Bluetooth Low Energy (BLE) is the lightweight wireless technology providing a low-power connectivity solution for Internet of Things (IoT) applications. Though protocol architecture is intended to consume low power, every implementation may not provide same performance in terms of power consumption. There are several factors that impact power consumption in a BLE-based design. Tweaking few parameters in connectivity sub-system can reduce power consumption significantly. Most IoT applications are battery-operated, making power consumption optimization an essential part of the system design. The pattern of drawing current, both instantaneous and average, impacts the battery life. Drawing high peak currents can potentially result in less battery utilization than the rated capacity. In this article series we will talk about challenges in battery operated IoT system design and some of the design principles that can help in reducing power consumption to get the best out of batteries.
Powering a device using batteries puts restrictions on how much current can be drawn by the system – not just over the period of time but also instantaneously. Based on the application, different battery types can be employed in the system. A majority of the IoT applications like wireless sensor nodes, beacons, remote controllers etc. employ non-rechargeable (primary) batteries. The main reasons for same are to keep product cost low and because the rechargeable batteries have higher self-discharge rate than the primary batteries. But designing a load for the non-rechargeable batteries is more challenging than the rechargeable ones. One of the main reason is the degradation in output voltage (increase in internal resistance) in non-rechargeable batteries as battery drains and drain current increases.
In this part of the article series, we will talk about commonly used primary batteries in IoT applications, their advantages and limitations, and what impacts their selection in these applications.
Though non-rechargeable batteries are available in several form factors, most common in IoT applications are AA/AAA and coin-cell batteries due to their small size and low cost.
Most common AA/AAA batteries are available in three variants – Zinc carbon, Alkaline, and Lithium batteries.
- Zinc Carbon battery – Zinc Carbon batteries are in use from the beginning of the 20th century. They have the lowest cost amongst three. However, they offer much smaller capacity compared to the other two. Also, their disadvantage is their short shelf life.
- Alkaline battery – Alkaline batteries offer several times higher capacity than Zinc Carbon batteries even at higher drain current. They have longer shelf life compared to Zinc Carbon batteries. But, they are priced higher than Zinc Carbon batteries. Currently, these are the most commonly available and preferred primary batteries in AA/AAA size.
- Lithium battery – These are the best amongst all three in terms of capacity, performance at high-drain current and shelf life. Also, they are lighter in weight compared to the other two. Main disadvantage is their high cost. Lithium batteries are generally available with 3V output. It can help in designing systems to operate using a single cell that would have required two cells if Alkaline or Zinc Carbon battery were used. However, variants are available with 1.5V output as well to allow them to be used be in applications that are designed to worked with one or more 1.5V cells.
Concluding the AA/AAA batteries, Alkaline batteries clearly stand-out against the Zinc Carbon batteries in terms of shelf-life, battery capacity and drain current. However, they are priced a bit higher than the Zinc Carbon batteries. As majority of devices come with a pre-installed (or included in product box), cost of the batteries tend to be an overhead and thus, to keep product cost low, many manufacturers opt to ship product with Zinc Carbon batteries. Also, most consumers don’t understand battery capacity specifications and their performance at different load types. In most cases, while buying batteries, they only care about battery’s output voltage and its dimensions. Now, the question that comes is, what does a designer design for? Even though battery technology has advanced over the period of time, for a good user experience, system needs to run for long time even for older battery types like Zinc Carbon.