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What are the possible causes of a battery charging only when the headlights are on in a car with both a new battery and a new alternator?

A couple months ago, my 2012 Honda Civic LX wouldn't start and needed a jump start. Testing the battery showed the battery was fine.

A couple weeks ago,the incident repeated, and the battery again tested fine, but it wasn't charging unless the headlights were turned on, and was diagnosed as having a bad alternator.

To keep the battery charged, I began the habit of driving day and night with the headlights on, and sitting in the parking lot idling with the headlights on if the trip was very short, but it didn't seem to help much, so I got the alternator replaced with a new one but kept the old battery since the battery still tested to be OK. This was 10 days ago.

The car was driven once after that for a short trip the following evening and then wasn't driven at all for the next four full days. After a few trips over the following couple days, it needed one more jump start because the battery was drained again, but hasn't needed a jump start since.

As a precaution, I resumed the habit of driving day and night with the headlights on but relaxed that yesterday.

After several short trips yesterday morning and without the headlights on, I noticed near-stuttering at one more start but it did start, at which point I took it to the parts store where they reported the battery was bad and needed to be replaced as well.

Even with a new alternator and now a new battery, the test showed the battery was not charging (Voltage Reg Failed) unless the headlights were turned on.

  • Can I just check how you know when the alternator was operating? – mx5_craig Oct 9 '18 at 9:18
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This sounds like some kind of 'smart' charging fault.

I am not very familiar with this vehicle, but have you tried checking for error codes?

There may be some global current measurement sensor that the ECU uses to determine when to charge and this is reading low for some reason?

  • ^this. Hondas use an electronic load detector. Typically hondas don’t put out charging voltage unless there’s load on the system. If you are unfamiliar with the system it may seem like the alternator has failed as in OPs case. ELD failure sets a code and illuminates the MIL. The system is also sensitive to non OE alternators as well. – Ben Oct 9 '18 at 17:03
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I suspect you've got a bad ground somewhere, so the circuit is only being completed when the lights are on. Check all the earth connections, particularly those to and from the alternator, battery and lights to make sure there's no corrosion that might be affecting it.

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