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,

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.

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.

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© 2007 Penton Media, Inc.