Rice-Sized Micro-Battery Will Enhance Salmon Tracking, Fisheries Monitoring

A new micro-battery design will allow researchers to power the acoustic tags used to track the movements of fish around the world with a device about the size of a long grain of rice.

The technology was developed not to create one of the world's smallest batteries - a rice grain-sized battery cannot contend with the human hair-sized micro-batteries that exist - but to create a power system with the longevity to withstand the long migratory routes of fish such as salmon.

A lighter battery requires a smaller transmitter, which can be inserted into younger fish. Such a device enabled researchers to track fish from their early stages of life, and the battery's longevity allows for a greater tracking range.

"The invention of this battery essentially revolutionizes the biotelemetry world and opens up the study of earlier life stages of salmon in ways that have not been possible before," said M. Brad Eppard, a fisheries biologist with the Portland District of the US Army Corps of Engineers.

"For years the chief limiting factor to creating a smaller transmitter has been the battery size. That hurdle has now been overcome," Eppard said.

Tagging and tracking salmon and other fishes is important for conservation research, and environment and wildlife managers use such data to determine when to open and close dams and chart the welfare of endangered fish species.

The new rice-sized battery and transmitter can be injected into the fish, rather than being surgically inserted. Injecting a transmitter is less stressful for the fish, and is less costly and faster for researchers.

"This was a major challenge which really consumed us these last three years," said Z. Daniel Deng, an engineer based at the Department of Energy's Pacific Northwest National Laboratory. "There's nothing like this available commercially, that can be injected. Either the batteries are too big, or they don't last long enough to be useful. That's why we had to design our own."

Jie Xiao, a materials science expert, was enlisted to help design the system. Xiao's design relied on what's known as the "jellyroll" technique, which is employed in common household battery design.

"It's a bit like flattening wads of Play-Doh, one layer at a time, and then rolling them up together, like a jelly roll," Xiao said. "This allows you to pack more of your active materials into a small space without increasing the resistance."

The new battery weights about 70 milligrams, about half the weight of the current standard weight for batteries used in similar acoustic transmitters. The device has an energy density of about 240 watt hours per kilogram, which is almost one and a half times greater than the current standard. The battery lasts long enough for the acoustic transmitter to send out a signal every five seconds for about one month.

Preliminary tests of the battery have been successful, the researchers said. They detail their work in the journal Scientific Reports.