Power management: power is nothing without control in IoT device design
Battery life is one of the most critical constraints for an IoT device - suboptimal power management design that causes excessive power consumption can stimulate several negative effects. First, and perhaps most damagingly, the lifespan of a relatively low value device can be shortened. This is because replacing or recharging a battery causes human interaction that can be too costly for the business case to bear. In this scenario, complete device replacement is often more cost-effective but this in itself can make the business case hard to justify.
A second effect is the need to charge batteries regularly which, again has a similar negative impact on operating cost. An additional impact for battery devices is deciding to leave out features and functions in order to save power, thereby diminishing the value of the IoT device and limiting its potential value. There are further issues to consider even when devices can be hard-wired to mains electricity because electricity itself can also be a cost that is too great for the business case.
The good news is there are a range of power management techniques that can be adopted at the device design stage to ensure power consumption is minimised. Technologies such as extended discontinuous reception (eDRX) and power saving mode (PSM) help limit consumption by either cutting out unnecessary device communication or sending the device to sleep. The sleep and wake-up mechanism of a system is therefore a critical enabler of device performance and ultimately the success or failure of the entire deployment.
In IoT, many devices have a deployment lifecycle measured in years and it’s therefore vital to extend battery life for as long as possible in order to meet both current and future needs. With LTE, power saving technologies are stable and the same approaches have been used from Category 1 to Category 18. These rely on sleep mode to effectively put the device to sleep in a mode that minimizes power consumption.
Of course, an essential aspect of managing this is to ensure the device can be woken up after it has been put into sleep mode. There are three possible ways to do this at the module: by incoming SMS, by incoming voice call or via incoming data. With USB-powered devices, there is a standard way to suspend and resume wake-up functions, according to optional conditions.
The power management benefits are significant. In an idle state a USB disconnected LTE-FDD PF=64 module could consume around 20 micro-amps, while in sleep state it would consume 2.0 micro-amps. This contrasts with consumption of 10 micro-amps in power down/OFF state. A further advance is possible in the AT+CFUN=0 USB disconnected mode which achieves consumption of 0.85 micro-amps The power management techniques available including for Cat-M and NB-IoT devices were explored in depth with particular reference to LTE modules in the recent Quectel Masterclass ‘Power Management in Modules by Sleep and Wake-Up Mechanism’.