The battery life was still good according to the reading, though.

I do have a bentley, but in terms of car work, electrical work is probably what worries the most. Any suggestions as to where I should go next with this problem?

[DesktopDave]

Determine if the problem is the alternator or the electrical system.  Try the test I outlined above:

With the car off, disconnect the negative cable & put the  DMM in series with it (hold one lead on the battery post, the other on  the cable lug you just pulled off).  Set it to DC amps, check if it stabilizes under 250mA.   The car's computers will pull more than that when you re-connect them,  but the car should pull much less than 250mA at rest.  If you have more  than a small current running, a short might be causing your problems.

I'd also be tempted to take that alternator for a test.  Some places will test them just like they do with batteries.  You might have a generator/starter rebuilding service close by, but they're a dying breed.

[seiferguy]

Turns out the alternator nut was ever so slightly loose, I tightened it and the battery started getting the correct voltage.

I guess I'll just have to check the nuts on that every once in awhile to see that they're on correctly.

[seiferguy]

Turns out the alternator nut was ever so slightly loose, I tightened it and the battery started getting the correct voltage.

I guess I'll just have to check the nuts on that every once in awhile to see that they're on correctly.

[DesktopDave]

Sweet!  Glad it was an easy fix!

On a related note, several alternator belts I've used (all non-OEM) tend to be a bit too long.  I'd suggest OEM, but I have never installed one myself.  I just have that alternator adjuster cranked way out, nearly to the limit.