Study on Simple Lead-acid Battery Charging Circuit

The MAX668 PPM controller limits the output current , and the flyback transformer provides isolation and flexibility for input voltages above and below the battery voltage. The MAX4375 current -sense amplifier monitors the charge current and uses its internal comparator to switch the flyback converter to trickle charge mode below the threshold.

The lower charging voltage circuit shown in Figure 1 charges a lead-acid battery the traditional way: a current-limited power source maintains a constant voltage across the battery (about 2.4V/cell) until the charging current drops below a current threshold defined by the battery's capacity. At this point, the charger is placed in a trickle charge mode. The current threshold is typically 0.01C, where C refers to the battery's capacity, specified in amp-hours. When charging a battery, "C" refers to the current required, theoretically, to charge a battery to its full battery capacity of C in one hour. In practice, the power lost during the charge cycle ensures that all batteries charged at their C rate require more than an hour to reach full charge. Ideally, you could charge a 5 amp-hour battery in one hour if the charging current was 5A. Also, ideally, a C/10 charge rate (500 mA) would charge the same battery in 10 hours. However, the power losses mentioned earlier increase beyond these two time spans for these charging times. Figure 1. This lead-acid charger applies a high voltage (15V) until the battery is charged, then applies 13.4V to maintain a small trickle charge. The charging voltage involves a trade-off between the life of the cell and the charging time. A high voltage minimizes the time required to fully charge, but it produces a large overcharge current, shortening the life of the battery by oxidizing its grid. To save battery life at the expense of charging time, this current can be lowered by reducing the charging voltage. The ideal compromise is to apply a high voltage until the charging current drops to about 0.01C, then reduce the voltage to maintain a low trickle charge current (<0.001C) until the battery is fully charged. The battery manufacturer's "Tafel" curve determines the voltage necessary to maintain 0.001C. In Figure 1, the step-up converter (IC1) applies a constant voltage nominally 15.4V to a 12V lead-acid battery until it is fully charged. To keep the trickle charge (overcharge current) less than 0.001C thereafter, the charging voltage is reduced to about 13.4V. Using a flyback transformer, the inductor is not isolated from V, and allows the charging voltage to range above and below V. To start a charge cycle, apply 5V to SHDN to be active low. The OUT terminal (pin 2) produces a voltage proportional to the battery charging current that IC2 measures. R2 drops the voltage across pins 3 and 4. For example, when the charging current drops below 0.01C, this voltage crosses the threshold of the internal comparator and drives COUT1 low and sets COUT2 to high impedance. By disconnecting COUT2, the feedback level shifts, thereby changing the charging voltage to approximately 13.4V. The maximum available charging current depends on V, the transformer saturation current, and the output voltage versus a load current, rather than a battery resistor. The load current of the load test circuit in Figure 1 is the current sense resistor R1. Figure 2. From right to left, this graph shows the change in charging voltage as the battery is charged. Initially, the converter is regulating because the battery voltage is below 12V and is therefore current limited (maximum current supplied). As the battery voltage rises, the charging current changes as shown. Figure 2. In the circuit of Figure 1, the applied voltage and charging current vary as shown, during a charging cycle.