Since Arduino Nano is very small in size and cheap, it might be a good choice for a battery-powered project. Clones are abundant on ebay and other websites for around 4USD and mostly come with CH340 series USB/TTL adapter, instead of the original FT232.
To see how effective it is as a battery-powered platform, I made some measurements and would like to share my results.
First, to save power, you will need a boost converter (or DC-DC converter) instead of using siz AA batteries or 9V batteries on VIN pin. The on-board converter is a 1117-5.0 linear regulator that is not very efficient. I recommend something like this:
You can also find some similar boost converters on ebay for less.
You can use two AA batteries to power the converter and get 5V output. Supply it to 5V on Nano. I measured currents on the battery side (@3V), not the 5V side, since I was trying to estimate battery life. I also measured currents on the USB side in case I use a power bank.
When Arduino is sleeping with POWER_DOWN sleep mode, the whole board consumes 15.5mA @3V from the battery. If you are using USB to power it (say you use a power bank), it consumes about 18.5mA. This is definitely not good enough for batteries. Even with a gigantic 20,000mAh power bank, the maximal time is 45 days. The efficiency of the boost converter and activities (other than sleeping) will reduce this to just a handful of days.
Next, the power indicator LED consumes a lot of current, if you want to use batteries, such as two AA batteries. You will need to remove the chip resistor next to it.
My clone has a 1kohm resistor in series with the LED. Once removed, the consumption went down from 15.5mA to 8.8mA (@3V). The resistor I removed was the 3rd one from the right side of the white reset button:
This is 50% saving, which is good and easy to do. You just hold your iron on the chip resistor until it is hot enough to remove it. Having a pair of tweezers helps a lot. Your project may now last weeks on two AA batteries.
After this, you will have to remove the 1117-5.0 regulator to get more power saving. This step is difficult. I used a hot air rework station to carefully heat up the regulator and removed it with only a little damage on the male headers' plastics. If you don't plan on putting the regulator back, get a small flush wire cutter and just cut out the regulator at the pins. This should do:
The wide tab might be difficult to cut but once you cut the three small pins, you can just hold your iron on the wide tab and remove it.
Once removed with the Arduino put to sleep, only 0.74mA of current is consumed at 3V on the battery. This will project into months of operation, if you are logging data at moderate rate.
So if some of you thinks this is not really what a "pico" processor promises, then I am sorry to tell you that the Nano just wasn't designed for battery operation. The CH340 consumes a certain amount of power even when sleeping. About 0.15mA at 5V so it could translate to 0.25mA at 3V. Its pull-up and pull-down resistors also cost similar amount of current draw. There is no way to remove the CH340 chip and still use the USB port. Using an arduino mini will solve this problem since it has no USB chip.
On a data logger I designed around ATMEGA1284P, I used FT232 but it will not receive power from the battery. The power consumption is about 0.4mA when sleeping. Not as low as it can go but good enough for 6 months.
So the conclusion is to at least remove the LED's resistor if not 1117-5.0 regulator as well. If you want to use AA batteries etc. get a boost converter.
Then, if you have other devices, such as sd card, rtc, sensors, you need to power them with a transistor so you can but their powers when arduino goes to sleep.
Hope this helps. I'll write about parasitic power and how you eliminate it another day.