THE INTERNET of THINGS FEATURE
Extending battery lifetime in remote IoT applications
Andrew Leake, managing director, at Dengrove Electronic Components demonstrates the latest smart solutions, which significantly prolong battery lifetime of an existing wireless IoT sensor solution, offering the possibility of a maintenance-free application lifetime
M
any IoT applications will be remote, meaning that they will not have
access to grid power. Therefore remote energy sources such as renewable energy or batteries must be considered; and these are in need of alternatives to running additional cables to supply sensor applications, creating a demand for components that make remote energy solutions feasible to modern sensors. In the year 2020 over 30 billion smart sensors are expected to be sold - more than double the number sold in 2015. New versatile and efficient electronic
components, like the R-78S from Recom open the possibility to use energy more efficiently in IoT applications, therefore prolonging the lifetime of these sensor applications and also allowing multiple and intelligent sensors to be considered. Battery-powered IoT applications need to be optimised for efficiency. One way of achieving this is operating the applications in the lowest possible load range. The R-78S switching regulator module has been optimised for an impressive efficiency of 93% under full load. To meet the specific requirements of IoT applications, the efficiency is still above 80% in the crucial light load range of around 10%.
COIN CELL LIMITATIONS One case where this device prolongs the lifetime of an IoT application is with the flagship product of SensiEDGE - the SensiBLE v1.0 module. This sensor board offers functions such as 3-axis accelerometer, 3-axis magnetometer, 3- axis digital gyroscope, pressure, micro- phone, relative humidity, ambient light and temperature sensors. However, one downside of the module is that it’s powered from a coin cell battery. This is fine for most cases, but in more power hungry applications, a coin cell can be quickly drained, and the battery for these remote applications must be changed frequently. Button and coin cells are used, because the voltage is above the threshold of the sensors and microprocessor in the application. The voltage just needs to be
Figure 1:
The SensiBLE sensor board connects to the R- 78S evaluation module through a fitting
breakout board, allowing battery lifetime calculations
lower input voltage from 3.15V to as low as 0.65V. Applications such as microcontrollers, WLAN, Bluetooth, LoRaWAN modules or IoT devices can now be supplied from a single 1.5 V cell. When connecting a buffer capacitor
between GND and CTRL, the R-78S also can be put into sleep mode, where it is then powered by its own output so that it only consumes 7µA from the buffer capacitor. After a certain discharge time, the voltage of the capacitor drops below a predefined threshold, waking up the application yet again. When the application is in sleep mode for too long, a watchdog function wakes it up, stopping the voltage of the capacitor from decreasing further and preventing the shutdown of the microprocessor.
regulated down to an appropriate level, and the circuit works fine. However, as a battery’s energy is depleted, its voltage drops. But once the battery voltage drops below 2V, the SensiBLE stops working, meaning that the last-gasp of energy from the coin cell battery is wasted. A single cell AA battery has as much as 10x the storage capacity as a standard coin cell. But a single cell battery can only deliver 1.5V of energy, which is too low to operate most micro processors. The typical solution is to either stick with the coin cell batteries and sacrifice energy capacity or to put two or even three single cell batteries in series to get the voltage high enough to power the application. But since the trend is to shrink the size of applications, using 2 or 3 single cell batteries is simply too bulky. Recom's R-78S is a highly efficient
switching regulator, specifically designed for these single-cell powered IoT applications. Due to its wide input voltage range, high efficiency and low stand-by losses, it extends the working lifetime of the cell. Based on the simple plug-and- play concept of the standard R-78 buck regulator, this boost regulator is able to provide a stable 3.3VDC output from a
/ ELECTRONICS Figure 2:
This circuit diagram shows the basic function of the R-78S module to supply a 3.3V application from a 1.5V cell
10 YEARS BATTERY LIFE GOAL In certain applications the microprocessor could also be triggered by an external “alarm” signal, waking the application up from sleep mode. When the application is active it draws about 200µA, which allows the capacitor to be ready again for sleep mode; thus saving valuable battery life. The size of the capacitor has to be scaled according to the desired minimum sleep mode time and depending on the application, the battery life can be prolonged drastically. The R-78S Evaluation Board
demonstrates how the R-78S and the SensiBLE can be connected through a fitting breakout board. It is equipped with several jumper headers to measure input and output current and voltage - allowing typical battery lifetime calculations in both active and standby mode. You can also use external input voltage from other low voltage sources such as energy harvesting devices, which are typical in remote applications. This offers the possibility to calculate the functionality and lifetime of the application based on a single AA battery power supply.
Dengrove Electronic Components
www.dengrove.com 01525 237731
ELECTRONICS | DECEMBER/JANUARY 2019 21
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