Another data point on the Gemini battery:
I charged a new one for the first time Monday evening with a MS charger that output 8.49 volts. When the charger light turned green, the battery measured 8.31 volts (might have been 8.39, but I doubt it - see earlier post).
I used the Gemini battery with the MS lighthead. I rode 1 hr 15 min on high with temps in the upper 30s Tues night. Battery measured 7.92 volts afterward. Rode another 2 hrs tonight on high with temps in the mid 40s. Battery measured 7.52 volts afterward.
Now the messy part: the MS lighthead was still green after the combined 3 hrs 15 min on high. But as soon as I pressed the button to go from high to medium, it turned red and stayed that way through all the cycles and after I unplugged and replugged it. Why's that messy? I recall several posts last year complaining that some MS lightheads would turn red prematurely but still be usable for some time. I know my own has never completely shut off, and I've run it at least an hour regularly on low after the red light comes on.
So was I at the end of the usable charge in Gemini battery? Dunno. What voltage does shutoff occur at? Mine's down to 7.52 volts after 3 hrs 15 min in cool temps. Draw your own conclusions, but share them, esp. if you have any experiences with the shutoff voltage for the MS lighthead.