Nov 21, 2023

10 min read

Batteryless Electronics: Starting Cold and Sleeping Deep

While we've discussed batteryless and self-powered electronic devices before, have you ever wondered what it really takes to bring them to life? In this article, we will take a peek at two key performance parameters within the ultra-low power electronics world - cold start and sleep power - and how important they are to nail true batteryless products.

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Imagine you’re back home from a well-deserved week's vacation. You’re a cautious fellow, so left all systems down before leaving the house: curtains closed, lights off, Wi-Fi router unplugged, because, hey, even internet routers need a break right? Now you’ve just turned systems on, turned your back on the unpacked bags and, you’re ready to melt into your sofa for a well deserved TV-binge. But then, plot twist! Your trusty remote control decides it’s its turn for a vacation - dead as a doornail! Humm… this is the moment you kind of start regretting having fallen for the whole tale of ditching batteries for the sake of Mother Earth! 

This is also the worst nightmare of your TV OEM… Yes, they’ve been dreaming of being crowned the greenest of the green in the whole tech-universe. Yet, let’s face it, those pesky little triple A’s jumping out of your remote and into your trash bin every couple of years are not helping their cause. But, above all, they want to keep customers happy! Tough call, right?

In their quest for sustainability and a green profile, major brands have been growing an appetite for energy harvesting options. Hence, it is nowadays possible to find remote controls powered by photovoltaics (PV) or radio frequency (RF) waves harvesting. Yet, when you look closely, most will still have a rechargeable battery and backup options, like a USB-C port for fast charging. One of the main reasons is the sort of corner cases as described in our intro, i.e. what happens when the remote stays in complete darkness and/or away from RF sources for long, completely draining all its energy reserve.

Mastering Cold Start

Starting or restarting an electronic system from zero energy available (power off) is what electronic engineers usually call “cold start”. The power necessary to kick start the system is generally called “cold start power“ or “startup power”. If you’re running your electronics from a battery, this is not a major concern - the comfort of a constant energy source is granted (at least until it’s time for the next battery change or recharge!). But if you’re relying only on energy harvested from the ambient, then you want your cold start to be as economic in power as possible. In addition, you need your system to live with a relatively small energy buffer - as the time to charge an energy storage element, say a capacitor, determines the cold start time. Without proper attention to these you run into the dead remote control situation: if your PV-powered remote hasn’t seen light for a while, because all lights are off or it’s been forgotten under a pillow, it will simply not work before you expose it to light for tens of minutes. This is simply not acceptable for most users - you don’t want to be light-bathing your remote to make it start! 

Basically, you want your remote ready in a couple of seconds, or even milliseconds, after you rescue it from under that pillow! For this to be possible from energy harvesting, the electronics in such remote need to be prepared for extremely low cold start power and energy buffer at any time and condition. For instance, we need to make sure we’re getting power from the PV panel extremely quickly once it’s seen any bit of light and that we’re filling up quickly our energy buffer element -  while obviously making extremely efficient use of such energy to run the electronics.

From Remote Controls to Electronic Shelf Labels

Problems with cold start are not exclusive to remote controls. We’ve been hearing similar complaints across many other applications and use cases. Let’s look into another favourite in energy harvesting and indoor PV communities: ESLs, short for Electronic Shelf Labels

When deploying tens of thousands of ESLs in a store, retailers do not want to be kept waiting for the devices to wake up from their zero-energy beauty sleep. Once they pop out of the box, they should be up and running in a few seconds. Also, retailers want their ESLs to be active virtually at all times, being able to push updates at any time of the day. In fact, early morning updates on pricing and/or product information on the ESL screens are common. Hence, quickly after the lights in the store are turned on, ESLs shall be fired up and ready to display that new update. Even those ones on the bottom shelf taking the dimmest light.

In turn, retailers do not like big PV panels messing up the arrangement of their shelves. Hence, PV-powered ESLs must respect the design and sizing that the industry is used to. This means very small panels seamlessly integrated into the ESL casing/frame, and the consequent need for rigorous use of the little energy these will feed into the ESL system.

All things considered, just like the remote controls, self-powered ESLs must be able to wake up fast from very little amounts of harvested energy - i.e. extremely efficient cold start is a must!

The list could go on and on - from all kinds of smart home gadgets and building automation sensors to asset tracking and smart labelling devices. In all these, very efficient cold start is a keystone for batteryless operation.

We have recently heard from an emerging player in the energy harvesting ecosystem stories about unboxing experiences where devices take tens of minutes to become awake. And we’re not talking about configuration troubles here - rather just about gathering enough energy to turn electronics on. These are definitely issues that the energy harvesting communities need to solve in order to beat the dominance of battery-powered devices.

Sleep Power

Another key parameter for energy harvesting systems is sleep power. We’re all familiar with the sleep mode or standby mode in computers, printers, TVs and all other sorts of electronic devices. This means that, once it is not used for a certain amount of time, the electronic system will go into a low power mode, saving very significantly on energy consumption compared to fully active mode. For wireless devices, typically powered by a battery, it is important that systems enter into a deep sleep mode whenever not in active use, in order to save energy. This becomes way more critical in batteryless systems powered by energy harvesting - where energy is even more precious. 

Many IoT applications typically have a long duty cycle, i.e. the time between actions is very long. Let’s take for instance a smart water meter. It may only need to measure and communicate once per day, meaning it will be inactive, i.e. in sleep mode, for the vast majority of its time. Another example could be an ESL, calling for a couple of updates per day or even less than that. The ESL only requires significant power during update actions (i.e. to wirelessly receive data and change the info on the e-screen accordingly), so the rest of the time it can be sleeping. Even remote controls can be sleeping most of the time, although their duty cycle is typically shorter and much less predictable. Bottom line, most IoT and small electronic devices that can possibly be powered by energy harvesting must be put to sleep for a substantial part of the time - for an efficient management of the precious energy resources available. And, at the end of the day, sleep power can actually be the largest responsible for overall energy consumption in many use-case scenarios.

When entering the realm of true ultra low power, one of the major contributors to sleep power consumption is leakage current, also known as quiescent current. Leakage current corresponds to electric current that flows in a circuit through alternate pathways, namely parts normally viewed as insulating. So, leakage might be for instance the spontaneous discharge of a charged capacitor or a battery, or the flow of current across a transistor in the “off” state. In semiconductors, leakage corresponds to mobile charge carriers (electrons or so-called holes) tunnelling through insulating regions. Keeping an efficient energy harvesting operation means minimizing leakage across all system components. Without proper attention to it, one could end up in a situation where leakage gets close or even supersedes sleep power consumption - which will obviously ruin any energy budgeting intentions.

Power obsessed: Redefining Ultra Low Power

At ONiO we have an obsession for power efficiency. Our technology has been crafted from the very beginning and across all aspects for self-powered operation, using energy harvested from surrounding ambients - which called for such an obsession! Accordingly, we’ve put quite a lot of effort into minimizing leakage current across all subsystems, enabling very low power deep sleep modes, and quick start-up/wake-up with minimal power/energy needs. In doing so, we are literally redefining the benchmarks of ultra low power electronics.

As an example, in a cold start test, was up and running in just a few seconds from a 2 cm2 PV panel with 20 Lux indoor LED lighting. At normal office illumination (400-500 Lux), starts in the sub-second range. At 1000 Lux, it could start from as little as 5 mm2 PV active area. This level of performance allows us to solve the corner cases across applications - like the remote control between pillows, the bottom shelf ESL, or the unboxing experience.

As discussed in this article, low sleep and cold start power consumption are key fundamentals to enable energy harvesting and batteryless operation. stands right at the sweet spot, boasting impressively low cold start and sleep power consumption levels. It's so efficient that it can jolt a PV powered remote control to life in just a few seconds, hinting at incredibly low energy requirements. These figures are so minimal that they pave the way for a new era of self-powered IoT and small electronic devices, setting a new standard in the field.

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