Solar complexity- mixing different watt panels mounted on tow vehicle and trailer

[dreadlocks]

is that it is constantly manipulating the panel voltage as conditions change, in order to extract maximum watts to feed the DCDC buck converter.

Kinda, MPPT is constantly putting maximum load on the panel/array so it can extract maximum watts to feed the buck circuit.. this dynamic load does influence the panels voltage, but thats just a consequence of full load on it, the voltage sags a little.. you can watch your panels fire up as the sun gets more intense, in the mornings they start off at a higher voltage when they output less watts, but by solar noon when its draining its max power the voltage has gone down because the load is higher and its sagging more as the wattage goes up.. however some of that is due to the panel heating up too.

a PWM is just banging on/off (Pulse) as a static load regulate the voltage down to the desired output, if your load needs 12v and the panel is producing 18v its effectively discarding the 6v it dont need and the wattage associated with it.. the panel is still producing that 18v, its just being wasted..

Neither PWM or MPPT have any direct control over input voltage, MPPT just changes the resistance (load) dynamically until max output is achieved, and keeps doing this as the inputs change.. If your MPPT controller dont have a load on it capable of sinking the panel's full wattage into then its no longer tracking maximum output, like when your battery is full or only capable of using a fraction of the available power.

 

[john61ct]

The maximum load (power extracted) is the goal.

Varying the voltage, sometimes by a lot many times in a short period is the method, the MPPT means to that end.

Which functionality is impeded by having to deal with multiple non-identical panels.

 

[rruff]

Varying the voltage, sometimes by a lot many times in a short period is the method, the MPPT means to that end.
Which functionality is impeded by having to deal with multiple non-identical panels.

As in the example I did earlier, it won't be much of a loss if the panels have similar voltage and current curves, even if they are facing in different directions or one is shaded.

 

[rruff]

the MPPT is constantly adjusting to the constantly varying voltage coming out of the panels, not the other way around..
A power source (panels) determines the voltage it runs at, not the consumers of that power (controller)

So, I have a 40W 12V panel that runs at ~20V with no load.

What is the voltage if I hook it up to my car battery? What is the voltage if I hook it up to a little 5A-hr Lipo cell?

 

[dreadlocks]

If the load is higher than the supply its going to cap the supply out in Constant Current where its outputting the most watts it can at the voltage being put in depending based upon the loads resistance, but as those batteries fill up its power is going to drop below your supply's max load output and the voltage read at the batteries is gonna climb up to the constant voltage and fry em @ 20V

Unregulated 20V charge source for either your car battery or a little 5A LFP cell is gonna murder em.. its no different than taking that 40W 18vdc laptop charger you found in a junk bin and trying to use it on a 12v battery.. it would work if you tossed a MPPT or PWM regulator on it, but otherwise its just a matter of time before a cell fails and they all cascade.

 

[rruff]

Unregulated 20V charge source for either your car battery or a little 5A LFP cell is gonna murder em..

The correct answer is that the panel will run at ~12V for the car battery and 3.3V for the LFP cell, and they will stay around these voltages until they are fully charged... because the battery voltage determines the panel's voltage, not the other way around.

Sure, you can kill a battery if you keep pumping current through it after it's fully charged... but that isn't what we were discussing.

 

[dreadlocks]

you dont pump current into any battery when its fully charged unless your charge voltage is higher than its fully charged voltage, which is exactly what were discussing.

The only way you get away with the no solar charger crap is with very large lead banks and very small solar panels, like a 5w panel on a 100AH battery, it will always have enough resistance to suck up that half amp even when full, it will never not have more load on it than it can supply.. its dumb as heck because you have no control over this 'float' voltage but it can be done and thats how those lil tiny "dashboard" solar panels work, but they still typically boil away the battery, just a very slow death counting on the fact that they rarely even see direct sunlight.. and if you hook that 5W dashboard solar panel up to a small battery in say a lawn mower or motorcycle it'll toast it just the same because the'll draw less than a half amp just chilling fully charged.

LFP is even worse, when they are full they'll drop to basically zero current if you hold voltage so the'll let a raw solar panel toast their BMS and cook the cells.. you never wanna high volt those expensive things.