November 20, 1997

Shunt battery charger provides
1A continuous current

Mitchell Lee, Linear Technology Corp, Milpitas, CA

Most battery chargers use nothing more than a series-pass regulator with current limiting. In solar-powered systems, you can't count on having sufficient head room to keep a series regulator alive, so a shunt method is preferable. An example of a simple shunt battery charger consists of an op amp driving a shunt transistor and ballast resistor (Figure 1). The heart of the circuit is IC₁, an LT1635, which contains an op amp and a reference. Operation is straightforward. A feedback divider comprising two 1-Mega-ohm resistors senses the battery voltage. The circuit amplifies the internal 200-mV reference to 7.05V and compares this voltage with the feedback signal.

Temperature sensor RT₁ introduces a temperature coefficient that accurately tracks the battery's correct charging voltage over a wide temperature range. Because RT₁ compensates for changes in battery temperature, position it close to the battery and as far as possible from the shunt elements. When the battery charges to 14.1V, the op amp's output voltage begins to rise, turning on the Darlington shunt and resisting further changes in voltage. Full panel power divides equally between the transistor and the 7.5 ohm resistor when the battery is completely charged. Provide adequate heat sinking and airflow for dissipation as high as 15W.

The charger can handle 1A continuous current, which is compatible with a 20W panel. It's unnecessary to disconnect or diode-isolate the charger during periods of darkness, because the standby current is only 230 µA--less than 10% of the self-discharge current of even a small battery. If you need a different or an adjustable output, you can easily modify the feedback ratio at the 1-Mega-ohm divider. A good compromise between an aggressive charge voltage and a conservative float voltage is 14.1V. Given the cyclic nature of insolation, periodic charging at 14.1V is not detrimental to Gelcells (Johnson Controls Inc, Milwaukee).

The circuit in Figure 1 works with larger or smaller batteries than the one shown. As a rule of thumb, you should size the panel from 1W per 10-Ahr battery capacity (a float charge under good conditions with a good battery) to 5W per 1-Ahr battery capacity (a one-day recharge of a completely discharged battery under favorable conditions of insolation). (DI #2114)

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