The advantages of batteryless power

Frank Schmidt, CTO and co-founder, EnOcean


Moore's law does not apply to batteries

Many fields of research focus on achieving a combination of more powerful batteries and devices that consume less and less energy. However, Moore's law does not apply to batteries.

The performance and component density of processors and memory chips double approximately every 18 months. Gordon E. Moore predicted this trend back in 1965, and it continues to this day. Since that time, improvements have been made by an impressive factor of 10,000. Unfortunately, this does not apply to batteries. Despite intensive research, their performance has improved by a factor of only three over the same period of time, and experts do not expect to see any quantum leaps forward.  

In the meantime, the individual components of electronic devices have significantly improved in terms of power consumption. However, more and more powerful functions have increased the products' hunger for energy, and this development has canceled out all energy-efficiency effects.

Combined benefits

Nevertheless, battery-powered devices that have a lifetime of a few years seem to be an attractive option for wireless products to make a consumer’s life easier. Batteries are cheap to buy, that’s true, but changing them isn’t. There are many hidden costs with regards to maintenance and disposal.

Particularly in the field of building automation, OEMs should consider a batteryless approach for their wireless products as a strong competitive advantage. Energy harvesting solutions free building owners, facility managers or contractors from the burden of batteries. They combine the benefits of highly flexible wireless solutions with the same maintenance-free attributes as wired-in devices at a fast and sustainable return on investment (ROI). 

Challenge of change

Today, wireless solutions are very popular for building automation systems. They can be flexibly placed and eliminate the need to install wiring and conduits. This significantly eases and reduces the planning and implementation effort.

However, there is the challenge of powering the devices. A larger system can comprise hundreds to thousands of these sensor devices all requiring power and communication capability. Battery-powered wireless devices can prove to be a drawback. Batteries last for only a limited time, depending upon the application, and must therefore be replaced regularly and disposed of properly.

This can be a challenge, even in a consumer home. Smoke detectors, for example, can save lives and there are major public service campaigns that remind consumers to change smoke detector batteries. Nevertheless, one in five homes has dead battery smoke detectors. The National Fire Protection Association, NFPA, estimates that 20% of U.S. homes have smoke alarms present but none that are working. Nearly all of this 20% involves dead or missing batteries.

Monitoring battery health

If people don’t even change the batteries for devices that can save their lives, they certainly won’t do it for a temperature sensor. Furthermore, there are often different types of batteries required, and each device has a different battery access method. In a larger system, the detection of the sensor’s position spans across several floors and offices and the devices are mounted obtrusively at places that are difficult to reach, e.g. on or above drop ceilings. Consequently, employing batteries means implementing a system to monitor battery health and determine when to change. 

There are typically three methods of battery maintenance:

1.         Build in a low battery monitor and alarm – such as the beeping smoke detector. Security systems have a supervisor signal every hour to inform the base station they are alive.   

2.         Change all batteries on a preset timetable, usually at the shorter end of their expected lifespan.

3.         Wait for the battery to fail, and then debug the failure as a dead battery. This presents some problems if the user is unaware that the device is battery-powered, e.g. thermostat or light switches that are typically not battery-powered. The consumer only knows that the heat does not come on or the light is out.

Environmental impact

When considering battery costs, the environmental factor needs to be included, too. Batteries contain heavy metals such as mercury, lead, cadmium, and nickel, which are detrimental to the environment. At the end of their lifetime, batteries are hazardous waste due to the toxicity (chromium D007), ignitability (D001) and reactivity (D003) and need to be carefully and expensively disposed of by the manufacturer or the user.

Depending on the battery technology in use, a user needs to dispose of between 200 and 1,600 batteries over 20 years in a residential home (50 nodes). A large system, for example in an office, comprising 10,000 wireless units, each powered by two batteries with a lifetime of two years, could require the facility manager to change approximately 30 batteries each day.  

Even the storage of batteries applies high standards and can be a real challenge. Randomly stacked batteries on top of one another can burst or generate heat.

Energy from surrounding sources

All of the negative environmental impacts and challenges related to battery maintenance, storage, and disposal are eliminated by energy harvesting technology. Here, the wireless modules gain their power from the surrounding environment and therefore work without batteries (see Figure 1). There are a variety of sources, an electro-dynamic energy converter uses mechanical motion, or a miniaturized solar module generates energy from indoor light. Combining a thermoelectric converter with a DC/DC converter taps heat as an energy source (see Figure 2).

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Figure 1: OEM design engineers can choose from different types of energy converters to harvest energy from motion, light and differences in temperatures

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Figure 2: The ECT 310 Perpetuum is a low-cost ultra-low-voltage DC/DC converter for powering battery-less EnOcean radio modules by thermal energy

In general, traditional battery-powered technology consumes ten times the amount of energy as self-powered devices. However, due to an optimized energy management, energy converters are unexpectedly powerful. Inside buildings, with eight hours of average light intensity (400 lux), an economical mini-solar cell with an efficiency of less than 5% and an area of 10 cm2 supplies approximately one ampere-hour (Ah) of energy over the course of 15 years – the same amount as five CR 2032 Li button cells. Outdoors (8000 lux), this intensity is as much as 20 Ah over the same period of time, which equals the power of more than 100 Li button cells. While in this typical example users must change the batteries of battery-operated devices every three years and every two months, respectively, energy harvesting makes equipment truly maintenance-free.

No maintenance for 20 years

Batteryless sensors and switches free users from maintenance effort and costs for 20 years. This “no maintenance” characteristic significantly reduces the TOC (total ownership cost) over the system’s whole lifecycle. It enables wireless components to be installed and forgotten just like wired-in controls, also at places that are difficult to reach.

The elimination of battery access doors also simplifies embedding the wireless sensors and switches in door locks, window panes or cable locks while the energy harvesting modules allow smaller device form factors and environmentally sealed enclosures at the same time. That way, OEMs can realize wireless sensors for harsh environments.

Energy harvesting offers significant advantages over batteries when it comes to ecobalance and maintenance-free operation. Based on the batteryless wireless technology, a wide range of self-powered applications are available today, including batteryless switches, intelligent window handles, temperature, humidity and light sensors, as well as occupancy sensors, relay receivers, heating valves, control centers and smart home systems. For all of these devices, the self-powered technology can substitute the battery (see Figure 3). For more power-hungry devices such as programmable thermostats with LCD panel and backlight, the energy-efficient radio, optimized for ultra-low power applications, can extend battery lifetime significantly over traditional radios. 

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Figure 3: The ECO 200 is an energy converter for linear motion

Technology with a future

Batteries will never disappear and for some applications they will remain a necessity. But from a design, environmental and reliability standpoint, energy harvesting is the technology with a future. In the years to come, energy harvesting will increase its lead over batteries even more – in particular, since energy converters continue to improve their performance.

This will solve one of the major challenges of an Internet of Things, where 10 trillion wireless sensors are expected to deliver the needed data: 10 trillion battery-powered sensors would require 1 million tones of Lithium – the combined worldwide Lithium production of 10 years. Based on a 10 year average battery life time, 10 million maintenance workers would be requested to change batteries, each of them 100,000 per year.