Batteries Not Included
James Holmes, Head of Electronics & Software, explores the issues surrounding battery usage within industry and the alternatives available.
The proliferation of connectivity into the world around us continues to accelerate with the IoT market set to grow to $457Bn by 2020. In industrial applications, this connectivity makes it possible for devices to remotely monitor anything from air quality to machine performance delivering valuable data in real-time. Retrieving data from these devices however still comes with a cost. Wired sensors require costly and sometimes complex installation of cabling. Wireless sensors have removed the need for cabling but still require a power source, this typically means batteries and the on-going maintenance cost of their replacement or charging. Across 10,000 installed sensors nodes, replacing batteries once a year could cost upwards of £40k annually in labour, transport, overheads and part costs making a battery-free design highly attractive.
In other, more rugged applications, batteries can present additional headaches when trying to meet safety requirements. For example, fundamental limitations in lithium battery chemistry place strict restrictions on the operating environment and temperature of a lithium battery powered product. In more hazardous environments such as those with potentially explosive atmospheres, the ATEX Directive means batteries must be very carefully selected and incorporated to avoid any risk of explosion.
However, with the power consumption of portable electronics continuing to fall, it is becoming possible to operate an increasing range of low power devices without the need for batteries at all, instead relying on energy harvested from other sources.
Solar technology has long been established as a means of powering small electronic devices and as device power consumption has dropped, the efficiency of solar cells has also increased. Related developments such as increasingly sophisticated DC-DC converter controllers which perform Maximum Power Point Tracking (MPPT) mean that small solar cells are now a practical means of harvesting sufficient energy to power simple sensors and radios even in indoor environments.
Dye Sensitized Solar Cells (DSSCs ), such as those produced by GCell are well suited to indoor applications and offer an interesting further advantage in that unlike batteries, they are thin and flexible. DSSC can enable completely wireless sensor-based products in indoor environments whilst also enabling exciting new form factors such as flexible or wearable electronics. This technology could be combined with sensors to remotely monitor air quality in public spaces and control air circulation or filtration systems without ever requiring replacement batteries.
Thermo-electric generators (TEGs) exploit differences in temperatures across a generator element to convert heat into small electric currents. Whilst typically a more expensive solution than solar cells, when combined with MPPT technology, thermo-electric generators can deliver sufficient energy for a range of sensor solutions. Such technology is ideally suited to sensors or other low power devices which harvest energy from waste heat from machinery or pipework. For example, a sensor could be attached to a remote and inaccessible heating system, indefinitely harvesting energy and reporting performance information, alerting maintenance staff if a performance defect is detected.
Short range transfer of energy between electro-magnetically coupled devices has become commonplace and is most widespread in the wire-less charging of phones. Whilst mobile phone chargers typically operate over distances of a few millimetres, they also require thousands of times more current than many low power sensors. As a result, it is quite practical to harvest small electric current from electro-magnetic fields even at distances of 1 meter or more. For shorter range applications, NFC (a subset of RFID technology) is supported by almost every recent mobile phone. It is possible to harvest sufficient energy from a smartphone or other RFID source to operate without an internal power source. As a result, some devices which would otherwise rely on lithium batteries and Bluetooth radios could instead communicate wirelessly via RFID and harvest the energy they need in the same process.
Modern batteries have revolutionised portable electronic devices and remain a solid choice for powering a wide range of connected devices. Batteries aren’t without their disadvantages though, and for remote devices in particular, the maintenance burden of replacing batteries in the field can become a significant cost over the product lifespan. For low power devices, battery-less designs are becoming an increasingly practical alternative which is opening up opportunities for entirely new types of devices.