The circuit was designed with the following goals:
Single board smart light switch. Analog simplicity with common parts for ease of repair. High efficiency: low loss charging and minimal idle current. Radio-quiet operation, low frequency charge control switching.
Nominal battery voltage: 12V Maximum solar panel current: 6 Amps Maximum load current: 6 Amps, more with an external solid-state relay Night time battery drain current: approximately 10 micro-amps Temperature compensation for use in variable climates Radio-quiet operation
The upper half of IC1 is the heart of the charge controller. It acts as a combined comparator/oscillator circuit. When the battery voltage is well below the float voltage setting, IC1 turns on, this causes the LED to turn red and the 4N35 to turn on. The output of the 4N35 activates FET T1, which connects the solar panel power to the battery. When the float voltage is reached, the circuit oscillates above and below the float voltage setting as the charging current gets switched on and off of the battery. The oscillation frequency is mainly set by the battery charging characteristics and the current that is available from the solar panel. The 10n capacitor across the upper half of IC1 limits the maximum switching frequency. The 4.7M resistor across IC1 causes the circuit to have some hysteresis, seperating the charge/float switch points.
The thermistor modulates the float voltage setting slightly, the full voltage setpoint rises in colder temperatures. The lower half of IC1 always produces the opposite output from the upper half of IC1 for driving the bipolar LED. Shorting the equalize terminals causes the circuit to stay in the charging state, this is useful for occasionally overcharging (equalizing) a battery.
Diode D4 prevents the battery from draining back into the solar panel at night. Diode D5 is a crowbar, if the battery is connected in reverse, it causes the fuse to blow, this saves the rest of the circuitry from destruction.
When the momentary switch is turned on, transistor T4 is turned on. This activates the comparator circuits formed by the two halves of IC2. As long as the battery voltage is above the LVD setpoint, the upper IC2 comparator goes high and FET T2 is switched on. Once the LVD circuit has been turned on, T4 continues to stay on via the current through the 1N4148 diode.
The IC4 regulator provides a reference voltage to compare the battery voltage to. The voltage on IC2 pin 8 tracks the battery voltage. When the battery voltage drops near the shutoff point, the lower half of IC2 turns on, causing the yellow low voltage warning LED to light. When the battery voltage drops further, to the cutoff point, the upper half of IC2 goes low, causing FET T2 to turn off, cutting power to the load. The bias current through the 1N4148 diode also shuts off, T4 turns off, and the rest of the circuitry loses power.
If the LVD circuit is on, switching the momentary load control switch to off lowers the comparator inputs, causing IC2 pin 1 to go low. This shuts down the LVD circuit as described above. High capacitance loads (above several thousand microfarads) will tend to keep the circuit on for a while when the off switch is pressed. Adding the circuit shown in the dashed box will help to speed up the discharge of the capacitance.
Resistor R16 has been added to the circuit for the SPC2-a mod. It adds positive feedback to the LVD circuit to improve the speed of the shutoff operation and reduce heating on MOSFET T2.
As sun shines on the solar panel, the circuit will pass charging current to the battery. The dual color LED will turn red while solar charging is taking place. When the battery voltage rises to the full setpoint, the charging current will be periodically cut off and the LED will alternate red and green.
The on/off switch controls the load like a normal power switch. If the battery voltage drops to near the shutoff point, the yellow LED will light up. If the battery voltage drops further, the circuit will shut the load off, preventing deep discharge of the battery. This greatly extends the life of the battery.