Measure the current that the devices draw. Battery capacity is measured in Amp hours, where 1Ah means the battery will supply 1A for an hour or half an amp for 2 hours, etc. Often capacity is stated in milliamp hours, that's 1000mAh = 1Ah.
Measure the current, do the maths. Any battery of any chemistry degrades over time so select a battery of twice the capacity you calculate.
PerryBebbington:
Measure the current that the devices draw. Battery capacity is measured in Amp hours, where 1Ah means the battery will supply 1A for an hour or half an amp for 2 hours, etc. Often capacity is stated in milliamp hours, that's 1000mAh = 1Ah.
Measure the current, do the maths. Any battery of any chemistry degrades over time so select a battery of twice the capacity you calculate.
Quite right. If a battery is told to be of 2000 mAh that is tested by using a discharge current of using C10 current, 200 mA. Using a higher load current the capacity is less, using a lower discharge current the capacity will be higher.
If you power the Nano directly from a lipo battery, you would connect the battery to the 5V pin, not the 3.3V pin. The 3.3V pin is an output of the Nano's USB adapter, not an input. The problem with doing this is that, per the datasheet, running at 16MHz, as the Nano does, needs something above 4V. But it works fine at 8MHz. You might look at an 8MHz Pro Mini instead of the Nano. The other question is what the other four devices need - they may need 5V, and wouldn't work well at lipo voltages.
The usual recommendation on low power operation is Nick Gammon's writeup:
For truly low power the Nano is the wrong part; use a Pro Mini with regulator and power LED removed (quite easy to do with a soldering iron). In power down sleep it uses <1µA. Wake up using the WDT, an external RTC, or other external input signal.
Mind that most boost converters have quite high quiescent current, draining your battery even when not in use.