Batteries

Introduction

Many IoT devices can be powered from fixed supply such a mains-derived source, or a vehicle electrical system. However, a key development in IOT has been the reduction in power requirements and technology advances, which has enabled use of batteries to reduce wiring costs to supply power. This is dramatically increasing the range of things that can be monitored and controlled. 

There are two main classes of batteries used in IOT - primary non-rechargeable batteries or secondary rechargeable batteries.

Primary batteries

Non-rechargeable batteries that are designed to last for 10 or 15 years are expensive, in the order of AU$10 or AU$20 for AA cells or a C Cell. Example technologies are a LiPolymer and LiFe. LiFe batteries perform quite well in much greater thermal extremes.

Secondary batteries

Rechargeable batteries are often used in remote applications, typically solar powered.

The main rechargeable battery technology is Lithium polymer and Lithium Ion. Lithium battery technology has encountered problems in some high power applications, where malfunctions of other kinds of products have made the news. However, the main issue for IoT has been in the supply of such batteries because Lithium batteries over a certain size have been banned from air freight. This can impact on the manufacture and maintenance of IoT devices.

A safer alternative to Lithium polymer with a longer cycle life and constant discharge voltage are lithium iron phosphate (LiFePO₄) cells.

Engineering challenges:

Lithium polymer which make up the bulk of rechargeable batteries, such as those found in supermarkets, technology, are very cheap, in the order of a few dollars. However, batteries of the same technology designed to last 10 or 20 years can cost $20 to $30.  

The choice of a battery technology which suits the environment is important. In designing IoT systems, a trade-off exists between battery cost and the cost of more frequent battery replacement.  In applications where it is difficult to access the device, or the labor cost of changing the batter is prohibitive, a good quality primary battery makes sense. Where devices are visited regularly for other purposes, a cheaper rechargeable battery technology may be more appropriate.

One particular challenge for IoT applications is in outdoor environments exposed to direct sunlight. The inside of device enclosures can get very hot in such circumstances, in the order of 70 degrees Celsius. Using the batteries during these conditions can significantly degrade their performance and life expectancy. This needs to be taken into account when designing the system and the enclosure. 

In some applications, it is possible that a battery is not needed at all and energy harvesting options can be explored many of which use alternatives such as super capacitors.