hey tinhorn,archy or any other of you electrical geniuses. i currently am using 3 9v batteries wired in parallell as my source of power for all my units w/ms20 sensor. will 6 aa's wired in series last longer than the 3 9volts. i know there is a formula that you guys probably know off the top of your heads . what about D batts. or C batts in a series. those 9v are pretty expensive thanks in advance.