Steady Voltage Supply for extended runtime: Batteries in Parallel?
If you use 12 cells of alkaline batteries with 1.5 V each instead of 2 9 V blocks you get also 18 V but with much more capacity in amphours. The 9V alkaline blocks have only about 0.5 Ah, AA cells 2 Ah, size C 8 Ah and size D 18 Ah. If the 9 V blocks last about 3 days, C cells will last about 16 times longer or 48 days. D size cells will last about 100 days. Using the small AA cells, a run time of only 12 days will result.