Radical Redesign Nears for Battery Chargers

Apr 1, 2007 12:00 PM
By Chris Calwell, Policy and Research Director, and Suzanne Foster Porter, Senior Research Analyst,

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.

References

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.