The biggest growth in connected things is currently taking place in the area of smart metering. These include billions of minimalist sensors that keep an eye on electricity, water and gas, sit on shipping containers and in medical devices and form the 'living' part of the Internet of Things. The IoT only works thanks to these sensors, which collect information from their surroundings, process it and send it to the receiving device. As these sensors only work with electricity, every development in the IoT requires a reliable source of energy.

Although a single sensor only consumes a few µA of energy in idle mode, the actual measurement, processing and transmission of the data requires a lot more: Data must be compiled, analyzed and ultimately disseminated in order to perform tasks such as controlling irrigation systems. Where does the energy come from?

The weakest link in the IoT chain

The suppliers of the required energy are batteries. According to analysts such as IDC and Gartner, up to 80 billion 'things' are expected to populate the earth by 2020 - which will then require billions of batteries that need to be purchased, maintained and disposed of. However, batteries need to be recharged regularly or replaced directly, which leads to maintenance problems. This makes the energy supply the weakest link in the IoT chain: it is both the foundation and the biggest challenge in the Internet of Things.

Electricity in every form - from button cells to rechargeable batteries.

© Duracell

There are several ways to tackle this weak point. One approach is to replace all batteries with an alternative energy supply. This would involve converting energy from the environment. Solar cells, piezoelectric and thermoelectric elements can be used to generate electricity from light, vibration and heat. Although the performance of generator elements has increased from year to year, it is still considered almost impossible to supply an IoT device with sufficient power in this way. Even the intermediate connection of a capacitor, in which the generated electricity is collected, is not sufficient to use this form of energy generation exclusively.

A second possibility to replace the battery is to be offered by the wireless technology itself. In this case, for example, researchers at the University of Washington are working on a solution to use the energy from existing radio, TV and radio signals. The technology used is called 'WiFi backscatter'. It describes a communication mechanism that enables radio frequency-powered devices to transmit data by reflecting or not reflecting the WiFi signal of a router. For this purpose, the sensor to be networked is equipped with an antenna that can modulate the intensity of an existing Wi-Fi signal. It reflects the Wi-Fi waves, but changes them minimally. At less than 10 µW, the power required for this is so low that it can be drawn directly from the electromagnetic field of the Wi-Fi. In this way, low-energy devices such as sensors and wearables can go online without cables or batteries. However, with transmission rates of up to 1 Kbit/s and a radius of 2.1 m, the power and range of the backscatter are not yet high enough to be used as the sole energy source.

Batteries as energy suppliers in the IoT

To date, the alternatives for batteries as energy suppliers in the IoT have therefore proved to be immature overall. What's more, they often involve additional mechanical parts that can cause faults. They increase the maintenance effort, especially for hard-to-reach devices. For the energy supply in the IoT, it is therefore a matter of reducing the energy consumption of applications, improving conventional batteries and continuing to use them. To eliminate their weaknesses, the battery must be made more efficient in order to make better use of the available energy.

The chemical composition of the batteries is responsible for achieving this goal, as it influences the area of application and the overall design of the circuit. Some compositions provide long-term energy storage, but are severely affected by load peaks. This problem can be mitigated by interposing a supercapacitor as a buffer between the battery and the circuit. Other compositions can provide sudden bursts of energy to enable long range radio transmissions. However, they only offer a short storage period and therefore reduce the efficient performance of the sensor node.

Based on their chemical basis, batteries can be broadly divided into two categories as they are either zinc or lithium compositions.

One example from the zinc family is the zinc-air battery. It has a high energy density of almost 1.7 MJ/kg, but discharges itself very quickly. It can therefore only be used for a few months.

Alkaline batteries, based on a combination of manganese dioxide and zinc powder, are another option. This composition is widely used for applications with low duty cycles. The nominal voltage of a single cell is 1.5 V and falls below 0.9 V shortly before full discharge.
Other zinc-based battery types are usually only suitable for use in the IoT environment to a very limited extent due to their low energy density, high self-discharge and low power.

The second category - lithium-based batteries - is usually used in the IoT as button cells. All battery types in this category use lithium for the anode, but differ in the filling of the cathode and the technical properties. The two types of lithium are referred to as 'BR' and 'CR'. BR cells usually consist of a carbon monofluoride gel and a lithium alloy. They offer a very low self-discharge rate and are used for applications such as measuring systems that combine very long maintenance intervals with relatively low power requirements. The nominal voltage is 3 V, dropping to 2.2 V shortly before discharge. Lithium CR cells are available in various capacities - from 50 mAh to over 500 mAh.

The CR form of the lithium cell uses manganese dioxide as the cathode material instead of carbon monofluoride. Manganese dioxide helps to reduce the internal impedance of the battery. A lithium battery with a mixture of heat-treated electrolytic manganese dioxide and conductive agents as the cathode, for example, is characterized by its particular thermodynamic stability, which ensures high reliability and performance even after long periods of storage.

Such lithium manganese dioxide batteries are used in demanding environments and could therefore serve as a power supply for the Internet of Things. They offer a high pulse current load capacity, high energy density, high temperature resistance and long shelf life. Due to their special spiral cell design, their performance is particularly high, as the spiral winding increases the anode surface area and thus their energy output.

Lithium-ion batteries are a practical choice for hubs and gateway devices or sensors that have both high energy and performance requirements. They can withstand many charging cycles and work reliably even at extreme temperatures. Several types of these rechargeable batteries exist - compositions of nickel, manganese and cobalt (NMC) promise reliable operation over large temperature differences; they work between -40 and +85 °C.

Author:
Anna Dahlke is responsible for press relations at Duracell Germany.