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Researchers have developed a microchip, named BATLESS, that can continue to operate even when its battery runs out of energy.

One of the key challenges of moving Internet of Things (IoT) devices from concept to reality is the need for long-lasting operation under tightly constrained energy sources, which demands extreme power efficiency. IoT devices—such as sensors—are often deployed on a massive scale and in places that are usually remote and difficult to service regularly, thus making their self-sufficiency essential.

Currently, batteries in IoT devices are much larger and up to three times more expensive than the single chip they power. Their size is determined by the sensor node lifetime, which directly affects how often they need changing. This has an important bearing on maintenance cost and impact on the environment when people dispose these batteries.

To extend the overall lifetime, the batteries usually recharge slowly by harvesting some limited power from the environment, such as using a solar cell. However, existing IoT devices cannot operate without batteries, and small batteries are fully discharged more frequently. Hence, battery miniaturization often results in highly discontinuous operation of IoT devices, as they stop functioning every time the battery runs out of energy.

The researchers designed BATLESS with a novel power management technique that allows it to self-start and continue to function under dim light without any battery assistance, using a very small on-chip solar cell.

This research substantially reduces the size of batteries required to power IoT sensor nodes, making them 10 times smaller and cheaper to produce.

No batteries required

“We have demonstrated that batteries used for IoT devices can be shrunk substantially, as they do not always need to be available to maintain continuous operation,” says research leader Massimo Alioto, associate professor from the electrical and computer engineering department at the NUS Faculty of Engineering.

“Tackling this fundamental problem is a major advancement towards the ultimate vision of IoT sensor nodes without the use of batteries, and will pave the way for a world with a trillion IoT devices.”

Battery indifference is the ability for IoT devices to continue operations, even when the battery is exhausted. To achieve this, devices have to operate in two different modes—minimum-energy and minimum-power. When the battery energy is available, the chip runs in minimum-energy mode to maximize the battery lifetime. However, when the battery is exhausted, the chip switches to the minimum-power mode and operates with a tiny power consumption of about half a nanowatt—this is about a billion times smaller than the power consumption of a smartphone during a phone call.

Power can be provided by a very small on-chip solar cell that is about half a square millimeter in area, or other forms of energy available from the environment, such as vibration or heat.

The chip’s ability to switch between minimum energy and minimum power mode translates into aggressive miniaturization of batteries from centimeters down to a few millimeters. The BATLESS microchip enables the uncommon capability to uninterruptedly sense, process, capture, and timestamp events of interest, and for such valuable data to be wirelessly transmitted to the cloud when the battery becomes available again.

Despite being in minimum-power mode when battery is not available, the reduced speed of the microchip is still adequate for numerous IoT applications that need to sense parameters that vary slowly in time, including temperature, humidity, light, and pressure. Among many other applications, BATLESS is very well suited for smart buildings, environmental monitoring, energy management, and adaptation of living spaces to occupants’ needs.

“BATLESS is the first example of a new class of chips that are indifferent to battery charge availability. In minimum-power mode, it uses 1,000 to 100,000 times less power, compared to the best existing microcontrollers designed for fixed minimum-energy operation,” Alioto adds.

“At the same time, our 16-bit microcontroller can also operate 100,000 times faster than others that have been recently designed for fixed minimum-power operation. In short, the BATLESS microchip covers a very wide range of possible energy, power, and speed trade-offs, as allowed by the flexibility offered through the two different modes.”

Self-starter

BATLESS is also equipped with a new power management technique that enables it to ‘self-start’ operations, while getting power directly from the tiny on-chip solar cell, rather than battery assistance.

The team has demonstrated this at 50-lux indoor light intensity, which is equivalent to the dim light available at twilight, and corresponds to nanoWatts of power. This makes BATLESS indifferent to battery availability, addressing a previously unsolved challenge in battery-less chips.

The team is now exploring new solutions to build complete battery indifferent systems that cover the entire signal chain from sensor to wireless communications, thus expanding the current work on microcontrollers and power management.

The research team aims to demonstrate a solution that shrinks the battery to millimeters, with the long-term goal of completely eliminating the need for it. This will be a major step towards the realization of the Smart Nation vision in Singapore and IoT vision worldwide.

The researchers presented their work at the International Solid-State Circuits Conference (ISSCC) 2018 in San Francisco.

Source: National University of Singapore