Battery replacement challenges

Component suppliers and wireless sensor companies often estimate a  battery lifetime of 5-10 years for their devices. However, actual deployments and experiences with batteries have left many potential users skeptical of theoretical battery-life calculations. Using disposable, primary batteries for wireless sensors involves two major challenges: maintaining sensors in hard-to-service locations and scaling a sensor network to hundreds or thousands of nodes. These challenges result in significant maintenance costs to replace batteries, growing concerns over the consumption of raw materials to produce batteries, and the continued environmental issue of battery disposal in landfills.

Hard-to service locations

Hard-to-service locations can add significant cost to battery replacement in wireless sensors. Costs for changing batteries in wireless sensors have been estimated to range from $10 per node for easily accessible nodes up to $100 per node.

In large-scale commercial deployments, users may want sensors to be located above ceilings, behind walls, inside sensitive or hazardous environments, and on the exterior of structures. Replacing batteries is simply not a practical or cost-effective solution for these applications.

Scaling the sensor network

Scaling sensor networks with power supplies that have limited life is an even greater challenge. To ensure performance, users must either replace batteries on a regular schedule, which would waste battery life, or accept the risk of changing batteries only when a low-voltage threshold is indicated. This might work for tens to a few hundred nodes, but consider the potential maintenance cost of replacing batteries for several hundreds or thousands of nodes: battery replacement would essentially become a weekly or daily activity.

ON World estimates the labor cost for changing batteries in wireless sensors will be greater than $1 billion during the next several years. The undesired maintenance task of battery replacement is a significant force acting against the growth of wireless sensor networks.

RF energy harvesting

Energy harvesting holds great promise for solving the problem of limited battery life and subsequent replacement. The wireless sensor community has had many discussions about solar, vibration, and thermal energy harvesting solutions. While these technologies can harvest useful energy, they share the common problem of being reliant on ambient sources generally beyond their control. Solar requires light, vibration requires motion, and thermal requires a heat source.

A wireless power solution based on RF energy harvesting overcomes this lack of controllability because power can be replenished when desired. The impact for wireless sensors is profound. Instead of design and operational constraints for maximizing battery life, devices can be recharged with energy repeatedly and perpetually, enabling greater functionality and more frequent use. Energy storage can be rechargeable batteries, solid-state/thin-film devices, or capacitors, and the recharge cycle can be continuous, intermittent, or on-demand, depending on the application.

RF energy harvesting works by converting radio waves into electricity. This is done by receiving the radio waves at an antenna and rectifying the signal. Conversion efficiency for RF into DC has historically been low and requires tuning the circuitry to match the load resistance of a specific device for optimal performance.  These problems can be resolved with circuits that provide high conversion efficiency across a wide band and for varying load profiles. RF harvesting circuitry (Figure 1) can be coupled with low power components to optimize charge management for batteries and other forms of energy storage.

Figure 1: A Powercast Powerharvester module can optimize charge management for batteries and other energy storage devices.

Wireless sensors with short duty cycles and long sleep cycles can spend most of their battery energy sleeping. This type of scenario aligns well with using small form factor energy storage devices and resupplying the power wirelessly on a scheduled basis (usually daily or weekly) or on-demand when a sensor reading is needed. In some applications, the sleep state can essentially be replaced by the sensor being inactive, requiring even less average power.

Widespread wireless power coverage can be created using a single or multiple power transmitters (Figure 2). The power from multiple transmitters is additive and produces a more even power footprint throughout the desired coverage area. Facility managers can think in terms of wireless power infrastructure and provide coverage for an entire facility similar to lighting or Wi-Fi. This infrastructure can power thousands of devices and enable mobility within the coverage area.

Figure 2: A wireless power distribution system can provide coverage for an entire facility similar to lighting or Wi-Fi. (click graphic to zoom by 1.4x)

Wireless power sensors at work

In one recent application, wireless power technology based on RF energy harvesting was used to recharge batteries for a wireless sensor operating in a perpetually cold environment – the penguin exhibit at the Pittsburgh Zoo. The batteries in existing sensors needed to be replaced every 3-4 months, an unacceptably short time frame. A sensor with four AAA primary batteries was retrofitted with a receiving antenna, a Powercast Powerharvester module, and two rechargeable alkaline AAA batteries. The results from a battery-only node and the Powercast-enabled node are shown in Figure 3.

Figure 3: A performance comparison of a battery-only node versus a wireless recharging node shows  the advantages of RF energy harvesting. (click graphic to zoom by 1.9x)

As illustrated in Figure 3, the battery-only node lost power between 90-120 days, while the Powercast-enabled node maintained a near-constant voltage with no need to replace the batteries. The average power consumption for the sensor was about 1 milliwatt. Powercast has also powered wireless sensor nodes without batteries at a distance of 85 feet from a single transmitter with a one-second sleep period. The operating distance can be even greater for applications that require less frequent data transmissions.

Sidebar 1: Evaluating the concept (click graphic to zoom)

Making more predictable power

The ability to eliminate batteries from sensors using energy harvesting is proven. Wireless power enabled by RF energy harvesting can not only be used to recharge or replace batteries, but also to provide a more predictable power supply for wireless sensors and other micropower systems than uncontrollable, ambient energy harvesting. Additionally, wireless power enables these devices to be completely untethered and mobile.

Harry Ostaffe is director of marketing at Powercast Corporation, based in Pittsburgh. He has 20 years of experience in the fields of broadband and wireless networking, industrial controls, and computing and has prior experience with Ericsson, Marconi, Lucent Technologies, AT&T Network Systems, Bayer, and IBM. Harry holds an MBA from Carnegie Mellon University and a BS in Electrical Engineering from Penn State University.

Powercast Corporation
800-963-6538
hostaffe@powercastco.com
www.powercastco.com