Energy harvesting is a diverse field encompassing many technologies. Storage battery technology is equally diverse, with as many battery types for storing energy as there are mechanisms for extracting it from the environment. A wide range of possible conditions must be considered when selecting a means of storing energy in batteries, from fluctuating energy input levels to the potential demand for the harvested energy. Fortunately, a wide range of battery types and performance are available to match storage to required energy production and load profiles, and this article will examine some of the principal choices.
Batteries are identified by their basic chemistry such as lead-acid or lithium-ion. There is continuous development of the technology in each basic chemistry, leading to new materials to improve the basic electrode structures. As such, it is important for the designer of an energy-harvesting system to fully understand battery specifications in order to make the right choice for the application among currently available products.
Today, there are many types of secondary cells available in a wide variety of form factors. Battery packs can be designed to squeeze precisely into a portable electronic device right from the factory, and are available in standard and familiar cell formats such as AAA, AA, C, or D.
Secondary cell chemistries — from the venerable lead-acid type to advanced thin film cells — are available in configurations ranging from uninterruptible power supply batteries for server farms filling entire rooms, to tiny thin-film batteries that can be embedded into integrated circuit packages to provide distributed storage on a much smaller scale.
Figure 1: Today, lead-acid batteries are available in sizes such as this Cyclon D cell allowing integration into a number of different systems.
With many rechargeable battery chemistries available, and various performance-optimized designs in each chemistry, it is useful to have a basic understanding of performance metrics in order to make meaningful comparisons.
The primary measure of any battery is total capacity. This is probably the main design consideration in any system employing a secondary cell battery, and this specification is highly dependent upon the discharge conditions. When choosing a battery solution, it is important to know that different manufacturers specify capacity for different conditions. How closely these match energy-harvesting system design criteria should be carefully considered.
Capacity is defined as the total charge the battery can deliver from its fully charged state to its cutoff voltage, or the point at which the battery is considered empty. Most cells can continue to deliver charge past their cutoff voltage, but this is the point at which continued safe, reliable operation remains viable. Furthermore, the capacity is specified for a certain discharge rate of the battery. Battery capacity will be reduced as discharge rate is increased.
It is convenient (and nearly standard) to define many measurements normalized to battery capacity. These so-called C-rates simplify comparisons between batteries. Power or E-rates may also be used. Charging and discharge curves and rates are specified as C-rates. For example, a discharge current of 1 A from a 500-mAh battery is a discharge rate of 2C. Likewise, discharging the same battery into a load drawing 50 mA is a rate of 0.1C.
|Nominal Voltage||Specified reference voltage||Volts (V)|
|Cutoff Voltage||Minimum specified voltage for operation; The point at which the battery is considered empty||Volts (V)|
|Capacity||Total charge that the battery can deliver stated at specified rate of discharge from 100 percent state-of-charge to the cutoff voltage||Ampere-hours (Ah)|
|Energy||The energy the battery is capable of providing from full charge until the cut-off voltage is reached stated at a given discharge rate||Watt-hours (Wh)|
|Specific Energy||The battery capacity per unit mass||Watt-hours per kilogram (Wh/kg)|
|State-of-charge (SOC)||The remaining battery capacity at a point in time, expressed as a fraction of the maximum capacity||Percent|
|Depth of discharge (DOD)||The percentage of the maximum capacity of the battery that has been discharged||Percent|
|Cycle Life||The number of discharge cycles the battery can provide to a given DOD before it fails to meet specified performance criteria. Higher DOD reduces cycle life. Criteria can vary widely between manufacturers’ specifications.||Cycles|
|Charge Voltage||The voltage the battery is charged to when it reaches maximum capacity||Volts (V)|
|Float Voltage||The voltage at which the battery can be maintained at maximum capacity to compensate for self discharge||Volts (V)|
Table 1: Summary of key storage cell battery specifications
Next to capacity, the second most important specification is internal resistance. This 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. Internal resistance is also temperature dependent, which further constrains the operating environment. Internal resistance is dependent upon the total capacity contained in the battery at any given time.
Battery performance needs to be taken into account for more than just the end points of total capacity (fully charged and completely empty). State of charge (SOC) is the percentage of maximum battery capacity. Since all secondary cells lose capacity over their useful life, SOC is calculated using the present value of cell capacity.
Battery conditions can also be measured in terms of the state of discharge of the cell. Depth of discharge (DOD) is stated as the percentage of maximum battery capacity that has been discharged. A cell discharged to its lowest recommended operating level is said to be at 100 percent DOD. Terminal voltage is simply the voltage delivered by the battery regardless of the load. Open circuit voltage is the battery voltage with no load connected. Open circuit voltage can be used to determine the state of charge of a battery when controlling charging currents.
Battery specifications are rarely consistent between products, so it is particularly important to understand discrepancies in the way figures of merit are measured. This is especially true of rated cycle life for the battery. Stated lifetimes are often a function of the battery chemistry and the application niche. For instance, deep cycle battery manufacturers may give cycle life for 100 percent DOD, while other applications requiring shallower discharge and more frequent charging cycles are more likely to measure cycle life for lower DOD values.
The venerable D cell
Whether or not the manufacturer quotes the cycle life for conditions matching the energy-harvesting system, comparing battery types and manufacturers is still a challenge. To illustrate the variation in battery specifications, it is useful to look at a given physical size of cell. Both much smaller and much larger storage batteries are available, but the “mid-size” D cell could be very useful for various energy-harvesting designs requiring only a single cell, or when ganged for higher voltage or capacity.
It may seem an unusual format for the workhorse of the automotive industry, but today D size lead-acid batteries are a viable option. The 842-1000-ND from Cyclon is a sealed lead-acid battery in standard D size. Perhaps less surprisingly, D cell rechargeable batteries are manufactured using other chemistries as well. For example, Energizer produces the N701-ND using nickel-metal-hydride (NiMH) technology.
Figure 2: Energizer N701-ND using nickel-metal-hydride (NiMH) technology.
Comparing some key specifications for these two secondary cells, the raw performance numbers are not only very different but also are presented in different formats and specified for very different discharge conditions. Cyclon specifies the capacity of the 842-1000-ND as 2.5 Ah for a 10 hour discharge rate (or 0.1C) at 25˚C. Energizer states capacity of the N701-ND as 2.5 Ah at 0.2C and a temperature of 21˚C.
Briefly comparing several available D cells from several manufacturers and chemistries showed a range of capacities from 2.5 to 9.3 Ah. The discharge currents ranged from 0.5 to 1.3 A. If the system design requires a different C-rate than the manufacturer’s specification, it is important to check the impact it will have on the cycle life of the battery.
Although the rechargeable battery manufacturer may quote specifications that closely match the energy-harvesting system’s operating parameters, it is equally likely that meaningful comparisons between chemistries and various designs, as well as manufacturers, will require the system designer to normalize the specifications from the products under consideration. This should be easier with a better understanding of some battery specifications and terminology that this article has provided.
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