Simple Solar Charge Controller

Simple Solar Charge Controller For Lead-Acid Batteries

Hi, This is a simple solar charge controller unit(CCU) that i designed for a solar street light. It would cost less than Rs 150 to build.I would say the performance of this is really a bang for the buck.
It has the following features :

• Low voltage load disconnect
• High voltage charging disconnect( over charge protection )
• Dusk-dawn(automatic) load control.
• Reverse current protection

CIRCUIT DESCRIPTION :

Note: sorry that the schemtic quality is low, the diagram was drawn long before i was exposed to the design softwares

The circuit uses two comparators , One for the low voltage disconnect and the other for the high voltage disconnect.Both the comparators come in a single 8 pin IC  LM358 package.Same 6.2 v zener diode connected through a 10 k resistor is used to provide reference voltage for both the comparators. There are two 10 k presets wired as voltage dividers which can be adjusted to set the high voltage and low voltage disconnect thresholds.  The NPN( BC547 ) transistor along with the MOSFET( IRF540 ) provides a inverting action which enables the dusk-dawn control of the load . There are two output MOSFET's, one for low voltage load disconnect and the other for overcharge protection. The switching MOSFET's IRF540 used here are high current rated MOSFET's capable of handling 28A if proper heat sinks are provided.

This is PCB version of the circuit i designed.

IMPROVEMENTS REQUIRED : 

• This circuit may sometimes cause oscillations around the voltage threshold points. Wiring the comparators in a hysteresis  mode can avoid this. But this requires a bit tweaking with the current design.
• The low cost presets available in the markets are generally not stable,their values vary with the temperature and atmospheric moisture content. Try replacing the presets with fixed voltage dividers using resistors to ensure stability.