Calling on Solar Techies
I have been thinking of going solar for some time and decided to pull the plug. My wants are a 400 to 500 watt system. I have four 12 volt batteries with 340ah. I am not planning on changing b...
Forum Discussion
I think we need to start with a basic understanding of what a PWM solar controller is and what it does and how it is different from an MPPT solar controller. In simplified terms the PWM controller is just a switch (a MOSFET transistor) and a micro controller that measures voltage and controls the switch.
In bulk charge a PWM controller isn't doing anything - the switch is on all the time and the panels put what ever current they can produce into the battery at the voltage of the battery.
In Absorb and Float, the controller starts turning the switch on and off (rapidly) while varying the ratio of time the switch is on relative to when it is off in order to reduce the *average* voltage at the programmed level (eg 13.6 V for float). The current is determined by the battery/load and the controller does not regulate or modify the current in anyway. *This is PWM and the current out always equals the current into the controller*
In an MPPT charge controller you have a switch mode power supply, commonly a buck converter. This is different to a PWM system because in addition to the switching MOSFET it also has a large inductor and a diode (and a bunch of other stuff, but that is not important). This allows the the controller to do two things, control the impedance it presents the solar panel to match the maximum power point from the panel AND to convert the voltage from the panel in excess of the battery voltage to extra current. *This is NOT PWM as the controller is doing more than just switching on and off*.
Now if the battery is drawing less power than the panel can supply, then the controller does not need to track the maximum power point and can just present a fixed impedance to the panel. This can be the case when the controller is operating in constant voltage mode (absorb and float), but even in this mode the power could exceed what is available from the controller without MPPT, in which case a well designed controller will begin hunting for the maximum power point again to provide the battery and loads with the power they need.
It is easy to tell if a controller is operating as a PWM controller - if the current out of the controller is not equal to the current in then it is NOT operating as a PWM controller.
In bulk charge a PWM controller isn't doing anything - the switch is on all the time and the panels put what ever current they can produce into the battery at the voltage of the battery.
In Absorb and Float, the controller starts turning the switch on and off (rapidly) while varying the ratio of time the switch is on relative to when it is off in order to reduce the *average* voltage at the programmed level (eg 13.6 V for float). The current is determined by the battery/load and the controller does not regulate or modify the current in anyway. *This is PWM and the current out always equals the current into the controller*
In an MPPT charge controller you have a switch mode power supply, commonly a buck converter. This is different to a PWM system because in addition to the switching MOSFET it also has a large inductor and a diode (and a bunch of other stuff, but that is not important). This allows the the controller to do two things, control the impedance it presents the solar panel to match the maximum power point from the panel AND to convert the voltage from the panel in excess of the battery voltage to extra current. *This is NOT PWM as the controller is doing more than just switching on and off*.
Now if the battery is drawing less power than the panel can supply, then the controller does not need to track the maximum power point and can just present a fixed impedance to the panel. This can be the case when the controller is operating in constant voltage mode (absorb and float), but even in this mode the power could exceed what is available from the controller without MPPT, in which case a well designed controller will begin hunting for the maximum power point again to provide the battery and loads with the power they need.
It is easy to tell if a controller is operating as a PWM controller - if the current out of the controller is not equal to the current in then it is NOT operating as a PWM controller.
