New Technology Demands New Battery Specification
Performance advantages of NiZn battery demonstrate need for mWh—a new measure of battery capacity
Challenge: The Need for a New Measure of Battery Capacity
Rechargeable batteries were first commercialized in the mid-1960s and have since undergone numerous technological advances. Unfortunately, the way in which battery capacity is measured—typically in amp hours (ah) or milliamp hours (mAh)—has not evolved to keep pace with the introduction of new technologies.
The mAh rating is a product of a battery’s current and time rating, omitting an essential element of battery power and capacity: voltage. mAh served as a valuable measurement gauge of battery performance when all batteries provided equal or similar voltage. However, an alternate measurement called milliwatt hour (mWh), which multiplies mAh by voltage, delivers the more informative and precise measurement needed to analyze and compare today’s differentiated battery market. The mWh specification can help businesses and consumers understand the true capacity of batteries to provide energy to a load.
Limitations of mAh
Today, batteries and battery packs are commonly described based on two separate characteristics: voltage and mAh (the product of current capacity over a finite time period). For example, an AA-cell Nickel-Cadmium (NiCd) battery may have a rated capacity of 800 mAh, and an open circuit voltage of 1.2 volts. These separate conventions have served the industry well when comparing batteries with similar chemical characteristics, but they are far less useful when comparing battery systems with dissimilar chemistry.
A significant limitation of the mAh designation becomes evident when mAh is used to compare two batteries with varying voltages, such as NiCd and Nickel-Zinc (NiZn) batteries. Because NiCd batteries have an open circuit voltage of 1.2 volts and NiZn batteries have an open circuit voltage of 1.6 volts, a NiCd cell rated at 800 mAh does not deliver the same performance as a NiZn cell rated at 800 mAh,. To compare and understand the performance capability of the two batteries, open circuit voltage must be factored into the measurement, which by definition the mAh specification does not do.
A better alternative: mWh
To detail more precisely the performance differences between batteries, the mAh rating—which is the product of current and time—must be enhanced to include voltage. As the product of current, time and voltage, the mWh rating delivers this much-needed measurement improvement.
Returning to the example of NiCd and NiZn cells that are each rated at 800 mAh, the mWh rating reveals the substantial performance difference between the two batteries. With an open circuit voltage of 1.2, the mWh rating of the NiCd cell would be 960 mWh (800mAh x 1.2v). Comparatively, the NiZn cell would have a much higher rating of 1280 mWh (800mAh x 1.6v).
Yes, only by employing the mWh rating does it become evident that there is a significant capacity difference between NiCd and NiZn—a difference that is masked by the limitations of the mAh rating.
Solution: Practical Applications of the New mWh Specification
To demonstrate the many benefits of the mWh specification, we will examine a few common electronic equipment applications for which businesses and consumers depend on a thorough understanding of battery capacity and performance.
Example 1: Digital Photography
Digital photographers want battery packs that provide a high number of total camera flashes with minimal reboot time between each shot. Using mAh, typical NiMH and NiZn battery packs appear nearly identical; only by implementing the mWh designation do the performance advantages of the NiZn battery packs become evident.
A typical NiMH battery pack may offer a digital camera user a total of 250 flashes before needing a recharge. The NiMH battery pack might be comprised of four AA-cell NiMH batteries with a rating of 800 mAh. A comparably sized NiZn battery pack (four AA cells) may also have a rating of 800 mAh rating, and yet the NiZn battery pack can achieve 400 digital camera flashes and lower reboot time throughout the cycle life.
Representational view of NiZn vs. NiMH discharge in a camera application
One reason for the performance difference between the two battery types is that digital cameras, like all electronic devices, have a minimum operating voltage that must be sustained to ensure successful operation. Because NiMH systems typically start at a significantly lower pack voltage than NiZn systems, the NiMH system will fall below the minimum operating voltage of the camera before its NiZn counterpart. Despite the fact that the NiMH battery pack may not have used the entire energy capacity of the pack, the minimum voltage cut-off of the device renders the additional capacity useless.
Because NiZn delivers greater mWh capacity it’s able to provide steady-state optimal performance over a larger portion of the system discharge. This correlates to not only more shots or flashes per charge, but also more responsive electronic performance. In the case of a digital camera this translates into quicker flash recharging and processor speed to allow for shorter recovery time between shots.
The bottom line is that battery pack voltage is not accounted for by the mAh rating convention. The four-cell NiMH battery pack in this example has a pack rating of 4.8 volts (4 x 1.2 volts/cell), compared to the 6.4-volt pack rating of the four-cell NiZn battery pack. This is a significant difference that affects user experience but is overlooked by the mAh rating—and would be accounted for by the new mWh rating.
Example 2: Flashlights
Flashlights provide another example of the variable performance possible between two batteries with identical mAh ratings. A two-cell AA battery system powers many conventional flashlights, and for the sake of this illustration, both the NiCd and NiZn systems have the same mAh rating of 800.
A consumer who purchases either a NiCd- or NiZn-powered flashlight would expect similar performance, based on the identical mAh rating. However, the flashlight with the NiZn cells will produce brighter light throughout the cycle life of the batteries. The flashlight with the NiCd batteries will produce a light that is not as strong, and the light will dim more over time.
Comparison of flashlights using NiCd vs. NiZn batteries
Again, the reason for the performance disparity is the NiCd battery pack’s rating of 2.4 volts, compared to the NiZn battery pack’s higher rating of 3.2 volts. Only by factoring in the battery pack voltage using the new mWh rating can a business or consumer understand the significant difference between the two types of cells.
Example 3: Power Tools
The value of the mWh rating becomes most evident when considering battery packs such as those commonly used in cordless power tools. A battery pack is a combination of cells, and voltage (rather than mAh) is commonly used to measure expected performance. As compared to voltage alone, the mWh rating provides a more meaningful and detailed way to describe, compare and analyze battery packs.
Assuming mAh capacity of 800 for both the NiCd and NiZn cells, a 19.2-volt NiCd battery pack would be rated at 15,369 mWh (16 cells x 960 mWh), while a NiZn pack would be rated at 15,360 mWh (12 cells x 1280 mWh). If a battery pack with a minimum capacity rating of 15,000 mWh were needed for a cordless tool application, the mWh measurement makes it evident that 16 NiCd cells (15,000/960) would be needed, compared to just 12 NiZn cells (15,000/1280).
Using voltage alone for battery packs might suggest that the performance between NiCd and NiZn cells would be nearly identical. In fact, because fewer cells are required, a NiZn pack is more reliable since fewer inter-cell connections are needed. In addition, the battery pack is smaller, weighs less and costs less—all while delivering equal or greater performance. Furthermore, the benefits of reduced size and pack complexity are amplified as the power requirements of a device increase, making the differentiation in performance measured by the mWh rating greater as battery pack size increases for devices such as in electric bikes and vehicles.
Performance gains as battery pack size increases
Conclusion: mWh Specification Offers Greater Clarity and Precision
For decades, the mAh rating has provided a helpful guide to the relative performance of different batteries, but today’s new technologies require a new battery specification that delivers more detailed and comprehensive information about battery capability and performance.
As shown above, the value of the mWh rating is particularly evident when considering batteries of different chemistries that have similar or identical mAh ratings, and yet deliver vastly different performance. The comparison between NiCd and NiZn cells is particularly revealing because their mAh ratings are often similar, and yet NiZn cells are technologically advanced, offering high energy density, superior power density, and longer cycle and shelf life, as well as being nontoxic and easily recyclable.
For businesses and consumers, information is power. By factoring in voltage as well as current and time, the mWh rating offers a superior alternative to mAh—one that provides all the information necessary to make smart, informed decisions about battery capability and performance.
About PowerGenix
PowerGenix has developed and patented a high-energy density, high-cycle life and low-cost Nickel-Zinc battery targeting the market for energy intensive electronics and hybrid electric vehicles (HEVs). Specifically designed to utilize existing Nickel-Cadmium manufacturing processes, techniques and equipment, PowerGenix offers the supply chain scaling and security OEMs require. With its patented rechargeable Nickel-Zinc battery technology, PowerGenix is pursuing applications to replace existing Nickel-Cadmium and Nickel Metal Hydride batteries in the multi-billion dollar rechargeable battery market.
For more information about the advantages of Nickel-Zinc battery technology, contact PowerGenix today.
(858) 547-7300
info@PowerGenix.com
www.powergenix.com
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