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,

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%.

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