This topic is one of the trickiest topics. There is no one size which fits all solution. The power requirements for the same device may vary depending upon how it is used by the sketch and how the circuit has been designed. Power source capacity is usually measured in terms of how much current can be supplied by a battery and for how long it can supply the current. Precise power consumption is an advanced electrical engineering topic; we will focus on learning how to determine approximately how many batteries we should use in our projects. Learning this should help you to become ready for the challenge of building standalone devices.

How many batteries will my device prototype need? How long will it run given x number of batteries attached to it? Let's try to understand the answers to these questions. The power source capacity is usually measured in terms of how much current it can supply over a finite period of time. The unit to the measure capacity of a battery is mAh (milli-Ampere-hour). 1000 mAh means a battery can supply a current of 1000 milli-amperes for 1 hour. In other words, if your device requires 1000 mA of current continuously, then a battery of 1000 mAh can last only for 1 hour.

Therefore, depending upon how much current is required by the device prototype and for how long, we will need to calculate the capacity of an independent battery power source. Any prototype that we build will have multiple parts. Each part will have its power rating in terms of current consumption. Based on the total power rating of all the combined parts, you will have to determine the type and capacity of the power source to be used.

Let's take a very basic example of blinking a Red LED with the Arduino Uno R3 board. Assume that the Red LED should be blinked at 1 second periods, for 3600 times over a 24 hour period - so, effectively, the Red LED would be blinked for 3600 seconds, which equates to 1 hour. From a Red LEDs datasheet, we know that the Red LEDs consume around 20 mA (on the higher side). Therefore, the amount of current drawn by the Red LED would be calculated as:

Current consumption = Current Rating * Blink Duration * Number of blinks per day

= 20 mA * 1 second * 3600 times per day

= 20 mA * 3600 seconds

= 20 mA * 1 hour => 20 mAh per day

Therefore, the red LED alone would draw 20 mAh of current/energy from a battery every day.

Similarly, we will need to calculate the current/energy drawn by all the other parts in our device prototype. In this example, the only other part is the Arduino Uno R3 board, which has a variable current draw between 20-50 mA depending upon various onboard parts being in use or not being in use and the operation mode it is in. For the sake of understanding the battery capacity topic, let's assume that the Arduino Uno board consumes an average of 40 mA and it remains operational throughout the day (without being sent into sleep mode); then the current/energy drawn by the Arduino Uno R3 board would be calculated as:

Current consumption = Average Current Consumption * Number of hours per day

= 40 mA * 24 hours per day

= 960 mAh per day

Therefore, the Arduino Uno R3 board itself would draw 960 mAh of current/energy from a battery every day.

So, as a whole, the energy/power consumption of the device prototype, on a per day basis would be calculated as:

Total current consumption = Arduino Uno R3 + Red LED

= 960 + 20 mAh per day

= 980 mAh per day

Arduino's DC IN port recommends a voltage of 7-12 volts; therefore, we would need at least 5 AA sized 1.5V batteries connected in series to achieve a voltage of 7.5 volts.

1 AA sized 1.5V alkaline battery rated capacity = 1800-2600 mAh ~ 2000 mAh

So 5 AA sized 1.5V batteries would be able to supply = 10000 mAh

Therefore, the number of days the 5 pack battery would last = Total energy / Daily need

= 10000 / 960 days

~ 10 days

This is how you can determine the approximate power source requirements for your device prototype. You must also remember that the preceding calculation is tricky because there are a lot of variables at play, such as whether the Arduino is in sleep mode or not. The additional current will also be lost as a result of dissipation, wire lengths, and so on. It is usually a good practice to err on the side of caution and provide for an extra 10-15% capacity than the calculated value.

Now, let's start exploring how to build a standalone device using the Arduino platform in the following sections of this chapter.