Then I decided to see if I can use a 9v battery to power my UNO
And if you're running delay, you've got the processor running full time too, got the power LED on the whole time, got the at8mu2 running full time, if your unused pins are left floating as inputs you've got potentlal switching losses there (best to set unused pins as inputs with internal pullups enabled).You'd be way better off running of 3 AAs direct to the 5V header pin, bypassing the regulator.
A rechargeable 9V battery reduces the cost over alkaline---but they are normally 150mAh or so, whereas AA's are 2000mAh (13 times the life). And if using rechargeable AA's then use 4, not 3, as they are about 1.25 to 1.3V each rather than 1.5V for alkaline.
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