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.
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.
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.