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I'm under the impression that the battery light indicates that charge is flowing "out of" the battery. Normally the battery is being continuously overcharged by a couple volts, meaning that electrons get shoved into the negative terminal and perform work electrolyzing the water in the battery. When this "reverse flow" stops and the battery has to discharge, the current flow is sensed and the battery light indicates this.

I have a condition where the battery indicator is on despite there being over 14v available. It only happens during mid-range RPM, and happens more often when using high-power accessories (headlights and defrost). I suspect that the battery indicator doesn't toggle instantly - trying to exacerbate it works, but only after ~10-20 seconds. Raising or dropping engine RPM resolves it, and usually turning off the mentioned accessories does too. Throughout all of these events the voltage indicator stays at 14.0±0.3v and seems to have no connection with the state of the battery indicator.

Most advice related to the indicator implicates the alternator being unable to supply overcharging voltage. I also suspect that my understanding of the battery indicator operation is incorrect, as it would not be possible for it to be on in my situation.

2005 Ford Focus 4dr Sdn ZX4 S

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    What voltage meter are you reading? Is it oem or are you connecting a multimeter to test? – kyle_engineer Jun 26 at 16:39
  • @kyle_engineer I was using an OBD2 monitoring application, but I re-measured it with a multimeter. The faster update time seems to have made a difference and indeed it went as low as 13.7v at times, and I think there was a medium-strength correlation between the voltage and the indicator state. My running hunch is that there's a low and high threshold to keep the indicator from flashing briefly. It seems like it goes on around 13.7v and off at 14.2v. – Vasiliy Sharapov Jun 27 at 15:21
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So from what I've found, the ECU handles voltage sensing and determines if there is a fail state for the charging system. From what I understand, there are low/high thresholds, and from what I was able to find quickly, they are typical 13.6~15v. If that's the case for your car, you should be within range, but it could be slightly different.

From my experience, sometimes a really bad battery can cause a light when it creates too much voltage drop, however this battery usually can't start the car either. Also, with a Ford, I had a belt snap completely (no alternator charge at all) and it took a few minutes of running for the battery light to come on. So I think (at least with the '90s explorer) the sensor was detecting overall circuit voltage, not isolating the alternator.

A few things to check are:

  • belt properly tensioned and in good condition

  • alternator mounting bolts fully tightened and secure (no ability to pivot or loosen)

  • all terminals and connectors to/from alternator fully seated and in good condition

  • terminals to/from the battery are secure and have clean contact surfaces without corrosion or rust

  • connect multimeter leads at battery and position the multimeter so you can read it from the driver's seat. Then see what the voltage reads when you hold the motor at 2000-3500 rpm. Try to sample a few different speeds, but also try to hold each sample for a few seconds. (BE CAREFUL! Watch where you stick your hands and where you lay the leads! Lots of hot or moving parts in there so try to drape them outside the engine compartment if you can and keep your hands clear of the engine and fans when it's running. I don't mean this warning to sound demeaning if you know what you're doing, just fair caution. )

There could be something totally different causing the issue (like a damaged or shorted component) but these are some simple things to check.

Also, it is totally possible that the belt is loose or in bad shape, and thus losses friction at higher rpm. This would explain why it only happens at mid range rpm. At low rpm it holds friction and generates sufficient voltage. At high rpm, it does give out a bit, but the surplus of voltage generated can be regulated down to an acceptable level by the VRR. At mid range is just the right range to keep functioning, for a while, but not enough to be happy. While running high power accessories, the alternator has to supply more voltage to keep the circuit charged because of the draw of those accessories, ergo why it is more common with these accessories on. Also, if 13.7v was your idle reading, then it could even be slipping a bit there to.

Finally, I don't understand the way in which the ECU detects or reports voltage, so I'd stick to using a multimeter that you know is in good working shape for your testing.

  • Thank you for your though answer, I'll make sure to update as I try these things. Really hoping the belt is slipping, that one makes sense. – Vasiliy Sharapov Jun 27 at 17:35
  • Yeah, I'm leaning toward that, but it's gotta be verified. Hopefully this gives you a solution, but worst case it points you in the direction of a solution. – kyle_engineer Jun 27 at 18:19
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The battery indicator light is not to show "charge flowing out of the battery" but instead it indicates a battery charging issue, so generally a problem with the alternator or wiring from the alternator.

It is triggered by the voltage from the alternator being too low.

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