I don't know if this was clear from above, but I just wanted to confirm that this is the intended behavior, and not a bug. This is very typical in aircraft with alternators and analog voltage regulators. It's actually often one of the first things that I hear real world pilots remark about when they test my aircraft, "you even got the low voltage light correct!" In my own flying, the cabin lights dimming and pulsing at low RPM while taxiing is actually a great indicator that you forgot to return the engine to an idle RPM of 1,000-1,200, which keeps the battery charged, and the engine healthy. Always happy to answer if you have any more questions!