A lot has been written about solar panels and chargers but it appears that there is still a lot of confusion. I would like to clarify this by giving an overview on solar panels followed by the different charge regulators. *SOLAR PANELS*
Solar panels are made up of P-N junctions stringed in series and parallel. A single P-N junction produces about 0.7V at 25 deg C. Thus a panel with 36 cells/junctions will produce an open circuit (not loaded) voltage of about 25.2V at 25 deg C. This voltage is highly temperature dependant and becomes less at elevated temperatures.
Secondly and most often misunderstood is the fact that solar panels behave as current sources and not voltage sources. This means that the output current is automatically limited by the panel wattage. There is therefore NO NEED FOR A CURRENT LIMITER in the controller as long as the correct controller is chosen for the solar panel output. (A battery is is voltage source of which the maximum current is limited by its internal resistance resulting in hundreds of amps when shorted.
Thirdly the maximum output power of a 36-cell solar panel occurs at about 16-17V. When not loaded the output power is zero (Vo*Io) where Vo is more than 20V and Io is zero. When shorted out Vo is zero and Io is equal to the specified short circuit current. The output power is again zero in this case. It is thus clear that the maximum output power occurs somewhere in between she shorted and open circuit point which happen to be around 16-17V. This operating point is temperature dependant and generally applies for a panel surface temperature of about 50 deg C. *SOLAR CHARGE CONTROLLERS*
There are four types of charge controllers on the market.
*Bang-bang type (selling for R100 to about R350)*
These controllers use either a relay or transistor and connect the panel directly to the battery while the battery voltage is below the fully charged voltage. When a voltage sensing circuit sees that the full voltage is reached (14.2V depends on manufacturer) the solar panel is disconnected until the voltage drops below the full voltage. This process repeats itself over and over. THESE REGULATORS CANNOT BE USED TO POWER LOADS (lights, etc.) WITHOUT A BATTERY BECAUSE THE PANEL IS PERIODICALLY CONNECTED AND DISCONNECTED FROM THE LOAD *PWM (Pulse Width Modulated) Step down (Buck) converters*
These can be used to power a load without a battery. The conversion efficiency is in the order of 95% which means that 95% of what is put into the controller will be available on the output) There are three versions of PWM controllers.
* *Basic PWM controller – (selling for about R300 to R1,000) *These controllers pull the solar panel voltage down to about one volt above the battery voltage (to complicated to explain here) which means that the panel voltage is in the range of about 13V to 15V which is slightly below the typical maximum output power point of most solar panels. This means that you will be losing about 15% of the output power of your solar panel because you are not operating at the maximum power point. This loss should be weighed up against the cost of a bigger panel of power point tracking controller.
* *Power Point Tracking PWM controller – (Selling for R1,000 plus) *There are mainly two types namely MPPC and MPPT controllers.
* *MPPC (Maximum Power Point Control)* These units contain circuits that pre-sets the operating point of the solar panel at about 16.7V which is correct for a 36-cell panel with a surface temperature of 50 deg C. The lost 15% of solar panel could be harvested under these operating conditions as long as the panel surface temperature is around 50 deg C. This implies that the maximum power cannot be extracted from a solar panel in very cold or extreme heat conditions but it should be weighed up against the cost of a MPPT controller. The selling price of these units should only be slightly more than the price of a basic PWM controller.
* *MPPT (Maximum Power Point Tracking)* These controllers are complicated and contain circuitry that constantly calculates the maximum power point by measuring both panel voltage and current and then shifting the operating point to track the maximum power point. *BUT BEWARE -* *the cheap versions are often operating above or below the maximum power point with little time spend at the maximum power point. *Claims made that these controllers can extract 30% or more power from a panel should be questioned. Expect a power extraction gains of about 15%.
I hope this helps to clarify some panel and controller issues because we live in a world of buzz-words and sales gimmicks intended to sell products to customers that are often not in a position to verify the claims. Members that require more technical detail can contact me directly. Best regards
Manie du Preez