Design Ideas

February 3, 1997

Simple circuit charges lead-acid batteries

Dana Davis, Maxim Integrated Products, Sunnyvale, CA

The circuit in Figure 1 charges lead-acid batteries in the conventional way: A current-limited power supply maintains a constant voltage across the battery (2.4V/cell or so, as specified by the battery manufacturer) until the charging current decreases below a certain level (also specified by the manufacturer but typically 0.01C). Normally, C represents the battery's capacity in coulombs. However, in this context, C is a rate in coulombs per second (amperes) that is numerically equal to C.

Choosing the charging voltage involves a trade-off between cell life and charging time. High voltage minimizes the charge time, but, at full charge, the high voltage produces a large overcharging current that shortens the battery's life by oxidizing its grid. To save battery life at the expense of charging time, you can lower this current by lowering the charging voltage.

The ideal compromise is to charge at high voltage until the current drops to 0.01C or so, and then lower the voltage to maintain a low trickle-charge current (less than 0.001C) after the battery is fully charged. You can find the voltage necessary to maintain 0.001C from the battery manufacturer's Tafel curves.

In Figure 1, a boost converter (IC₁) applies a constant 15V (2.5/cell) to the 12V lead-acid battery until it is fully charged. To maintain a trickle charge (overcharging current) of less than 0.001C thereafter, the circuit lowers the battery voltage to 13.2V (2.2V/cell, from the Tafel curve in the Gates Application Manual). Using a flyback transformer instead of an inductor isolates the battery from V_IN and allows V_IN to range above and below the charging voltage. Momentarily pressing the start switch begins the charging cycle.

IC₂ measures the battery-charging current by generating a smaller (1/2000) but proportional current at the OUT terminal (Pin 8). The resulting drop across R₂ produces a voltage at Pin 5 of IC₁. When the charging current drops below 0.01C, for instance, this voltage crosses the internal comparator threshold and drives LBO low. This action turns off Q₁ and shifts the feedback level to maintain a charging voltage of 13.8V. The maximum available charging current depends on V_IN, the transformer's saturation current, and current-sense resistor R₁.

How you adjust or select a fixed value for R₂ depends on the point at which you want the circuit to shift from the higher charge voltage to the lower float voltage. For a rate of 0.01C, you can use the following formula to calculate R₂ as R₂=V_LBI(2000)/0.01C, where V_LBI is IC₁'s low-battery input threshold; typically, 1.5V. Thus, for a 10-Ahr battery, R₂ is around 3 kilohms.

You can test the circuit by producing a graph of output voltage vs load current using a resistive load instead of a battery. From right to left, this graph gives the variation of charging current with battery voltage as a battery is charged. At first, the converter is out of regulation because the battery voltage is less than 12V, and, therefore, is current-limited (supplying maximum current). As charging commences and the current decreases from 1A to 0.1A, battery voltage rises, reaching 15V at around 100 mA (V_IN=5V) to 400 mA (V_IN=15V). Charging current continues to decrease, and the voltage stays at 15V until the current reaches approximately 40 mA. By 20 mA, the voltage decreases to the trickle-charge level of 13.8V. (DI #1979)

FIGURE 1

This lead-acid-battery charger applies high voltage (15V) until the battery is charged and then applies 13.8V to maintain a small trickle charge.