Laptops, cordless tools, electric golf carts, cordless phones, electric toothbrushes, portable music players, cell phones – all are dependent to some extent on rechargeable batteries. These products offer substantial economic and environmental advantages over products powered by throwaway batteries and are more convenient than corded or gasoline-powered products. Certain products, like cordless tools, might not even be feasible without custom rechargeable batteries. However, every rechargeable battery-powered product wastes some amount of energy in the process of charging a battery – energy that cannot be used by the product, but which still appears on the user’s electric bill and results in increased air pollution from power plants. Using more efficient rechargeable battery-powered products saves energy and ensures that these costs stay to a minimum.

What do we mean when we say "battery charger systems"?
It’s quite easy to name a handful of portable products that contain rechargeable batteries, but just what is a battery charger system and how does it work? A battery charger is a set of electronic devices used to convert high voltage AC electricity from a wall outlet into lower voltage DC electricity that is carefully supplied to a rechargeable battery. There, the electricity is further converted into chemical energy and stored. A battery charger may be a stand-alone device, such as chargers produced by battery manufacturers to recharge common battery types like AA, AAA, C, or D cells or chargers used with industrial fork lifts. On the other hand, battery chargers may be an internal subsystem of a larger product such as a cell phone. See the illustration for an example of how a battery charger might be integrated into a cell phone or similar product and how power from the wall outlet flows through the system. The red box shows the components – internal and external to the phone – that are considered part of the battery charger system.

Battery charger systems include the following three components:

• A power supply that converts high voltage AC electricity into lower voltage DC electricity
• Battery charger circuitry that controls the flow of electric current into a battery
• A battery that stores the electric current provided by the battery charger circuitry in the form of chemical energy

Battery charger systems are used in a wide range of products and differ mainly in size (the amount of power that is meant to flow through the charger when it is recharging batteries) and battery chemistry. Four battery chemistries currently dominate the market, including: nickel-cadmium (NiCd), nickel-metal hydride (NiMH), lithium-ion (Li-Ion), and lead acid (LA).

How many battery chargers are in use and how much energy do they use?
Today, over 800 million products that contain rechargeable batteries and battery chargers are currently in use in America’s homes, offices, retail spaces, and warehouses [1]. This is one of the most diverse groups of products currently covered on EfficientProducts.org, spanning everything from low-power cordless or cellular telephones all the way up to high-power battery-operated forklifts used in industrial facilities or even electric vehicles.

Current market research conducted by Ecos suggests that battery powered products consume over 70 billion kWh of electricity annually. Anywhere from 35% of that energy could be saved cost effectively through relatively simple design improvements.

How can I find today's most efficient battery chargers?
Consumers wishing to identify the most efficient consumer battery charging products can currently look for ENERGY STAR^® labeled products. ENERGY STAR only covers a subset of battery charging products, so EfficientProducts.org recommends some of the following additional guidelines when shopping for products that fall outside the scope of ENERGY STAR's battery charger specification:

• For products that are bundled with external power supplies, check the product’s packaging to see if its external power supply meets the criteria of the ENERGY STAR program. This will be signified either with an ENERGY STAR label or using a small roman numeral (roman numerals V or higher indicate that the power supply complies with ENERGY STAR's external power supply specification). Although this does not address the efficiency of the entire battery charging system, it does ensure that one component of that system is efficient compared to others on the market. NOTE: Some battery charger products (power tools, kitchen tools, lawn care devices, personal hygiene items, and other battery chargers that produce heat, light, or motion) are specifically exempt from this label.
  
  
• If you cannot find a suitable product bearing the ENERGY STAR external power supply label, look for battery charging products that utilize switch-mode power supplies like the one shown below. These power supplies are flatter, more lightweight, more compact, and more efficient than traditional linear power supplies.
  
  

• Try to purchase battery-powered products with indicator lights that indicate when the battery is fully charged. Then, remove the battery from the charger and unplug it after the charge is complete.

What is the definition of efficiency?
The definition of whole-product battery charger efficiency is still a topic of debate in the energy efficiency community, but we do know a lot about how battery chargers operate and where they can waste energy.

The operation of battery chargers can be broken down into three distinct modes. The first is charge mode. This is the time when the battery charger is actively charging a connected battery. So-called “rapid” battery chargers that force high amounts of charge into batteries in a short period of time can consume a lot of energy in this mode.

Once the battery is fully charged, the battery charger enters the second mode of operation: battery maintenance mode. Battery chargers usually consume less power in this mode than when they are actually charging a battery, but some inexpensive consumer appliances continue to draw a high level of power in battery maintenance mode, even after the battery is completely charged.

The third and final mode of operation is called standby mode. This is the time when the battery charger is connected to a wall outlet but the charger is not connected to a battery or a battery-containing product. This situation might occur, for example, if someone leaves their cell phone AC adapter plugged in by their nightstand so that they can charge their cell phone when they go to bed. The AC adapter will continue to draw power from the wall outlet during the day even when it isn’t connected to the cell phone.

Battery Charger System Test Methods
Different organizations have developed slightly different methods for defining and measuring efficiency in battery chargers, particularly when it comes to charge mode. In December 2008, the California Energy Commission’s (CEC) adopted a comprehensive test procedure for nearly all types of residential, commercial, and industrial battery charger systems. It addresses all the modes of operation of the battery charger and measures power factor. The development of this test procedure was funded by California Energy Commission PIER program and Pacific Gas and Electric.

A test procedure that focuses on power tools, lawn care products, hygiene products, and kitchen tools is available from the EPA’s ENERGY STAR program.

The U.S. Department of Energy stated its intent to modify the current national battery charger test procedure to include active mode in its July 2009 public meeting.  For more information, see the DOE website on battery charger rulemakings.  

California Test Procedure Revision History

Summary
Below is a history of revisions for the California Energy Commission Battery Charger Test Procedure. Although only the most recent revisions are shown, the drafting process began in late 2003. The first draft was reviewed by stakeholders at a workshop held in November 2003 at Pacific Gas and Electric's Pacific Energy Center in San Francisco, California. The most recent drafts, comments from stakeholders, discussion documents, and summary of recent workshops are shown for reference below to illustrate the development of the test procedure over time.

Test Procedure Stakeholder Participation: 2008Recent stakeholder comments and participation were addressed through the California Energy Commission's Title 20 rulemaking mechanisms. Public documents associated with Title 20 rulemakings related to battery charger systems, including discussions on the test procedure, can be found on the Commission's web site.

Test Procedure Stakeholder Participation: 2007In order to facilitate stakeholder comments to high priority issues, a working draft of the Battery Charger System Energy Efficiency Test Procedure was released on May 21, 2007. This identified changes that were proposed in response to the comments received on the February 2006 draft as well as additional testing and research that had been completed by the consultant team. Stakeholders may also reference Draft 2 Energy Efficiency Battery Charger System Test Procedure and Energy Efficiency Battery Charger System Test Procedure Comprehensive Comment Summary for a historical record of published drafts.

In addition, the consultant team informed stakeholders of the direction of the final draft and provided opportunity for comment at a Battery Charger System Energy Efficiency Test Procedure workshop on May 30, 2007.  A full list of presentations made during the webinar and other relevant documents may be found below. Written comments were also accepted until June 15, 2007.

• Battery Charger Stakeholder Meeting Agenda
• Battery Charger Test Procedure Development History and Scope
• Technical Context for Battery Charger Workshop
• Proposed Changes to Draft 2 Test Procedure
• Next Steps for Test Procedure Development
• Battery Charger Workshop Comprehensive Notes

Test Procedure Stakeholder Participation: 2005On Thursday, November 17, 2005 the Energy Commission and PG&E held a free one-day workshop at the Pacific Energy Center in San Francisco to discuss the draft test procedure and present energy efficiency data on a wide variety of battery charging products. The workshop also provided opportunities for stakeholders to give recommendations and ask questions about the test procedure. All comments on the test procedure were requested by May 15, 2006. Key documents presented at the workshop are listed below.

• Draft 2 California Battery Charger Test Procedure
• Battery Charger Technical Workshop Agenda
• Workshop Notes
• Workshop Introduction
• California Energy Commission Policy Overview
• ENERGY STAR Policy Overview
• Introduction to Battery Charger System Efficiency
• Test Procedure Overview: General
• Test Procedure: Battery Discharge Test

• Test Procedure: No Battery Mode Test; Charge and Maintenance Mode Test