Every year electronic components get smaller, and require less power. As electronic components and micromechanical technology improve, very small devices become possible. These miniaturized devices require miniaturized batteries, which are also known as “microbatteries”. While there is no commonly accepted definition of exactly how small a battery must be to be called a microbattery we take this to mean a battery with at least two out of three dimensions under 2 mm. And if you’re looking at developing a miniaturized electronic device, at some point you will need to think about adding a power source. Perhaps you need to develop a battery as small as a grain of rice for your application. For instance, a pill camera, drug delivery device, or implantable stimulator may need such a small battery. Conventional batteries may take up more space by themselves than your entire device can occupy.
At first glance, this seems easy enough. You can just take a conventional battery design and shrink it down to size, right? Unfortunately, it is not easy to simply scale down a conventional battery to millimeter size to create a microbattery. Various mechanical constraints, such as how thin components can be made and what tolerances are possible, limit manufacturability for batteries with dimensions under a few millimeters.
How Small a Battery You Need
The first, most obvious question to ask yourself is, “How small of a battery do I need exactly?” Various battery constructions become problematic below certain sizes. Manufacturing processes, such as crimping, sealing, and coating, become much more difficult to control as dimensions become smaller. So, the larger you can go, the easier it will be to find a battery that meets your needs. For that matter, if you can find something off the shelf that does everything you need that should be your first choice—you may not even need a “microbattery” at all. If you have a unique application that needs a tiny battery, however, and there is a business to develop one, you may need to go the custom battery route.
Power and Energy Requirements
As with any other battery, a microbattery can only supply so much power and so much energy. In fact, given the mechanical constraints involved in microbattery construction, the power and energy density available are somewhat less than a larger “conventional” battery. So, you need to figure out how high of an energy density (in terms of W-h/L) and power density (in terms of W/L) your device requires.
Energy density refers to the total amount of energy that one can extract from a given size battery, at ideal conditions. This is a thermodynamic number, which depends solely on the amount of active material within a given battery volume and the chosen battery chemistry. Practical energy densities are only a fraction of the theoretical values, and are dependant on the discharge rate, cutoff voltage and the operating conditions such as temperature. Power density refers to the power which a given size battery can source; this depends on the battery construction as well as the chemistry chosen. Knowing what energy density and power density your device requires will help determine if your application as you envision it is even feasible. Click here to determine power and energy density requirements for your application. You can check these numbers against known energy densities for standard chemistries.
When you think about electrical connections for a battery, two things come to mind. First of all, there are removable battery holders (everything from common coin cell battery holders to specialized mechanical connectors for laptops and cell phones). Secondly, there are bonded (welded or soldered) joints. Both of these can be problematic at the micro scale. Beyond a certain point, it can be hard to secure a very small battery in place, depending on device geometry. Furthermore, heat generated during welding or soldering can be much more problematic when it is closer to the active battery materials, as is the case when working with smaller devices. Gluing the battery in place using conductive glue is possible, but this requires accurate registration. Fixed mechanical holders are also possible—again, this requires assembly methods capable of working at the small scale required for microbattery integration. While connecting a microbattery into a device is possible, it requires some thought.
Beyond electrically connecting a microbattery to your device, you also have to figure out how to mechanically integrate it into your device. This can be as simple as placing it into your device’s enclosure. Sometimes, though, you may need to encapsulate the battery within your device. This means that the battery needs to be able to withstand whatever temperature and/or pressure you need to apply. Not every battery chemistry is capable of this. For example, conventional lithium and lithium-ion cells generally have a tough time with even brief exposure to temperatures above 70-80 °C. Furthermore, lithium and lithium-ion batteries require more robust packaging than aqueous chemistries, since the chemistry is sensitive to moisture contamination (thick packaging layer and wide seams); this packaging takes away valuable volume when dealing with a very small battery.
Whether the Microbattery is a Finished Package
There are a wide array of microbattery chemistries described in the literature. So why, then, are there so few batteries available, aside from in solid-state thin-film form? If you look closely at many of these reports, you’ll see that the battery exists only as a laboratory device with macroscale packaging; it is not available as a standalone finished package anywhere close to the size of the battery. Creating this package is non-trivial, and you need to consider whether or not any microbattery you consider is actually going to be available as a packaged unit. If your microbattery doesn’t have a package ready, it isn’t going to do you any good.
How—or Whether—to Recharge the Microbattery
Since you are looking at a microbattery, your device is going to be very small as well—recharging is not likely to be as simple as plugging the device into a DC power supply. There may or may not be space for a charger on the microbattery, and handling your device after its initial use may be problematic. You may or may not need a custom fixture for recharging. Furthermore, some applications, such as drug delivery devices, lend themselves to one-time use.
Desired Operating Environment
You need to think about what sort of environment the microbattery will be used in, just like with conventional batteries. Extremes of temperature, pressure, or humidity can play a role in what sort of battery you select. In addition, you need to think about how long the battery should last—anywhere from hours to years. For instance, a pill camera would only need to last for hours, while an implantable sensor may need to last for years.
Biocompatibility for microbatteries is going to be even more challenging than for conventional batteries. The smaller size of a microbattery means that a failure is less catastrophic than for a larger battery. Nonetheless, the smaller space available for packaging for a microbattery could make it less robust than a larger power source. Beyond that, the battery—and its packaging–must be compatible with any sterilization processes involved in preparing a medical device for use.
As you can see, there are a number of things you need to think about if you’re considering adding a microbattery to your device. Microbatteries are not simply scaled-down conventional batteries, and they require their own “care and feeding.” If you have an application that you need a microbattery for, feel free to contact us.