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I just changed the battery on my 98 Mazda 626 and now the battery light comes on and stays on after starting the car. I checked the battery and it's reading 12.8v with the engine off, and 13.8 with the engine on. Another time I tested it it showed 14v while running and the battery light was not on.

I also did a diode test as per Paulster's comment, and it showed 1 in one direction and -1199 in the other direction, which seems within spec.

It didn't do this with the old weak battery, or at least I didn't notice it. I just washed the engine compartment with degreaser, a brush and a water hose, but kept the battery, alternator and distributor covered - not sure if that's relevant.

EDIT April 7th, 2016

So this problem has occurred again. Everything was fine at 14.21 volt for awhile, then I went for my inspection. During the emissions part they hook up to the battery posts. As they were disconnecting, the battery light came on, but went back off after a few minutes.

Then today, I revved the engine hard, it stumbled and almost died, then the battery light came on. Got out to check and saw 13.75 volts, with a reading of between 0.02 to 0.04 volts between the negative terminal and the alternator casing.

I'm thinking maybe there is a problem with the connection to negative post, and both these events jostled it enough to cause a problem.

Here's a picture of the negative post:

enter image description here

I guess I'm going to try and clean the connector and maybe sand the inside of it to see if that helps.

EDIT 2: April 7th, 2016

OK, so cleaning and sanding down the inside of the connector and the post didn't help. I wonder if these two events might have caused some of electrical surge that screwed up a diode somewhere or something strange like that?

EDIT April 8th, 2016

I checked the tension on the alternator belt and it seems to be in spec. I also checked voltage between the B+ post and the negative battery terminal and it's also about 13.8.

  • If I am understanding your question, the battery light comes on, but goes out after a few seconds. Your concern is it is staying on longer than it did with the old/weak battery? – Pᴀᴜʟsᴛᴇʀ2 Sep 7 '15 at 15:56
  • @Paulster2 No, it didn't come on at all before. Now it comes on a few seconds after starting the engine, and then doesn't go out. – Robert S. Barnes Sep 7 '15 at 16:30
  • Have you kept the voltmeter on the battery after startup to see what happens? Does the power go up, down, stay the same, get erratic? – Pᴀᴜʟsᴛᴇʀ2 Sep 7 '15 at 16:31
  • @Paulster2 No, I've only tested with engine on and off, not during startup. What do you suspect? – Robert S. Barnes Sep 7 '15 at 16:34
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    Couple thoughts... Have you measure the voltage directly at the alternator, or at the ECU (or whatever triggers the battery light)? Wondering if there is a bad connection or broken wire somewhere. Do the battery connectors look good, no frayed wires, tighten down good, not able to wiggle when tightened down? Has the alternator been replaced? Might grab one from the JY to see if you get the same results. When there is a higher load, does the voltage drop? Lights, fans, radio, etc. – rpmerf Apr 7 '16 at 17:49
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Try this, key on engine off unplug the alternator connector. If the battery light goes off that tells you if the battery light circuit is OK and that the problem exists with the alternator or the circuit on the S terminal.

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It turns out that I had what appears to have been a faulty field control unit on my alternator. Voltage regulation is done by the ECU via the field control unit. I changed the alternator and now every thing's OK.

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