My project will consist of a Nano, a couple of I2C devices (an I/O expander and a display), some short LED strips, some input sensors (TBC) and possibly some other components.
I have an ideal power source in mind (a PP3 batery) but I assume it's worth looking at how much power the circuit will consume and how it will be used, in order to know whether a PP3 battery will run the circuit for a useful time.
The datasheet I've seen for a random PP3 suggested it's rated at around 500mAH.
The Nano's spec lists 19mA.
The datasheets for the I2C devices I've seen either doesn't list anything remotely useful (the display) or lists far more than I can get my head around (the I/O expander), so I don't know how to estimate their impact on battery life.
When I decide on the LED strips, I should be able to get their power consuption figures and I can guestimate how much time they will be illuminated.
So ... am I worrying to much about power consumption, is there a relatively quick way to guestimate/calculate battery life, or am I setting foot down a rabbit hole of mathematical hell, the bottom of which I have little chance of reaching?
CrossRoads:
5V LED strips? Each LED will draw near 20mA when on.
12 LEDs = 240mA - that + Nano = ~250mA, so the battery will be dead in about 2 hours.
9V/PP3 are not a good power source - if they can even put out 200mA of current - their capacity is based on much smaller current draw.
Yes, but I'm expecting current to be drawn by only a small number of the LED's (probably only 3) at any one time and for only a short peercentage of the time (maybe a total of 5-10 sec/min).
So my question remains about how to judge the current drawn by the whole circuit.
My thinking is that as I'd like to produce quite a few of these circuits, a PP3 has an apropriate voltage, and is both easy and cheap to get hold of. So avoiding wallwarts, is there a better power source?
CrossRoads:
Power the Nano's 5V pin with 3x AA, or Cs, or Ds. Anything would be better than 9V, the 5V regulator just wastes 4V x current draw as heat.
How does that power wastage relate to battery life?
Obviously it will reduce it, but will 3xAA batteries rated at 250Ah last a comparable time to a 500Ah PP3?
And wouldn't you need 4x AA batteries? (I suspect this is a silly question that underlines my lack of understanding :))
Recommend you use 4 x AA (or C or D if need larger capacity) and a low drop regulator such as MCP1700. The life of the battery depends on a number of factors. The AH quoted is a nominal value and varies by current draw. Generally, the lower the current draw, the higher the battery AH capability. The temperature of the batteries also has an influence as at low temperatures alkaline batteries perform poorly. If the application of your circuit is applicable, consider using low power modes of the Arduino when inactive to reduce the average current. Alkaline batteries last longer if they are used in pulse mode. Self discharge of alkaline batteries is 2-3% per year typically and increases at higher temperatures. As batteries discharge not only does the terminal voltage decrease but also the equivalent series resistance. Meaning if you have significant current pulses, there will be larger equivalent voltage drops.