I have not found that the red/green light does squat other than change color once in a while. If it's green, the battery is probably fairly fresh, apart from that it's random. I can put a freshly charged battery on it and have it come up red, unplug and plug back in, it's green, do it again, red, etc. It used to turn red about 3:10 into a 3:15 runtime. now it sometimes turns red 30 minutes into a 3:15 runtime, or maybe an hour, or two hours.
Random.
I just charge the battery every couple of days, and keep a P7 flashlight on the bars if I misjudge.
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Work: the 8 hours that separates bike rides.