Electronic Design

Lead-acid battery charger also monitors terminal voltage

The three-IC power-supply system shown charges typical three-cell lead-acid batteries of the 6-V sealed D-cell type, and provides a manually activated check of the battery capacity. For charging, a conventional switch-mode controller steps down Vin to provide a regulated charging voltage between 2.35 V and 2.4 V per cell (7.2 V maximum). IC1’s OFF terminal provides a means for disconnecting the charging voltage—a necessary condition for testing the battery’s capacity level.

According to Gates Energy Products, Gainesville, Fla., the typical opencircuit voltage for a fully charged leadacid battery is 2.18 V per cell. This voltage declines linearly, passing through 2.09 V at 50% remaining capacity and continuing to 1.98 V at 10% remaining capacity. The voltage imposed during charging raises the cell voltage above 2.3 V. To prevent damage, the battery’s load is disconnected at a minimum limit of 1.8 V per cell. Thus, the key voltage levels per cell are: charging, ≥2.30 V; fully charged, ≥2.18 ; 50% capacity remaining, ≥2.09 V; 10% capacity remaining, ≥1.98 V; battery dead (disconnect load), ≥1.80 V.

To sense these levels, a resistor string connected across the battery produces five voltages for comparison against a reference of 1.25 V (Fig. 2). The result of these comparisons either turns off the 5-V output (dead battery) or illuminates one of the four LEDs. For a proper measurement, you should first open the charging circuit by momentarily closing the Battery Test switch—an action that causes IC1 to turn off Q1. IC1 also limits the peak inductor current by turning off Q1 in response to voltage across the 0.075-Ω resistor Rsense.

The five-comparator IC provides a 33-µA supply current, open-drain outputs, and a guaranteed accuracy of ±1% for the trip levels and their inter-channel matching. In this application, the load is an LDO (low-dropout) linear regulator (IC3), which generates 5 V at 500 mA. It draws only 15 µA, yet the high-power SO package allows power dissipation to 1.8 W. IC3’s extra-low dropout voltage (320 mV) allows extraction of most of the battery energy. Finally, its shutdown capability (pin 4) lets the system turn off VCC to prevent a deep discharge of the battery.

See associated figures 1 and 2

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