Why is battery power key in creating the IoT?

The IoT is a hot topic right now. It promises a multitude of interconnected devices equipped with embedded sensors and intelligent decision making - storage tanks that create an alert when they need filling; household appliances that manage themselves, and bridges that monitor their own structural integrity are all examples. But the IoT concept is still a long way from reality and the underlying technologies are very much in development.

To qualify for the IoT, a device must have its own IP address and, in the industrial IoT realm, many devices will take the form of remote sensors; each made up of the sensor itself, a microprocessor and a transmitter. Some people estimate that there is scope for tens of billions of devices, many of which will be sensors embedded into the fabric of their surroundings, which feed performance data back to central databases for monitoring.

A typical sensor device will draw just a few μA in sleep mode, when it might be waiting for an external cue to take a reading. Alternatively, it might draw around 80μA in standby, running its internal clock between timed sensor readings. Data recording and processing might use 20mA and then transmitting a few tens of bytes of data might call on up to 100mA. Other devices acting as base stations or gateways will receive and relay data transmissions from many terminal devices such as environmental sensors. These applications will require higher currents and more frequent transmissions and so will consume more energy.

With many sensors being located in hard to reach, inhospitable and remote locations, it becomes challenging to provide energy to sensors or terminal devices on the IoT. Mains power is often impractical due to the physical location or installation budget. As a result, many IoT devices will rely on batteries to provide the energy for a lifetime of operation. Replacing a battery on a sensor embedded in a high ceiling might involve extensive scaffolding, a specialist access contractor or downtime of critical equipment, with the cost of the change far outweighing the cost of the battery. So it’s important to select a battery that will deliver a long and reliable life.

Battery selection

When selecting an energy source, engineers need to choose either a primary battery or a rechargeable battery operating in conjunction with some method of harvesting energy from the environment; typically a solar panel. Selection is governed by the energy requirements of the device and its application.

A rule of thumb is that if a device will use more energy than can be supplied from two D-sized primary batteries over a life of ten years, then a rechargeable battery is the most practical choice as it will free up the designer to use relatively high power consumption. And because not all D-sized batteries are created equal, Saft has set this threshold at between 90-120Wh; the energy stored in two LS batteries, which are based on lithium-thionyl chloride (Li-SOCl2) cell chemistry.

Finding the right battery for a new application can be extremely challenging. The power and energy requirements need to be considered against the technical performance of different battery types. Electrochemistry has a strong bearing on how a battery will perform, but other aspects such as the way a cell is constructed are also important to a battery’s performance. Quality of the raw materials used in battery construction also has a major bearing on life, as do the construction techniques used on the production line. And when a battery needs to operate in a potentially extreme environment for a decade or more, the proven reliability of cells becomes a vital consideration.

The ideal primary battery for an IoT sensor has a long life, requires no maintenance, has an extremely low rate of self-discharge and delivers power reliably throughout its life, with little degradation, even towards the end of its life. In addition, because many devices will be located in harsh environments, cells should deliver current reliably in extreme temperatures.

A number of primary lithium cell chemistries are available and of these, Li-SOCl2 is currently the best fit. This is because it has an extremely low rate of self-discharge and a long track record over 30 years, having been widely deployed in applications such as smart metering. Li-SOCl2 cells, such as Saft’s LS series, are available in the well recognised formats from ½ AA through to D, which makes them a direct mechanical replacement for conventional alkaline cells. But it should be noted that the lithium electrochemistry provides a significantly higher nominal cell voltage of 3.6V (against 1.5V). The energy cells are designed specifically for long-term applications of up to 20 years and deliver base currents in the region of a few μA with periodic pulses of up to 400mA. While the power cells can deliver pulses of an order of magnitude greater, up to 4000mA.

An important aspect is the continuity of performance throughout the life of the battery. When powering a child's toy, it doesn't matter so much if performance drops off towards the end of the battery's life but in the IoT predictable high performance can be vital. An IP enabled sensor might need to transmit data that is essential to safety or business continuity; Li-SOCl2 chemistry continues to deliver high performance throughout its life.

LS batteries are already commonly used in sensors and smart meter devices. In October 2013, Saft won two major contracts to supply the batteries to major OEMs in China for installation in gas and water meters. During the life of the meters, the batteries will provide ‘fit and forget’ autonomous power for a minimum 12 year service life. M2M specialist manufacturer Sensile Technologies also uses the same cell type in its SENTS smart telemetry devices for oil and gas storage tanks. The cells power the devices that measure liquid or gas tank levels, record the data and transfer it by SMS or GPRS to a central monitoring system. Over their life of up to 10 years, the SENTS devices provide data on the levels of fuel in tanks. This data allows Sensile Technologies’ customers to optimise their purchasing of fuel and other stored fluids.

Elsewhere, spiral wound LSH batteries have been selected to power telemedicine devices manufactured by SRETT. The batteries provide three years autonomous operation for the T4P device, which is designed to monitor sleep apnea patients’ use of their medical devices and send performance data every 15 minutes via GPRS communication, where it can be monitored by medical professionals.

Other primary lithium battery types may also have a role to play, particularly in applications that demand high pulses of energy. An example might be to provide relatively high power for the ‘ping’ of a corrosion sensor on a remote oil pipeline that uses an ultrasonic pulse to determine the thickness of pipework.

Rechargeable batteries

Li-ion batteries make a practical choice for IoT applications that call for rechargeable batteries because of their high cycling life and reliability in extreme temperatures. There are several types of Li-ion cell chemistry, which can be blended or used individually. Of these, Nickel Manganese Cobalt (NMC) is particularly interesting because it operates reliably across the widest temperature range.

Using NMC of Saft’s own formulation, Saft can deliver a rechargeable cell that operates at temperatures between -30 and +80°C, which means that it can provide reliable power for devices installed anywhere from an arctic blizzard to a pipeline running through a desert or integrated into equipment in an engine room. And while some Li-ion technologies suffer degradation if left on float charge (for example, consumer device batteries may degrade if left to charge continuously or for long periods), Saft NMC does not. This means that it can be paired with a solar panel and left to charge day after day without losing performance - an advantage that could represent cost savings for a sensor’s operator.

A typical application is AIA who are utilising a Saft rechargeable Li-ion battery in their Solar Battery product line for Class 1 Div 2 hazardous environments and extreme temperature operation. With a Li-ion battery operating in combination with a solar panel, AIA’s Solar Battery solutions provide power, data acquisition and wireless internet communication to simplify installation, maintenance and support of remote hazardous environment sensors. The AIA system allows customers such as Taqa North to achieve end-to-end monitoring and control of their fixed or mobile assets in extreme climatic conditions.

While IoT devices are not yet commonplace, the battery technologies are already available to power them effectively and reliably, thanks to the extensive field experience built up in comparable applications such as wireless sensor networks, machine to machine applications and smart metering. Ultimately, selecting the right battery depends on having a solid understanding of the base load and pulse current that a sensor, terminal device or gateway will draw. Designers can play an important role by optimising their application, ideally keeping the size and frequency of data transmissions to a minimum. In most cases, only a handful of bytes need to be transmitted, and existing battery technology can handle this comfortably to deliver upwards of ten years of reliability in even the most demanding environments.