Power Electronics

Radical Redesign Nears for Battery Chargers

As new markets for energy-efficient battery chargers emerge, designers can take advantage of techniques for testing and improving charger efficiency.

Energy-efficiency funding is expected to grow rapidly in future years as utilities respond to their internal goals for greenhouse gas reductions and legislation such as AB 32 in California. AB 32 mandates an absolute reduction in total state greenhouse gas emissions of 25% by 2020, at a time when the state's emissions are still rising by roughly 1% per year. An absolute reduction of 70% to 80% in greenhouse gas emissions is needed to stabilize the climate (see “Climate Context” on page 21).[1]

The California Energy Commission (CEC) has already proposed a series of initial policies for securing those reductions, including mandatory efficiency requirements for external power supplies and many other types of consumer electronics. Hearings on an additional round of energy-efficiency standards are scheduled to get underway in the last half of 2007, addressing battery-charging systems and other energy-consuming products.

The U.S. Department of Energy is also considering mandatory efficiency standards for external power supplies and battery-charging systems. Meanwhile, Europe is evaluating mandatory and voluntary policies to improve power-supply and battery-charger energy efficiency through its Energy-using Products (EuP) effort.[2]

It is only fitting that the consumer electronics, battery-charger and rechargeable-battery industries be asking: “What are we doing right now to reduce the amount of energy our products consume by at least 70%? What more could we be doing?”

Undertaking such innovation now could mean the difference between having a market-leading technology to sell during the voluntary phase of efficient battery-charger policies, or scrambling to buy or license one from a competitor when regulatory deadlines loom.

After tabulating market research and measured energy consumption results in the laboratory and in homes for hundreds of different electronic products, Ecos Consulting concluded that there are now more than 3.6 billion electronic products in use in the United States consuming more than 300 billion kWh per year. This is about 10% of all national electricity use.

In those products, the largest opportunity to save energy lies in improving the efficiency of ac-dc power conversion. This goal is already the focus of numerous mandatory and voluntary programs worldwide. The next largest energy-saving opportunity is in the battery-charging process, as there are more than 1 billion products that charge batteries.

Studies conducted by Ecos Consulting on actual battery-charger products reveal some of the opportunities for improving the efficiency of these products. The studies make use of a proposed procedure for testing battery-charger efficiency that is expected to become a standard test procedure for agencies such as the CEC when their battery-charger regulations go into effect.

The Battery-Charger Opportunity

Ecos Consulting estimates that products containing rechargeable batteries currently consume about 42 billion kWh of electricity per year in the United States.[3] That means the annual electric bill for operating these products is roughly $4 billion. The electricity use per product is often low, but the number of products in use is immense.

After measuring the energy consumption of 62 different battery-charging products in dozens of product categories, researchers from Ecos Consulting and the Electric Power Research Institute (EPRI) observed some clear differences between best-in-class and typical energy efficiency in each category. Certain qualities are typically found in the most energy-efficient battery-charger designs:

  • Best-in-class products typically convert high-voltage ac from the wall outlet to low-voltage dc very efficiently (75% to 90% efficiency), because upfront losses in the power-conversion process cannot be recovered later through clever circuit design. The linear power supplies typically found in low-cost battery chargers are often only 40% to 60% efficient, and can also consume multiple watts of standby power instead of the 0.5-W-or less level seen in highly efficient designs.

  • Best-in-class products carefully monitor the charging process to determine state of charge, providing only the minimum amount needed of battery maintenance energy thereafter. Episodic pulses of maintenance energy for those battery chemistries with higher self-discharge can improve efficiency significantly over nonstop charging, for example.

  • Best-in-class products charge their batteries rapidly enough to meet consumer demands for convenience, but not so rapidly that the batteries suffer substantial heating that requires mechanical ventilation to cool them.

  • Best-in-class products draw virtually no power when batteries are removed from the charger, employing a simple mechanical or electrical switch to disconnect power from the ac mains when no battery is present.

  • Best-in-class products typically use Li-ion-based or lead-acid battery chemistries to maximize coulombic efficiencies in the charge and discharge processes and minimize self-discharge losses. Nickel-cadmium (NiCd) and nickel-metal-hydride (NiMH) batteries are normally less efficient at discharging, charging and maintaining charge.

The widespread redesign of present battery-charging systems to incorporate at least some of these energy-efficient design practices would cut the products' annual energy use from 42 billion kWh per year to about 23.5 billion kWh per year. The resulting savings would be 5.5 Rosenfelds — the equivalent of an annual electrical output of 5.5 typical new coal-fired power plants, each of which would cost about $1.5 billion to construct today with the necessary associated transmission-line infrastructure. (One Rosenfeld equals 3.33 billion kWh per year.) More radical redesigns to maximize efficiency, minimize product size and favor portability could save even more energy, allowing many products to reach savings of 70% to 80%.

Measuring Efficiency

Before diving into the design consideration of particular types of battery chargers, it is useful to consider what metric and test procedure should be used to characterize and measure energy efficiency. The general notion of battery-charger system efficiency is straightforward: Divide the output energy by the input energy to determine the percentage efficiency (Fig. 1). Note that the ac-dc converting power supply, the charge control circuitry and the battery itself are all part of the battery-charger system, while the utility grid is external to it on the front end and the final end use powered by the battery is external to it on the back end.

But what functions should the battery charger be performing when input and output energy are measured? Two extremes have been considered to date, with neither being wholly satisfying from a technical perspective. The first would be to only compare dc energy that can be recovered by discharging a battery to the ac energy needed to charge that battery via its charger. This approach is theoretically simple and straightforward. However, it fails to capture maintenance-mode energy use, which can be significant, or standby-mode energy use, where chargers continue to consume power indefinitely with no battery present.

An alternative approach looks only at the energy use when no battery is present (standby mode) and when the power level in a fully charged battery is being topped off (maintenance mode). This approach may simplify the task of measurement, but misses the maximum power consumption of the device and fails to measure it while performing its intended function — charging batteries.

Fortunately, it is possible and actually advantageous to combine both approaches into a single test procedure. Some chargers can complete the battery-charging process rapidly (in 30 minutes or less) and then abruptly drop into a much lower-power, easily discernible maintenance mode. Other chargers draw a very similar ac power level indefinitely, regardless of the state of charge of the battery they are charging or the time required to charge it.

A combined test procedure makes no attempt to distinguish precisely between charge, maintenance and cell-equalization modes. Rather, the procedure allows the test to run for a long enough period that each can occur for the time needed, in whatever proportions the manufacturer chooses for that particular battery size, chemistry and application.

Fig. 2 presents the results of recent testing conducted in Ecos Consulting's laboratory, showing charge, maintenance and no-battery mode power consumption for two common battery-charger systems. The Li-ion cellular phone charger actually draws more ac power than the NiMH-powered handheld radio during their respective charge cycles. However, it completes the charge process more rapidly, drops into a lower level of maintenance-mode power when charging is complete and consumes less power when no battery is present. Its power supply has an average efficiency of 73%, compared to 43% in the radio, allowing its battery-charging circuitry to operate more efficiently across its full range of functions.

The NiMH product exhibits no discernible difference between charge and maintenance modes, thereby missing an opportunity to save energy by providing the battery with only the amount of current it needs to maintain its level of charge, rather than what it could tolerate without damage. It also continues to draw 0.8 W even when no battery is present.

The overall percentage efficiency differences between the two systems are significant. The overall system efficiency of the radio charger and its battery is about 6%, since its measured battery capacity at 0.2C is 3.9 Wh, and its total charge and maintenance energy consumption over a 24-hour period is 63 Wh.

By contrast, the overall system efficiency of the cellular phone charger and its battery is about 45%, because its measured battery capacity at 0.2C is 3.6 Wh and its total charge and maintenance energy consumption is 8 Wh. This comparison illustrates the magnitude of energy savings possible in battery-charging products with more efficient design and the role of a standardized test procedure in highlighting those energy-savings opportunities.

The table summarizes the 24-hour charge and maintenance efficiency and no-battery mode power consumption measurements from 195 battery chargers measured according to the proposed standard test procedure described previously.

Product category Count Devices tested in charge mode Typical chemistry Efficiency range on a 24-hour charge and maint. cycle (%) Avg. effi-ciency on a 24- hour charge cycle (%) No-battery mode range (W) Average no-battery mode (W)
AA battery charger 45 7 NiMH 2 to 16 11 0.18 to 3.09 1.10
Auto battery 1 1 LA N/A 25 N/A 1.86
Camcorder 1 1 Li-ion N/A 54 N/A 0
Camera 2 2 Li-ion 13 to 56 35 0 to 1.16 0.58
Cordless phone 5 5 NiCd/NiMH 3 to 7 4 0.98 to 3.06 0.04
DVD player 1 1 Li-ion N/A 42 N/A 1.39
Egress lighting 1 1 LA N/A 30 N/A 1.46
Forklift 2 2 LA 28 to 40 34 13.41 to 50.32 31.87
Golf cart 2 1 LA N/A 47 N/A 205.60
Laptop 3 3 Li-ion 59 to 69 64 0.52 to 3.29 1.87
Lighting 1 1 LA N/A 34 N/A 1.00
Mixer, cordless 1 1 NiCd N/A 7 N/A 0.50
Oral care 3 3 NiCd 4 to 11 7 0.59 to 1.66 1.21
Power tool 86 33 NiCd 4 to 54 18 0 to 10.95 2.50
Handheld radio 1 1 NiMH N/A 2 N/A 0.82
RV battery charger 4 4 LA 22 to 28 25 26.28 to 69.66 49.31
Shaver 9 4 NiCd 4 to 13 8 0 to 0.67 0.31
Sweeper, automatic 12 5 NiCd 11 to 26 19 0 to 3.45 0.92
Toys 4 2 NiCd 4 to 19 12 0.73 to 1.34 1.00
Wheelchair/scooter 2 2 LA 26 to 33 29 16.27 to 49.05 40.52
Wireless telephone 9 9 Li-ion 24 to 64 39 0 to 0.94 0.08
Total 195 89
Table. Battery-charger laboratory data. Data were collected by the Cadmus Group on behalf of EPA in 2005 for the development of an Energy Star specification and by Ecos Consulting and EPRI in 2006 on behalf of the CEC. N/A's appear where data were not collected.

The subset of products for which charge-mode testing was conducted appears in Fig. 3. The range of measured efficiency results is extraordinarily broad, from a low of 2% to a high of 69%, which indicates just how large the potential still is for improving battery-charger efficiency.

Which technical approaches were employed by the products that achieve very high charge- and maintenance-mode efficiencies and very low standby power consumption? EPRI researchers modeled the energy consumption of a basic, two-piece battery-charging system with a linear power supply and resistive regulating element to better understand typical approaches to consumer-grade cordless tool charging.[5]

As Fig. 4 illustrates, such products commonly operate at an efficiency of about 10%, as measured via the test procedure described previously. Note that most of the losses occur in the linear power supply itself.

Overall efficiency for linear chargers will always be limited by the fact that a dissipative element — a resistor or transistor — is being used to control the dc charging current. The replacement of the linear charger with a switch-mode charger improves efficiency significantly because this dissipative element is eliminated entirely.

There are two types of switch-mode chargers: single piece and multipiece. The single-piece switch-mode chargers are more difficult to design and manufacture than the multipiece, because they must be designed as a system tailored to meet the needs of a specific battery pack. Every new charger would need to be tested for UL approval and other engineering standards, making the redesign process relatively time intensive and costly.

Multipiece chargers that employ a switch-mode power supply in series with a dc-dc converter are simpler and less expensive to design (Fig. 5). A switch-mode power supply can be specified and purchased from a power-supply manufacturer. A separate dc-dc converter can be used to regulate the charging current to the battery.

Although this multipiece approach is not as efficient as using a single switch-mode converter, it is much easier to specify and manufacture, because the dc-dc converter and other control components can be packaged with the battery-powered product or the battery itself. The system efficiency of the two-piece charger and battery system is approximately 50%, an enormous improvement over the 10% efficiency estimated for a comparable linear charger.

Other techniques for improving power-conversion efficiency, such as synchronous rectification, resonant switching, increasing battery system voltage and hysteresis charging (Fig. 6), are discussed in more detail in the EPRI primers for power-supply efficiency and battery-charger system efficiency posted at www.efficientpowersupplies.org and www.efficientproducts.org, respectively. Gains of 20 to 30 percentage points in 24-hour charge- and maintenance-cycle efficiency are possible by carefully employing combinations of these design strategies.

Next Steps for Manufacturers

The proposed test procedure described previously was first drafted in 2003 and has been revised multiple times since then. Manufacturers have one more opportunity to provide input on the wording and technical details of the test procedure before it becomes final in the third quarter of 2007. To download a test procedure draft for review and read other materials associated with the test procedure's development over time, go to www.efficientproducts.org/bchargers.

Manufacturers may also wish to test their products according to the new test procedure to benchmark their efficiency against that of similar models.


  1. Eilperin, Juliet, “Humans Faulted for Global Warming,” www.washingtonpost.com/wp-dyn/content/article/2007/02/02/AR2007020200192.html.

  2. “Environmentally Friendly Design of Energy-Using Products: Framework Directive for Setting Eco-Design Requirements for Energy-Using Products (EuP),” http://ec.europa.eu/enterprise/eco_design/index_en.htm.

  3. Herb, Kim, and Calwell, Chris, Battery Charger Market Characterization. Prepared for the CEC by Ecos Consulting, 2006.

  4. Tackling Climate Change in the U.S.: Potential Carbon Emissions Reductions from Renewable Energy and Efficiency by 2030, ASES, January 2007.

  5. Kamath, Haresh, Geist, Tom, Foster Porter, Suzanne, and May-Ostendorp, Peter, “Designing Battery Charger Systems for Improved Efficiency: A Technical Primer.” Prepared for the CEC by EPRI and Ecos Consulting, 2006.

Climate Context

With each new update of the United Nations' scientific findings on climate change, the implications become more stark for those countries and industries that have not yet made commitments to reduce greenhouse gas emissions. In the face of continuing growth in the global population and economy, absolute reductions of approximately 70% to 80% in worldwide emissions of carbon dioxide, methane and other greenhouse gases are needed by 2050 to stabilize the global climate.

Some of these reductions will be achieved by cutting the amount of greenhouse gases that power plants emit per unit of electricity produced — accelerating the development of wind and solar alternatives to coal-fired power plants, for example. In other cases, we will continue to refine technology for capturing or sequestering carbon-dioxide emissions from conventional power plants.

However, both options can be quite expensive, placing multitrillion-dollar investment demands and significant time constraints on the global utility industry at a time when existing generation, transmission and distribution infrastructure is in great need of attention. Indeed, it would be challenging enough to meet anticipated growth in electrical demand with nonpolluting energy resources, let alone build enough carbon-dioxide-free electrical capacity to offset existing emissions by three-fourths.

The more likely scenario is that the majority of needed greenhouse gas reductions will come from improvements in the efficiency with which we consume energy — an approach that is faster, cheaper, cleaner and more effectively targeted to the problem at hand than building more power plants and power lines.[4] Energy savings can substitute directly for new generation, preventing the need for more fossil-fuel combustion and its associated greenhouse gas emissions.

U.S. electric utilities already invest $2 to $3 billion per year on programs to help their customers reduce electricity consumption. These programs deliberately shift customers' purchasing patterns toward more energy-efficient products through financial incentives, marketing programs, retailer training and customer education.

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