Battery Size Matters

Introduction

The Internet of Things (IoT) is upon us. Gartner recently reported a typical family home may contain more than 500 smart devices by 2022 (Gartner news, Sep 8, 2015). The promise is that every day home automation products will be smarter through the introduction of low-­cost IoT technology including wireless transceivers and innovative sensors. But these features take energy, and a majority of these devices will be powered by batteries.This presents an interesting challenge. To successfully design a product for the connected home, designers must generally build a small form-factor product, with the cheapest bill of materials (BOM), and reliable operation over several years without battery replacement.

1. Different Markets, Different Requirements

Before you can begin designing it is important to understand the target market, as each present different requirements on cost, reliability, and battery life. Below are two extreme examples of how market requirements force designers to make trade-offs.Subscription-based Service ProvidersA subscription-based service provider (e.g., cable or satellite internet service) places greater importance on long-term, reliable operation than achieving the lowest BOM cost or attaining the smallest form-factor.These companies weigh product cost versus the cost of sending a technician on-site for trouble shooting, which costs between $200 and $2000 per trip, according to variuos sources. Of course, if the issue is merely a drained battery, the cost trade-off is obviously to put a bigger battery or a lower-power design in place.Do-it-Yourself Home AutomationWhile this segment values reliability, these consumers often select products based on cost, size and appearance. This market is where batteries are typically small to accommodate an aesthetically pleasing enclosure. Additionally, the product cost must be low to achieve a low shelf price. If the do-it-yourselfer gets the product home only to have it fail, the replacement cost of a defective consumer product is much lower than the professional service provider market; therefore, product designers are willing to make these kinds of trade-offs.

2. Battery Efficiency and Wireless - Not Always an Obvious Choice

When a product needs wireless connectivity, the choice of which protocol to use will be a major factor affecting battery life. There are several wireless choices to consider, some of which may be preferred or even required by the target market.Wi-Fi is a commonly used protocol in consumer wireless products. Wi-Fi accommodates large streams of data at high throughput rates and is more power-hungry compared to other wireless protocols. Wi-Fi products are often plugged into power outlets or frequently recharged as they are not optimal for a product running off a battery compared to products that use very low throughput rates.For example, most home automation sensors such as magnetic door and window sensors and passive infrared (PIR) motion detectors, are static for long periods of time and would not benefit from large data stream Wi-Fi offers. These sensors can utilize battery-friendly wireless standard such as Bluetooth® Smart (also known as "BLE") or ZigBee®. Bluetooth Smart targets direct point-to-point communication, while ZigBee targets multi-node mesh-networking.As a point-to-point technology, Bluetooth Smart or "BLE" might be best suited for querying a smart device in the home such as a door lock. For mesh networking, ZigBee might be best suited for multiple smart devices such as thermostats, door sensors and even window shades that can be configured to communicate autonomously with very little user interaction. A mesh network also has built-in redundancy that can eliminate single points of failure, making it more reliable than point-to-point technology.