Board Thread:General Nerf discussion/@comment-27306930-20160422035231/@comment-27306930-20160425010155

My understanding is that IMRs start at 4.2 volts but quickly fall to 3.6 and stay there through most of the discharge cycle. Toward the end of the discharge cycle they rapidly drop to 2.8. IMRs with overload protection cut off at 2.8 volts. Apparently, the optimum point to catch them is at 3.1 volts, but they don't stay there for long.

On the plus side, unlike Ni-Cads there's no down side to recharging them as often as you like. IMRs don't have a "memory". So the long answer to your short question is anywhere between 3.6 and 3 volts.