if you check the manual, it calls the final voltage the lowest voltage possible before shutdown. so it should be lower than the regular voltage of your battery output. i run a twelve volt batt. i set it to 9 volts, and never see the low batt warning. it will turn off on you, too. i set the thing to twelve volts, and the ac adapter was connected, and i got the low batt message and then the machine shut down. that's when i posted one page back about this, and dug into the manual...
hth