Figure 4: Simple solar PV charging of a lead-acid battery for a woodshed light
The circuit is shown in Figure 4. In this case op-amp A does act as a comparator with hysteresis, similar to the one in Fig 1 but without the diode, but R3 and R4 draw 20μA from the ADR292 output, so that when the comparator output is high and approximately 6μA flows into the ADR292 from R6 its net reference current is still out rather than in and so there is no loss of accuracy.
As in the previous circuit the numbered resistors, R1-R6, must be 1% to eliminate the need for any other adjustments to the circuit. All are E12 values, except R1, which consists of two E12 resistors in series.
The comparator has 0V output when the battery voltage exceeds 13.8V, which turns off T1 and T2 and turns on T3 and the green LED in the tricolour indicator LED. With charge turned off, and the LED drawing 1mA, the battery voltage will gradually fall. The comparator changes state again when the battery voltage drops to 13.4V and turns the PMOS power switch back on, connecting the solar panel directly to the battery again, providing its output voltage is high enough. If the solar panel is unilluminated no current flows when T2 is turned on since the solar panel and the battery are isolated from each other by the 1N4001 diode.
If the solar panel is not producing power when the battery voltage drops below 13.4V it will continue to drift down, from self-discharge or from current drawn by the 4W white LED lamp. If it should fall as low as 12.1V the red warning LED in