RE: Safe TX battery level?
The system will operate fine down to the alarm. I recommend however charging earlier, I recommend if the system falls below 9.6V to charge. 9.6V is the nominal voltage of the battery and is in the flat part of the discharge curve of the battery, so it will stay at 9.6V for quite some time generally. This is a safer voltage so that the battery power will not suddenly drop possibly causing loss of control.