thank you for your input guys. its a project I'm working on with my father, he is across the country so i have to wait until he tells me how the changes help (im having him take the supply+ up to the 20V, which should give it enough room to do its Op amp stuff.
the speed is not important it will probably take several hours for the battery voltage to be drawn down. its for an automated robot cart thing that needs to know to go home and charge when its batteries get low.
I'll just have an external clipping circuit to keep the final voltage below 10V. which should never trigger, but just in case so as to not blow the analog input card on the PLC