IoT How to get power supply for IoT devices right

One familiar challenge many developers face in the design of internet of things (IoT) devices is reliable, long-lasting power for the device. Even if you select the perfect ultra-low-power microcontroller and do everything else right, the wrong choice of power source can cripple device performance and ultimately cause your device to fail.

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This article examines some of the factors that engineers should consider when selecting a power source and power management strategy for their device.

Factors to consider when selecting a power source for IoT projects

Below are key factors to consider when deciding what power source, you should use for different types of IoT projects.

1. The total energy required by the device/project

The total energy your device needs to run should be the first and foremost factor you consider before selecting a power source.

IoT devices usually comprise three units:

• 1. Sensors/Actuators

• 2. Processing Unit

• 3. Communication Unit

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The total energy consumed by a device is the summation of power consumed by each of these units (and the components they are made of). Therefore, it is important to choose the right technology and mode of operation for them.

One key thing to keep in mind when estimating device consumption is how it varies with different operation modes of the device. Operation modes like the communication mode, where the device is expected to transmit data to the cloud, are usually more power-intensive than others like the sleep mode. As such, putting a figure based on the sleep mode or the transmit mode only will lead to errors.

This error can be avoided by estimating the average consumption in cycles. Typical cycles for IoT devices revolve around; Wake Up - Take Readings - Transmit Data - Go back to sleep. By estimating the amount of power consumed by each of the stages in the cycle, and the amount of cycle the device runs in say 1 or 24 hours, an accurate estimate of the device consumption can be obtained. Only after this has been done, should the selection of the power option be considered.

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In battery-powered applications, for instance, the required battery capacity should only be decided on after the energy consumption of the device has been ascertained, although min/max constraints could be set before design commences. The reason for this is simple, the battery life (which determines how long your device stays powered) is determined by the battery capacity divided by the average rate of discharge (the amount of energy consumed by the device per time). As such, a 4000MAh battery lasting 6 days in device A, may not complete a one day run in device B.

It is also important to note that there are other factors like; the battery shelf life and temperature that affect battery life, and they must all be factored in before the battery rating is selected.

For battery applications, battery life can be calculated using the equation below:

Battery life = (Battery capacity in milli amps-hour/Total Load current in milli amps-hour) X 0.6

The multiplying factor 0.6 is a random figure to make up for other factors like temperature, etc., that could affect battery life.

So for devices with varying load current due to device switching between different operational modes, the load current for a complete cycle of, say an hour, can be gotten and then inserted into the equation above. The equation will give the number of complete cycles (in hours) that the device will be able to run using that battery.

Easier computation and more robust results can be obtained using online tools like the battery life calculator.

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2. Application suitability

Suitability takes into consideration the usage conditions of the device. While it makes sense to power devices like an Air Conditioner using a plugged-in power source, it will be very absurd to consider the same route for a device like the Fitbit activity tracker. Not the best choice of examples, but I hope it drives home the point. You must be sure the power option being used is the one that provides the best experience for users.

For example, showing an understanding of how tedious having to recharge an activity tracker can be, manufacturers like Fitbit and other smartwatch producers are beginning to seriously consider energy harvesting as a way of powering their devices. This ensures users don't have to plug in the devices every now and then.

3. Availability and life span

While availability is closely related to suitability, it deserves a separate mention due to its importance. The "Spotter" by "Spoondrift" is probably a good example to use in establishing this point. The spotter is a floating device used for collecting ocean wave data. For an IoT-based equipment like the spotter, it only made sense for it to be powered by solar-based energy harvesting since it's not just the most suitable, but it is available in unlimited quantities outdoors. It certainly would have been outright crazy to design for AC mains-based charging or battery replacement.

Another example will be devices designed for developing countries where 24 hours of electricity is not available. In such situations, it will be impractical to design IoT devices to be powered plugged in, especially devices that need to stay on for a long period. A combination of energy harvesting (solar, wind, etc.) and rechargeable batteries may be the most available and suitable option.

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4. Maintenance

One of the main disadvantages of battery-based systems is the battery life and the replacement cycle. In certain applications, the task of replacing batteries could be difficult and costly, especially for field sensors. It is thus important to consider easy access and serviceability before deciding on what power options to go for. Going back to the Spotter example, using a replaceable battery would have created a vast maintenance bottleneck as a team will need to be deployed on the sea whenever battery change is required.

5. Cost

Cost is a factor that should be seriously considered at every stage of product design and development. The cost implication of the power option to be incorporated in your device needs to be seriously considered. It's important to ascertain that customers will be willing to pay more for the additional feature (e.g., longer battery life) that you are adding to the device, as it may have a lot of impact on sales. However, as organizations like Apple have continually proven to us, it will all be about who your target users are and if they can afford the extra cost for a more durable battery or energy harvesting.

While there are certainly more points here and there that should be considered, they all will be related to these 5 points in more ways than one. However, it's essential to remember that selecting a power source for IoT devices is a game of tradeoffs, and all decisions should be weighed carefully to avoid cons that could mess up the product.

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