Carl about 14 volts is the minimum. The standard LiPo 4S battery shouldn't typically go lower than 14.4 volts. It is considered dead and likely can't be recharged at 12.8v. Also as it gets lower it begins to lose it's oomph and the gimbal may not respond to quick movements as well as a fully charged battery.
Hey Carl, 15.2 volts is the lowest I'll run a LiPo battery on an M10 before changing it. If we're about to do a long take or I have time at 15.4 volts, I'll change it then. A good rule of thumb is to change the battery shortly after the first LED light goes out on the power indicator on the back of the MoVI. I usually have 4 LiPo's in rotation for any shoot I'm on.