The circuit was designed with the following goals:
Single board smart solar power system with self-contained light. 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. Capable of handling resistive, inductive and capacitive loads.
Nominal battery voltage: 12V Maximum solar panel current: 9 Amps Maximum load current: 10 Amps, higher power loads can be connected directly to the battery. Night time battery drain current: approximately 150 micro-amps Temperature compensation for use in variable climates Radio-quiet operation
See the full SPC3 kit specifications for a more detailed list.
The upper half of IC1 is the heart of the charge controller. It acts as a 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 opto-coupler to turn on. The output of the 4N35 activates FET Q1, which connects the solar panel power to the battery through Schottky diode SD1. When the float voltage is reached, the circuit oscillates just above and below the float setting and charging current is switched on and off of the battery. The oscillation frequency is set by the battery charging characteristics and the current that is available from the solar panel. The maximum switching frequency of IC1 is limited by the hysteresis provided by R11.
The thermistor TM1 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, this is used for driving the bipolar state indicator, LED1. Shorting the equalize terminals causes the float voltage setting to rise, this is useful for occasionally overcharging (equalizing) a battery.
Schottky diode SD1 prevents battery power from leaking back into the solar panel at night. Diode D1 is wired as a crowbar circuit, if the battery is connected in reverse, the fuse to blow, saving the rest of the circuitry from destruction.
When momentary switch S1 is turned on, transistor Q5 is turned on. This activates the comparator circuits formed by IC2d and IC2c. When the circuit is active and battery voltage is above the LVD setpoint, the IC2d comparator output goes low and transistor Q4 is switched off. This allows gate drive voltage to reach Q3, Q3 connects battery power to the load. Resistor R16 bleeds off the gate drive voltage from capacitor C8 when the circuit shuts off.
When momentary switch S1 is turned off, the Q3 gate drive voltage is shorted to ground, causing load power to be switched off.
IC2a and IC2b form a square wave oscillator, the output of the square wave is fed to the D3,D4,D5 voltage tripler circuit to produce the gate drive voltage for Q3.
Once the LVD circuit has been turned on, diode D7 feeds the load power back to the circuit to keep it active. Power to LED3 and LED4, the two white LEDs is routed through R17, a current limiting resistor.
Regulator VR2 provides a reference voltage for the oscillator circuit and for the battery voltage comparator ladder IC2c and IC2d.
IC2c is the low battery sense comparator. When the battery reaches 0.6V above the shutoff point, the IC2c output goes high and turns on the yellow LED. Resistor R21 sets the difference between the low voltage indication and the power shutoff.
Leave the circuit connected until the battery voltage has reached or exceeded the desired full-charged setting, this is typically around 13.8V at room temperature. Turn the float voltage setting counter clockwise until the LED alternates red and green. Tweak the setting until the LED blinks and the battery voltage is where you want it to be at the full state. When the LED is alternating red/green, it is normal for the battery voltage to vary by about 10mV.
If a battery pack with a float voltage setting below 13V is used with the SPC3 (LiPO or NiMH for example), zener diode ZD1 should be changed to a 1N4740 (10V) and resistor R6 should be changed to 250K.
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 load can be operated during daytime charging and at night.
The on/off switch controls the load like a normal power switch. If the battery voltage drops to 0.6V above 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.
