We are all familiar with those coin and button cells that are available in a bewildering array of sizes and chemistries. Some are fairly common, such as the 2032 coin cell or the 357 button cell, but there are also many niche ones that are in limited use, and their replacements are hard to obtain. The proliferation of sizes and voltages can be especially frustrating when the one you can readily obtain is just a fraction of a millimeter larger than the one you need, and thus will not fit in the holder.
But these tiny batteries may soon be considered the "big ones." The growth in applications such as wearables (wristbands, stylus pens, smart glasses) and deeply embedded Internet of Things (IoT) devices is driving demand for even smaller primary and secondary (rechargeable) power cells. Despite their limited capacity rating, they have enough energy for a large class of situations, including devices that are recharged daily or that receive regular pulses of energy via a harvesting source.
A good example is the rechargeable G-320A pin-type Li-ion battery from Panasonic. It is a cylinder 20 mm long with a 3.65 mm diameter, with a nominal output voltage of 3.8 V (3.0 to 4.35 V range) and a capacity of 15 mAH, Figure 1. While that is not much, it is enough for many ultra-low-power end products. Despite its diminutive size, the battery has some carefully defined charge and discharge parameters that must be followed to ensure proper operation and avoid self-destruction.
Using these tiny batteries also means the production team needs to work closely with the manufacturing side—whether in-house or contracted on the outside—to make sure the pick-and-place board-loading system can handle them (they are available in surface-mount and through-hole packages). These teams will also have to study their reflow-soldering profile, which likely differs from other components on the board.
Such tiny energy sources bring another set of issues: how do you assess their performance? Measuring the miniscule currents they provide is not easy because you cannot just put a meter in the line and expect valid results. Adding a current-sense resistor brings problems as well, with the tradeoff between voltage drop (you want more) and wasted power (you want less). You may even have to go to sophisticated instrumentation such as this battery-drain analysis solution recently introduced by Keysight Technologies, which includes a DC power-analyzer modular mainframe, a two-quadrant source-measure unit, and control-and-analysis software, Figure 2.
Even if you master the use of these pin cells, there is no time to be complacent. Still-smaller batteries are already in use, and while they are not mass-produced, they are in low-volume hand production. The microbattery developed by Pacific Northwest National Laboratory (see "A battery small enough to be injected, energetic enough to track salmon") is about the size of a grain of rice, just 6 mm long and 3 mm wide, Figure 3. It is hand-crafted with multiple layers to increase the internal surface area and thus keep the self-impedance low. The staff has made about 1,000 of these and implanted over 700 into fish to power tracking devices.
Where will the quest for smaller batteries end? There is no single or simple answer, of course. Perhaps people will soon have electrodes implanted (300 to 500 W) in their bodies as a standard surgical procedure, so they can become their own power source for all of their wearable and wellness devices, and even their smartphones.