Storage Battery Solutions for Energy Harvesting Applications

Ambient light, thermal gradients, vibration/motion, or electromagnetic radiation can be harvested to power electronic devices. At the same time, all energy-harvesting-based systems need energy storage for times when the energy cannot be harvested (e.g., at night for solar-powered systems). Rechargeable batteries – known as “secondary” cells to differentiate them from “primary” or single-use cells – are usually specified for this task. This article will examine the various secondary cell technologies available to energy harvesting system designers looking for a cost-effective and powerful battery solution.

Primary and secondary batteries contain the same basic structure of a cathode, an anode, an electrolyte for moving charge between the terminals, and a means to separate them. Secondary cells are distinguished by the type of rechargeable chemistry employed, such as nickel-cadmium or lithium-polymer, or solid-state thin film.

Although the energy-harvesting-system designer may not be focused on cell structure and chemistry, it will be helpful to develop a working knowledge of the various technologies since it is the cell chemistry that determines all of the battery performance metrics and limits. (For more details and explanation about storage battery performance metrics consult the Energy Harvesting TechZone article, Understanding Storage Battery Specifications for Energy Harvesting.)

NiCd and NiMH

Nickel-cadmium (NiCd) batteries have enjoyed the longest market run among secondary cells for portable electronics, resulting in a mature technology with low cost. NiCd, however, offers performance advantages as well.

For example, the latest NiCd batteries have very low internal resistance, rivaling that of standard alkaline batteries. Next to capacity, internal resistance is the second most important specification as it determines much of the battery’s maximum performance in both charge and discharge, since it decreases battery efficiency. Internal resistance limits operating conditions such as temperature and determines the useful life of the battery. Its low internal resistance makes NiCd a good candidate where high power is required for short periods. High discharge currents of up to 50 times the battery capacity (or 50 A for a 1-Ah cell) are easily tolerated and have a minimal effect on capacity. Terminal voltage changes very slowly as the cell is discharged and NiCd cells do not deteriorate after long periods spent fully discharged. Self-discharge rates are approximately 20 percent per month at 21°C. A common NiCd cell size is AA. The Sanyo Cadnica SY115-ND (Figure 1) provides 70 mAh capacity at 140 mA (0.2 C rate). Internal impedance at 50 percent discharge is 16 mΩ (at 1,000 Hz) and charging current can range from 70 mA (standard, 14 to 18 hours charging time) to 1,050 mA (fast, about 1 hour charging time).

Figure 1: Structure of the Sanyo Cadnica SY115-ND 700 mAh NiCd AA cell (left) and the cell itself.

Nickel-metal-hydride (NiMH) technology has begun to displace NiCd batteries in many applications. The cell and electrode structures have a lot in common with NiCd cells, although NiMH replaces cadmium with a hydrogen-absorbing alloy.

Metal-hydride cells are available in sizes down to AAA and appear in many current consumer products, making them price competitive with more mature NiCd technology while meeting or exceeding NiCd performance. NiMH offers high discharge rate with low terminal voltage droop for relatively high depth of discharge. Energy density can be up to twice that of NiCd cells.

NiMH cells experience self-discharge rates similar to NiCd; some so-called hybrid cells incorporate a thicker electrode separator to reduce the effect. These cells are often pre-charged and shipped ready to use since some versions retain up to 80 percent state of charge for one year when stored at 20°C. Figure 2 shows the discharge characteristics of the Energizer NH-15-2300 AA cell.

Figure 2: Energizer NiMH AA cell discharge characteristics.

Lithium and thin-film

Lithium-ion (Li-ion) cells offer a factor of two greater power-to-mass density improvement over NiMH cells. Lithium battery prices are declining thanks to their success in portable computing devices. Panasonic-BSG, for instance, manufactures lithium cells in very small coin formats in a range of capacities from a few mAh up to 100 mAh (at 0.002 C discharge rate). The P088-ND cell achieves this high capacity in a package, 30 mm diameter and 4.2 mm thick, specified to deliver 200 µA for 500 hours. Li-ion cells exhibit a high internal resistance compared with other secondary cells.

Figure 3: Panasonic-BSG P088-ND 100-mAh lithium coin cell.

Li-polymer (often referred to as Li-Po) batteries are a specific class of Li-ion cell in which the electrolyte is held within a polymer material. Li-Po cells improve power output for a given weight compared to standard Li-ion cells. Li-polymer batteries are attractive for energy harvesting since they exhibit a very high discharge to charge efficiency (greater than 99 percent compared to less than 90 percent for standard Li-ion). But while lithium-Ion/polymer chemistry batteries provide the high-performance features necessary, they must be treated with care. For example, lithium-ion/polymer cells can become unstable if charged over 100 mV beyond their recommended float voltage. Manufacturers’ specifications should be carefully reviewed before designing the charging and discharging control circuits for all lithium-based secondary batteries.

Thin-film secondary cells are a specific class of lithium-ion cell in which the reduced dimensions of the components increase active surface areas of the electrodes, allowing manufacturers to squeeze more active ingredients into a smaller cell volume. The result is dramatically increased performance, evident most notably in the power and energy density ratings. The electrolyte is solid state.

Cymbet is one of the leading manufacturers of this type of cell. The size of the cell allows distribution of energy storage to the integrated-circuit level. Cymbet cells can be sold as bare die for packaging with ICs, extending the old concept of battery-backed SRAM to a new dimension.

Solid-state EnerChip thin-film storage cells are also available as individual cells in standard IC packages. Cymbet CBC050-M8C (Figure 4) ships in a QFN package. The rated capacity is 50 µAh at 2 C discharge rate. Even with this relatively high discharge rate and depth of discharge to 50 percent, the thin-film cell is rated for 1,000 cycles of lifetime at 25°C. The nominal cell voltage is 4.2 V, simplifying integration with a range of electronic components.

Figure 4: Cymbet CBC050-M8C 50-µAh 4.2-V thin-film cell.

Another unique benefit of solid-state thin-film batteries is their ability to work at low temperatures. Conventional batteries with a liquid electrolyte can freeze and even burst their battery cases. As the temperature reduces, internal cell resistance increases. Having a solid state electrolyte, there is no issue of the battery case breaking. While you still have internal resistances increase with lowering temperature, the battery continues to operate in very cold conditions, still delivering current at temperatures as low as –40°C. In such cases, you may couple the battery with a capacitor where a high-current pulse is needed, for example to power a radio transmitter in a wireless sensor.

Infinite Power Solutions (IPS) is another noted provider of solid-state, rechargeable, thin-film batteries that are well suited for autonomous, self-powered applications like Bluetooth Smart devices and other wireless sensors. Its THINERGY Micro-Energy Cells (MECs) feature near zero self-discharge current, and with 100,000 recharge cycles, can last the lifetime of the application.

The active materials in the IPS THINERGY MEC201 (Figure 5) include a lithium cobalt oxide (LiCoO2) cathode and a Li-metal anode. A solid-state electrolyte called LiPON (lithium phosphorus oxynitride) is used; the low electron conductivity within LiPON results in very low self-discharge, making this technology ideal for applications where energy must be reliably stored for long periods without the ability to recharge.

Due to its low internal cell resistance, the device is a good choice for energy storage in applications where only extremely low-current recharge sources are available; pulsed or continuous currents as low as 1 µA can be used to effectively recharge this device.

Figure 5: In the THINERGY MEC201, external terminals in the form of positive and negative nickel-plated tabs are located along the top edge of the cell for easy soldering to PCBs.

Summary

Depending upon the energy harvested, the load demands, and various other system design considerations, there are many options for a storage battery solution. High discharge rates, low self-discharge, high energy density, and other performance characteristics are vastly different among the secondary cell chemistries currently available. Matching battery characteristics to the energy input and output profiles will improve energy-harvesting system performance. Always consult manufacturer specifications to optimize performance and enhance operational life of the battery. Also remember to pay close attention to all safety guidelines to avoid any mishap when using rechargeable batteries.