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The main function of a solar charge controller is to prevent the solar panels from overcharging batteries. The algorithm or control strategy of a charge controller determines the effectiveness of battery charging and solar panel utilization, and ultimately the ability of the system to meet the load demands.
Additional features such as maximum power point tracking, temperature compensation and equalization can enhance the ability of a charge controller to maintain the health, maximize capacity, and extend the life of batteries.
Two basic methods exist for controlling or regulating the charging of batteries from a solar panel - Series and Shunt regulation.
A Shunt controller regulates the charging of batteries by short-circuiting the solar panels and therefore interupting the charging current . A blocking diode is required in series between the batteries and the switching element to keep the batteries from shorting when the solar panels are shunted. This blocking diode "costs" you about 0.6 of a volt to cross and adds to the known overall system wide voltage drop.
The shunt-interupting control strategy is commonly referred to as an "On/Off" type of controller. It will push the batteries up to a preset voltage regulation set point and then turn off. It will then wait for the batteries to fall to a preset reconnect voltage and then turn back on. These contollers are designed for solar panel currents of about 20 amps or less due to high-current switching limitations.
Some clever marketers now call this old "On/Off" type of charging strategy "Low Frequency Pulse Width Modulation" due to the pulsing effect when reaching the finishing charge state. However, this is not to be confused with true Pulse-Width-Modulation (PWM) which will be covered later in this discussion.
A series controller regulates the charging of batteries by open-circuiting the solar panels and therefore interupting the charging current . A blocking diode may or may not be required, depending on the switching element design.
The series-interupting, Pulse-Width-Modulated (PWM) control strategy uses a series element which is switched on and off at a variable frequency with a variable duty cycle to maintain the battery at the voltage regulation setpoint.
In effect, it will push the batteries up to a preset voltage regulation setpoint and then instead of turning off, it will begin to taper the charging current from the solar panels. This strategy has been proven to maintain the highest state of charge with the least amount of water consumption.
In comparison, the shunt-interupting (On/Off) controller is an older stategy that works reasonably well but has some serious drawbacks. They rarely (if ever) reach full charge and are famous for either consuming lots of water or for 'sulfating' battery plates depending on its setpoints. In other words, they require you to pay attention to your batteries' water level and finish charge voltage.
There is a "new kid on the block" which adds Maximum Power Point Tracking (MPPT) to the proven Taper charging strategy. This controller has the ability to "boost" the charging current coming from the solar panels under certain conditions. It does this, in essence, by converting some of the "excess" voltage developed by the solar panels into increased charging amperage. The biggest boost will come when your solar panels are cool and your battery voltage is very low. As the solar panels heat up and the battery voltage rises, the "boost" effect will be somewhat diminished.
Some versions of these controllers aren't truly "tuned" for the needs of the RVer. However, we worked with some manufacturers to design some new controllers with MPPT that are tunable for the needs of the RVer. Check out the 2512iX and the TriStar Maximum Power Point Tracking Controllers
OurPre-Configured SunRunner Systems
have taken into consideration everything discussed above and use tunable PWM and MPPT charge controllers to meet the needs of the RVer!