I'm working on an Arduino project that will likely rely on a 9v for its power. If I can get at least 40 hours of power, I'd be happy with that. Here's why I think it's doable:
First, rechargeable 9v lion batteries nowadays have substantially higher capacities than even a few years ago. I see one on Amazon rated for 5400mWh!
Of course, I'll want to reduce power consumption as much as possible, so I'm thinking an Arduino micro would be the best choice (though I will be attaching a shield to it, which will add to the current draw.
My understanding is that the linear regulator on the Arduino is capable of reducing the 9v down to 5v, but at great expense to efficiency through heat loss. My thinking was to bypass the linear regulator entirely, and rely on a much more efficient buck converter to adjust the voltage down to 5v.
Thinking it might be possible to reduce the core clock as well, and run the Arduino at 3.3v instead of 5.
Anything I should consider that I might have missed? I'm excited for the project, but definitely need to get the power consumption as low as possible so I can run it off a 9v and not be constantly swapping/charging.
IDK about setting clockspeed in an Arduino, but with a raw atmega328 it's definitely something you'd do. IIRC the atmegas uses fusebits which has to be set during programming. I suspect that you'd at least need to access the ICSP pins instead of programming over USB. At which point you could just build the needed part of the Arduino on perf board and ditch the atmega16u4 and LDO.
If you don't mind 4MHz @ 3V, then the datasheet claims that will draw 1.5mA https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-7810-Automotive-Microcontrollers-ATmega328P_Datasheet.pdf but if you refer to section 28 in the datasheet you'll see that you can go as low as 2.7V.
If you really have your heart set on the PP3 battery then sure, but I'd probably see if I could use 3xAA instead. I just spent 10s googling, so my research may be lacking. An energizer PP3 lithium ultimate claims 750mAh, if we, for the sake of the argument, don't care about discharge voltage curves and just use the nominal voltage, that makes 6750mWh. Varta has a 2900mAh AA, and 3 of those then become 13050mWh. I know that it's a crude comparison, and you may not get twice the capacity, but still.
If you went for the AAs then your buck will be more efficient as well. Buck converters become less efficient the higher your input voltage https://electronics.stackexchange.com/questions/528284/why-does-converters-efficiency-degrade-with-higher-v-in so you can double the capacity and increase efficiency with AAs.
With 3 AAs and at 8 MHz 3.3V (Arduino Pro Mini) you don't even need a buck converter.