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I have a brand new battery and alternator my car runs fine. When I am idling in traffic the battery light flashes on and off and on. It eventually goes out when I pick up speed. I had Honda do diagnostic. Checking and they could not find anything wrong. It makes me nervous if I get into a traffic jam when the light comes on what could be the problem.

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  • Welcome to Motor Vehicle Maintenance & Repair!
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Aug 22, 2018 at 23:21
  • Presumably there was a reason you got a new alternator, as well as a new battery? I would go back to whoever fitted those parts, and not give up pestering them till they sort out the problem.
    – alephzero
    Commented Aug 24, 2018 at 7:48

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Never a simple answer on these problems. Typically, I've found this to be a problem with the charging system not generating enough power to charge the battery. Bad alternator, possibly. Curious Honda could not find an issue... There may also be an issue with a short in the power system. Sometimes, auto parts stores will check out your charging system for free - might be worth a second opinion from them.

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An alternative possibility - If you've done a lot of modifications to your car, it's possible you're drawing more current than your alternator is able to provide at idle speeds.

Some aftermarket stereo systems can have this effect, as can some brighter-than-normal headlights.

Then again...If this were the case... You'd probably know, having been the one to put such things into the vehicle.

Either way - Try turning off any electronics accessories, headlights, and stereo and see if it's still doing that. If with your stereo / other accessories, even the lights off, it's still doing this... You have a significant charging system problem.

If adding a load (Stereo, headlights, cigarette plug drawing 10 amps) increases this issue (don't do this while in traffic, but while parked somewhere safe to test), it's indicative that your alternator isn't putting out enough current to power these devices and charge your battery while idling.

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  • If accessories are the source of the problem, you can usually find an OEM high-output alternator that bolts on or have an alternator shop rewind your existing alternator for high output. If you're drawing a totally unrealistic amount of extra power, dual alternator setups are available (and stock in some truck applications). Expect to take a little off the top of your mileage rating.
    – user16128
    Commented Jan 16, 2020 at 1:27

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