In have never had what you are describing on a Rebel. The stator output drops at idle, but you are right, the lights run off the battery. First, check the charging system. Get a multimeter, set it on volts, and connect the leads to the battery terminals. With the engine off, it should read right around 12V. With the engine idling it should drop down into the 11V range, because the lights and ignition are drawing more power than the stator puts out. Then rev it up a bit, and you should see readings in the 14V range, and they should be steady. Voltage should not fluctuate. If it does, you have a bad RR (regulator/rectifier) a bad stator, or a loose connection somewhere. It could still be a bad ground to the lights even if the battery has a good chassis ground. You could take a piece of wire and ground the lights directly to the battery and see if it makes any difference. The Rebel has a fairly simple electrical system, so with some patience it shouldn't be that difficult to track it down with a multimeter.
2016 Rebel, 2013 Royal Enfield B5, 2009 Genuine Stella, 2006 H-D Sportster 1200, 2006 Vespa GT200, 2001 Yamaha XT225
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