Microbatteries power electronics in humans, fish

Article By : Bill Schweber

There are applications where unusual chemistries, construction and form factor are priorities, such as safely powering devices inside humans.

For many reasons, batteries of various technologies and form factors get a lot of attention. Everyone is looking for a battery with higher energy density by weight and volume, with superior discharge (and even charge) specifications and low cost. As a result, there's been lots of attention on lithium-ion (Li-ion) chemistry and its many variations, since that seems to offer–at least for the foreseeable future–the greatest potential (pun intended) in meeting those density objectives.

Still, there are applications where unusual chemistries, construction and form factor are the priorities, more than just an incrementally better Li-ion cell. Consider the work being done by a team lead by Professor Christopher Bettinger at Carnegie Mellon University. They are developing edible, biocompatible batteries that use non-toxic materials already present in the body, with available liquids such as stomach acid as the electrolyte; see references below (Figure 1). His team has produced cathodes based on melanin, a pigment already in the body, and anodes made of manganese oxide, which is also already present; other versions based on body-friendly materials have also been developed.

The idea is that the electrodes will dissolve harmlessly after use. Most of the resulting batteries, using a variety of soluble cations, had modest voltages (between 0.5 and 0.7V); although definitive specifications are hard to find, there was one mention of 5mW of power for up to 20 hours.

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__Figure 1:__ *Prof. Christopher Bettinger of Carnegie Mellon University dramatically demonstrates what may be possible with the edible batteries his team is developing. (Source: Carnegie Mellon University)*

It’s not just humans who need special batteries. A microbattery developed by Pacific Northwest National Laboratory is just 6mm long and 3mm wide and weighs only 70mg (Figure 2), and is used to power acoustic fish tags. It is hand-crafted of multiple layers that are then rolled up (Figure 3), which increases the internal surface area and so reduces internal resistance—a weak spot in many real-world battery implementations. Although each is handmade, about 1000 of these have been constructed and they have implanted over 700 of these into fish to power tracking devices.

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__Figure 2:__ *These rice-grain sized batteries are hand assembled and used to power acoustic fish tags. (Source: Pacific Northwest National Laboratory)*

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__Figure 3:__ *The fish-tag batteries are constructed of layers which are then rolled up and inserted into tiny cans. (Source: Pacific Northwest National Laboratory)*

These grain-of-rice sized batteries can supply enough power to send a 744-microsecond signal every three seconds for about three weeks (or about every five seconds for a month). Energy density is specified at 240WHr/kg, compared to around 100WHr/kg for standard silver oxide button microbatteries–although you do have to wonder how these measurements are made and if they are fair comparisons.

These specialty batteries have other unique issues as well, such as providing for connection or attaching the leads. The Pacific Northwest microbattery has internal leads; it’s not clear how the edible ones from Carnegie Mellon are connected. Certainly, users of these batteries won't be able to go to battery-holder vendors and get a standard connector or holder.

Have you ever had to specify, design, or resort to a highly specialised, unique, custom microbattery? Was this an early-on decision, or one that came into play late in the design cycle?

This article first appeared on EDN.

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