Portable designs have long been hampered by the laws of supply and demand—an insufficient supply of energy and an excess of demand for it. Battery technology hasn’t progressed much since the advent of Li-Ion cells, and unless you’re comfortable with a thorium-based energy source there isn’t a lot of room for improvement.
On the demand side semiconductor engineers have made great strides over the last 10 years or so reducing power consumption. Moore’s Law has helped a great deal, but so have a host of other clever innovations including multiple clock and power domains, dynamic voltage and frequency scaling, power gating, and multiple sleep states. Today Renesas is well justified in referring to their RL78 and RX100 families of MCUs as “ultra-low-power.”
Still, no device can last indefinitely in the field unless its battery is supplemented by energy harvesting. In applications where it would be either dangerous (pacemaker), expensive (mountain top weather station), or impossible (satellite) to replace the battery, a supplemental alternative energy source is highly advisable.
Energy Sources
Energy harvesting is a promising power source but one that presents numerous challenges—the major ones being availability and quantity. Solar power is obviously only available when the sun is out; vibration/motion sources only work when the motor is turned on or the person being monitored is moving; and thermal sources only work when there is a reliable source of heat and a heat sink to generate the temperature gradient required by thermal electric generators (TEG's).
The level of energy available from energy harvesting sources is generally quite low, often a few orders of magnitude less than what even the lowest power applications require. With the exception of ambient RF, typical energy harvesting devices can generally supply anywhere from 10 µW to 1 mW, though rarely on a steady basis. The problem can be addressed by adding more solar cells or TEGs in parallel, for example, if your application will allow it. Still, you're dealing with micropower sources and there's a limit to what you can do with them.
You can sometimes address the problem by combining a variety of energy harvesting sources. For example, a remote wireless strain sensor on a building might combine a small solar cell with a TEG; on a bridge roadbed a similar device might combine a TEG with a vibration sensor. If your application can combine light, vibration, and thermal energy sources then it may be able to operate indefinitely by just harvesting these energy sources.
Energy Storage
The most common battery powering low-power devices is the ubiquitous CR2032 Li-Ion coin cell. The CR2032 is rated at 3.0V @ 225 mAh with a recommended continuous standard load of 0.2 mA. The chart shows the cell voltage staying relatively flat while delivering 190 μA into a 15 kΩ load for over 1,000 hours, after which it quickly fails.
Figure 2: Discharge characteristics for CR2032 coin cell
Coin cell batteries are not rechargeable but they still have a place in low-power devices that harvest ambient energy sources. A low-power sensor application could be powered by energy harvesting sources when they are available—with their energy stored in a capacitor or thin-film battery—switching over to the coin cell when those sources aren't available. Such an arrangement could greatly extend the life of the coin cell battery.
Thin film batteries are rechargeable, with a discharge curve closely resembling that of a CR2032. They’re quite small—in both physical dimensions and storage capacity—so they’re generally targeted at ultra-low-power applications such as wireless sensors, RFID tags, and standby supplies for NV-SRAM in real-time clocks.
Figure 3: Typical thin-film battery discharge characteristics
Because they’re so small thin-film batteries can be damaged by sudden power surges—such as when a normally quiescent wireless sensor node wakes up for a few microseconds every second to pulse out data before returning to sleep mode. To be able to handle sudden current surges thin-film batteries employ a large capacitor or a supercapacitor across their output.
Because of the extremely close proximity of the conductive layers supercapacitors—also called electric double-layer capacitors (EDLCs)—have an energy density hundreds of times greater than electrolytics. They have a lower energy density than batteries but a far higher power density, and unlike batteries they can be discharged almost instantaneously. Supercaps buffer the load for the power source, providing peak current when it’s needed.
Thin-film batteries require power management to regulate the charging rate and protect them from overcharging as well as voltage spikes. There are PMICs available that are specially designed to handle such energy harvesting applications.
While a thin-film battery could easily power an RL78G13 or RX110 in standby mode, if the application only requires active power quite briefly—as in the earlier wireless sensor node example—then a thin-film battery (or bank of them) used in conjunction with a supercap might fill the bill in an RL/RX-based energy harvesting application. Otherwise it’s back to Plan A: the coin cell.
Looking Ahead
It’s a natural for ultra-low-power MCUs to look for ultra-low-power energy sources. Their availability and variability make energy harvesting sources challenging to incorporate into low-power designs, but if you’re contemplating an ultra-low-power project then give energy harvesting serious consideration.