RE: Voltmeter for A123 cells
The only thing that I have seen that get "pretty close" is the CellPro 4S charger. When you plug in a battery to charge, it tells you APPROXIMATELY how much energy is still in the pack. Still, I would NOT use this indicator as a reliable fly/no-fly indicator.
The best way (right now) to determine how many flight you can get is simply to fly for three flights, charge your A123 cells with a charger that will tell you how many Mah it has put back in, and then divide that number by three. Do this a few times and establish a good average Mah per flight. Then, simply divide the Mah per flight in to 1900. Since the cells are 2300, I feel 1900 is a good safe cut-off that will allow for a few flights with increased Mah draw.