Solar Charge Controller Design - Help Needed!

[Arun M]

Hello Everyone!

I have started designing a solar charge controller to charge a 12V lead acid battery. Load for the battery will be a dc motor.

What I have done till now is that I had designed a Buck - Boost Converter for a output of 14.5V. For the feedback circuit I had to use op amps for controlling the mosfet. In addition to these I had to provide overvoltage, undervoltage and overcurrent protection all using op amps.

I have made a simulation using Multisim. I need your help for what are the things to be corrected and what should be added.



Specifications are:

1. To cut off the supply from Solar PV when Battery Voltage is higher than 14.5V
2. To disconnect the load if battery voltage is less than 10.5V
3. To disconnect the battery from the Supply if current is high

 

[BradtheRad]

1.

At first I would think it is logical to charge at 14.5 V. It automatically cuts off charge when Spec #1 is reached.

However you will find that charging current is greater when you apply a higher volt level, say 24 V. My solar panels had enough cells to output that amount, with no load attached. I think the manufacturer experimented until they discovered the optimum amount of cells to assemble in one panel.

I made my own charge controller. Basically it was an op amp controlling a mosfet. I designed it to halt charging when the battery reached about 14 V.

2.

Your schematic has a 50V battery. Will this be a real battery? Or is it a voltage obtained from a DC-to-DC converter? Simulated as a battery?

 

[Arun M]

Thanks for the reply!

That 50V is assumed to be the voltage from PV Cell.I had taken this value just for simulation.

Have you used any current sensing, voltage sensing and PI Controller circuit using op amp in your Charge Controller?

 

[BradtheRad]

Have you used any current sensing, voltage sensing and PI Controller circuit using op amp in your Charge Controller?

It was by simple voltage sensing. A zener diode at one input. Potentiometer at the other input.

The op amp turned on a mosfet. It conducted power from the panels to the battery bank.

I adjusted the pot so the op amp would go low when the battery reached 14.5 V (or so). This turned off the mosfet, and charging stopped.

However this is premature. The battery volt level routinely elevates when bulk charging is almost done. But the battery can still take more juice. A taper charge is recommended, so that maximum charging can occur, without overcharging.

However the project would have become quite complicated. So I tried a halfway measure.

I added hysteresis. The mosfet shut off as battery V rose after bulk charging. With charging off, the battery would gradually decline. After a few minutes it was down a volt. Then charging came back on.

On a sunny day the cycle repeated every few minutes. It seemed to be a workable method to continue charging, without causing bubbling and loss of electrolyte.