There's no way I have this math right, so please... show me where I'm being stupid.
I've got a simple circuit that all it does is wait for the micro to count up to some arbitrary number and then switch on a light. Kind of like a water filter monitor. That arbitrary number is yet to be determined, but probably in the order of months. This needs to be battery operated, so I tried to design it for very low consumption. I just took some measurements and did some math, and it suggests my circuit will run happily for over 34 years on a couple of AA batteries. That can't possibly be right, so where am I going wrong?
First off, how I'm measuring the current (perhaps my flaw is there): I have the Postive lead from the battery going to the positive terminal on the device. I have the negative lead from the battery going to the black lead on my multimeter. The red lead from the multimeter is going to the negative terminal on the device. My meter shows me 5.75uA.
Now for the math, which is probably where I'm going wrong.
5.75uA = 0.00575mA
Assuming 2500mAh battery capacity
Assuming 70% efficiency
2500 / .00575 * .7 = 304,347.8 hours????
divided by 24 = 12,681 days
divided by 365 = 34.74 years
No bloody way, so where am I stupid please? :)