Tech Briefs

These long-life batteries can be used to power unattended sensors in harsh and remote environments.

There is a great need for energy sources that can power unattended sensors for more than a decade. Unattended sensors can be located in harsh and remote locations that are often dangerous for personnel maintenance and power source replacement. The power source must last the lifetime of the sensor. Unlike chemical batteries, the higher energy densities of radioisotopes allow the sensors to operate for infrastructure lifetimes (~150 years). Isotope batteries (iBATs) have the potential to become reliable, robust, and maintenance-free power sources for remote, long-term, low-power sensors. iBATs are different from chemical batteries because they are self-contained energy sources using radioisotope decay.

Figure 1. The GaAs arrays before the layers are wired and glued together to make a single sandwich to surround the platelets.

Indirect power conversion is used for the commercial off-the-shelf (COTS) iBAT. The conversion process is based on a two-step process converting nuclear decay to optical energy, then optical to electrical energy. The isotope is encapsulated inside a phosphor. The beta decay excites the phosphor generating photon emission, usually at a narrow frequency bandwidth. Photovoltaics (PVs) surrounding the phosphor platelets convert the optical energy into usable direct current (DC) electrical energy. There are inefficiencies inherent in the two-step conversion processes.

The most difficult part of the design of the battery was selecting a solar cell that is sufficiently efficient when exposed to narrowband wavelengths and low light conditions. By bandgapmatching the PV to the optical phosphor output and identifying fabrication process effects on PV efficiency, the total device efficiency could be optimized. Silicon (Si) solar cells such as amorphous Si are the most available and inexpensive in the market. The highest conversion efficiency and specific power density was found in the indium gallium phosphide (InGaP).

The components used were:

  1. GaAs thin-film/InGaP solar cells. The PV cells convert photons to usable electrical energy, which trickle charges onboard backup batteries.
  2. Phosphor platelets.
  3. ABS cassette and enclosure case. The cassette adds additional mechanical support for the vital components. The shape and features of the cassette allow the user to individually slide the cases into the enclosure. The enclosure is the case for all of the cassettes, the female connector, and the energy harvester circuit. This adds additional mechanical support and other environmental resistance.
  4. Board-to-board electrical connectors that electrically and mechanically connect the components together.
  5. A thin-film battery that provides onboard backup and energy storage for the COTS iBAT to directly power sensors. The iBAT trickle charges the battery array. By definition, it is an energy harvesting system when coupled with any type of energy transducer.

Two different types of epoxies are used in the iBAT design. The first layer of epoxy is a flexible, translucent epoxy with a 90-minute work life and a high shear and peel strength. After that layer cures, another epoxy is applied. This is a high-impact-resistant epoxy that is a white, low-viscosity liquid that when applied, hardens in 20 minutes.

Figure 2. The iBAT GaAs cassette (left) and the 3D CAD of the iBAT assembly (right).

The assembly includes all of the materials listed, along with additional required tools for handling, safety, and precision. The GaAs solar cells are placed on a vacuum table so they can lay flat. Five cells are soldered in series, which makes up a single layer. Optical adhesive is applied on the PV surface using a tiny paintbrush. Using plastic tweezers, platelets are placed on the surface. After the platelets cover the surface, the layer cures underneath a UV lamp for 40 seconds. Another solar cell array of five cells is placed on top of the platelet layer. Figure 1 shows the two layers before they are wired and glued together.

The two layers are connected in series, which is considered a single sandwich. Optical adhesive is applied to the edges of the PV layers, which physically attaches them together after a 40-second cure. Double-side Kapton tape is placed on the other side of the sandwich. This process is repeated throughout the entire assembly. Each sandwich is attached to each other and connected in parallel. Four sandwiches make up one cassette.

The first adhesive is applied to the entire surface of the cassette and allowed to cure. Then a thin layer of the second is applied to the surface. After curing, the sandwich is slid into an ABS cassette. Connecters are soldered to plus and minus wire leads. The process is repeated 6-10 more times, depending on the necessary power needed. The individual cassettes are inserted into the enclosure, starting from the bottom to the top. The female connector is secured into the enclosure cover. The energy harvester circuit board is screwed into the cover stand-offs and platforms. Lastly, the cover is aligned and screwed onto the enclosure. The connectors are aligned to the leads protruding from each cassette, and electrically and mechanically connected with the enclosure’s cover being screwed and pressed in place. Figure 2 shows a 3D CAD view of the COTS iBAT assembly, its components, and the actual cassette.

This work was done by Johnny Russo, Marc S. Litz, and Dimos Katsis of the Army Research Laboratory. ARL-0177