An important part of an ultra-low power design is the energy storage device. This section covers a number of commonly used energy storage devices, their typical limitations and pitfalls, and applications that they excel at. It is not meant to be a definitive explanation of battery technologies, as that information is better described elsewhere.

Primary Lithium Coin Cell (Manganese Dioxide Lithium)

Perhaps the most commonly used battery family for ultra-low power applications is standard primary (i.e. non-rechargeable) lithium coin cells. They provide high energy density and very low self-discharge rates. Typically rated at 3V, they can provide voltage at typical ultra-low power electronics levels without the need for multiple batteries or a boost converter (which might be needed for a 1.5V alkaline cell.) The main limitations of these batteries are that they cannot be recharged (so energy harvesting is not possible) and they have very limited discharge current capability. For example, the Renata CR2032 has a maximum continuous discharge rating of 3mA. To achieve the quoted energy capacity, a lower current of 0.2mA must be used. For many designs, this is simply not enough power.

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Lithium Thionyl Chloride

Lithium thionyl chloride batteries are also non-rechargeable and have very low self-discharge rates. They have excellent performance over a wide temperature range and are among the highest energy density batteries available. Correspondingly, they are expensive. For the developer, it is worth noting that they have extremely flat discharge curves, so it is not practical to estimate remaining battery life simply from a voltage measurement. They can provide significantly more power than Manganese Dioxide Lithium coin cells.

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Lithium Ion Cylindrical Cells

The commonly used 18650 battery is a lithium ion cylindrical cell. These are also available in other sizes, such as the AA-sized 14500 and the AAA-sized 10440. They are rechargeable, but have lower energy density than lithium primary cells. They also have much higher self-discharge rates, up to ~10% of the capacity per month. These batteries can provide a lot of power, although some cells are designed for power and some for energy. Note that batteries sold from non-reputable sources often vastly overestimate their capabilities. 

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Alkaline Batteries

The common AA or AAA alkaline battery can also be a reasonable choice for ultra-low power electronics. These batteries are cheap, non-toxic, have low self-discharge, and relatively high power output. Over a long period, they can leak and corrode the electronics they are installed within, so they may not be a good idea for devices with multi-year battery life.

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Supercapacitors

Although the energy density of supercapacitors is far lower than all of the batteries here, they do have certain advantages. They are simple to charge using energy harvesting technologies and have very high power output capability. They are also relatively expensive and some have high self-discharge (leakage). Because they are not batteries, they do not have a flat discharge curve. It is usually only possible to use some portion of the energy before the voltage drops below a usable level.

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Closing Thoughts

When selecting an energy storage device, be aware of the operational temperature of your device. Batteries are significantly affected by their temperature, and rechargeable batteries can only be charged within a relatively tight range. If the design will be outdoors, be aware that the temperature can be far higher than ambient if under direct sunlight, but the low-end of temperature will be limited to ambient.