I have nothing definitive to offer. But in case it helps here's what I see with my dyno.

I have a HUB SHIMANO ALFINE DYNO 36H BLACK DISC dyno. I used to use it to charge a POWER CONVERTER PEDALPOWER+ W/ iCABLE AND V4i BATTERY battery pack. On that particular battery pack it would flash indicating a charge. But I'm pretty sure that it was not really charging, or not charging significantly. When my cycle speed increased from say, 5 or so mph to around 12 mph, it would start glowing steady instead of flashing.

That arrangement worked OK for a while. But I've since starting using a Mophie Juicepak Pro as my battery pack. This holds about 4-5 charges of my iphone. The Juicepak indicates charges with four bars. That's not very accurate but it's workable for me. The juicepak, unlike the iCable, always flashes - never glows steady. On average it will go up something less than two bars or so in a day of riding.