Solar Panel Regulator

Here's a snippet of the schematic that shows how the solar panel is regulated. It's connected to the battery when the battery needs to be charged and disconnected when the battery is fully charged (that's the simplistic explanation). Actually it's connected and disconnected many thousands of times a second in order to control the current flowing into the battery in order to get the optimum charge.

The part in the middle marked IRL3714 is the power transistor it's an N-Channel MOSFET transistor. The connection labeled RC3/CCP2 is the connection to the microcontroller's (uC) PWM peripheral (the CCP2 part). When the uC asserts that pin high (5 volts) then the MOSFET conducts from the Drain ( the top part, connected to the panel positive lead) to the Source (the bottom part connected to ground (labeled GND). So this is just shorting the panel positive and negative leads together. You can short a panel and it won't hurt it. It will just send it's maximum current through the lines.

When the uC pulls that pin low (0 volts) then the MOSFET does not conduct and the current will flow through the diode D3 to the battery, thus charging it. The diode keeps the current from flowing from the battery to ground (that would be very bad as it would waste all the energy in the battery and batteries don't like to be shorted and in some cases they can explode).

So it's just that simple to control energy from a solar panel. You can just build a cutout type charger and not use a PWM peripheral to switch at high frequency. It won't charge the battery to capacity but it will get you 70% of the way there and it will save the battery from over charging.

With the PWM you can control the voltage of the battery by varying the PWM percentage based on the battery voltage coming in from you battery voltage sensor (see previous post). This way you can hold the battery at whatever voltage you set. For a 3-stage lead acid battery charger it's something like this (check the data sheet for your battery to be sure).

1) Bulk charge - 100% until the battery reaches the saturation level, around 14.5 volts

2) Saturation charge - hold the battery at 14.5 volts for a set time (based on the batteries Ah rating (75 Ah would be about an hour).

3) float charge - hold the battery at 13.8 until ready for use

A benefit of the short type circuit shown here is that current through your panel lines stays somewhat constant, instead of turning on and off dramatically during every switching cycle. The current on/off can cause other problems in your circuit as well as creating an unintentional radio transmission antenna out of your wires that connect your panel to your controller.