My battery bank size

What is the Vmp and Imp of the solar panels?

And what brand/model/link of solar charge controller?

-Bill

MPPT has one practical advantage (in my humble opinion)--High Vmp-array voltage (upwards of 100 VDC for the "typical" ~140-150 VDC max input controller). This allows the owner to use either (or both) much smaller diameter copper wiring and/or placing the solar array farther from the battery bank+charge controller (upwards of 100's of feet is now practical--vs 10's of feet for a 12 volt PWM controller to array distance).

A secondary advantage is that MPPT controllers tend to be newer designs with lots of processing power... Better logging, computer networking, more "functions"/"Options", more sophisticated charging cycles, etc.

The third "advantage" to MPPT is, in my humble opinion, is the MPPT advantage with COLD arrays (typically freezing or colder weather).

However, this really only applies to folks with cold winters--And the advantage becomes a bit less because of the shorter number of hours per day of sun light (few hours of sun during winter, 10-15% more power from a low starting base of power, is not much power).

And it also depends on how people run their systems... As Photowhit says, if you cycle the battery only to 90% or 80% SOC, the battery charging voltage is going to be very near the Absorb set point--And either type of controller is going to be "throwing away power" at that point (PWM or MPPT). Secondary is that MPPT controller + Networking + etc. use more Watts just to run--Their percentage efficiency falls in morning/evenings when you have partial sun (i.e., operating power / harvested power = instantaneous efficiency) the average efficiency of a MPPT will be lower than a "simple" PWM controller.

After years of helping people here--I have stopped even trying to estimate the harvest differences between MPPT and PWM controllers. The efficiency (and panel losses--Vmp depression affecting MPPT--And Vmp-array vs Vbatt issues with PWM), that, on average, the difference is not enough to "care about" for the typical user (warmer/non-winter operation, usually run battery bank charging >~85% SOC, "over paneled" with 10% or greater rate of charge, etc.).

More or less, most people would be hard pressed to "see/measure" a 10% difference between MPPT and PWM output for their installation. And with solar panel being extremely low cost (compare to 10-20 years ago)--If 10% difference is "important", then just add 10% more solar panels and call it a day.

I suggest that people do two different paper designs (at least) for their systems. Do a MPPT and a second PWM design and see what works best for your needs.

In general, below ~400 Watt system, PWM can be less expensive. For systems >~800 Watts, generally a MPPT system will work out better ("cheap" >>140 watt panels with Vmp~30 volts or higher; less money on copper wire; various networking/logging options).

-Bill

lkruper wrote: »

One thing I don't understand. I thought that MPPT took the higher voltage and converted it to a lower voltage at a higher amp. So why would this not be true at the different stages of charging including the Absorb point. That being said I do recall reading someone say (somewhere?) that the final stage of an MPPT (float?) was actually PWM.

If you are familiar with AC transformers--MPPT controllers (an buck mode switching power supplies) are sort of like variable AC transformers (Variac). The MPPT controller matches the available AC voltage (and current) to the needed load voltage (and current). Efficiently (~95% efficiency).

https://en.wikipedia.org/wiki/Buck_converter

More or less, the governing equation for a MPPT charge controller running greater than 50% of rated output current (efficiency wise) is:

• Varray * Iarray * 0.95 charger eff = Vbatt * Ibatt

You can see, that as Varray approaches Vbatt--Then Iarray (array current) has to pretty much equal the Ibattery (battery charging current).

And the equation for a PWM controller is (roughly):

• Iarray * 0.99 charger eff = Ibatt
• Vbatt*Ibatt / Vmp-array*Imp-array = overall efficiency of PWM controller

And for the PWM controller--Varray has almost nothing to do here... Once Varray is greater than Vbatt, then Iarray will flow to charge the battery.

What does all of this mean? The "wild card" here is the operating voltage of the solar array (Vmp of the panel and Vmp-array).

More or less if you have a standard Vmp-array~17.5 volts or so at ~75F cell temperature, that voltage falls to towards 15 volts on a warm summer day (or even lower). And when you charge a lead acid battery, when the battery gets >~80% state of charge, the Vbatt approaches 14.8 volts (for a flooded cell battery at room temperature).

So, for two identical systems (Vmp~18 volt solar panels and Vbatt ~14.8 absorb voltage charging), the "spread" in the voltage between Vmp-array and Vbatt-charging is very small (voltage drop in wiring and through charge controller).

It means that MPPT controller has "very little voltage drop across the controller is small, and there is little "excess voltage" that can be converted into "extra" charging current.

And the "simple" PWM controller is (mostly) acting like a "closed switch" from array to battery bank--So it is wasting very little energy (no switching losses, the computer is much slower/lower power in PWM controller vs MPPT, etc.).

The only time that the two (otherwise "identical") systems would diverge is when the temperature of the solar cells are very cold--That causes Vmp and Vmp-array to rise, and gives the MPPT controller "more head room" to convert the "excess voltage of the solar array" into "higher charging current" for the battery bank.

OR--If you (for example) put two 17.5 volt panels in series and run them to a 12 volt battery bank through a MPPT charge controller, you can use much smaller AWG wiring from the array to the charge controller and can save lots of money on copper wiring costs.

If you where to connect a PWM controller in the above system (two 17.5 volt Vmp panels in series for Varray~35 volts), the PWM controller would be about 50% efficient as the "extra voltage" is completely wasted with a PWM controller.

As the batteries reach full charge, they accept less charging current. Once a MPPT controller feeds less current to the battery (battery is holding 14.8 volts and the current is running from 13% rate of charge down to below 1% rate of charge), then MPPT really does not "help"... Both the MPPT controller and PWM controller are not taking all of the energy available from the solar array.

However, if you have DC loads (inverter, water pump, etc.), that you are running during the daylight hours--The MPPT controller can still give you "more power"--Because even though the battery bank does not need the full charging current, the MPPT controller basically supplies the loads with operational current directly--And the "MPPT" function can still be useful and gather more energy from the solar array vs what a PWM controller can.

Many people will turn on "optional" loads after the battery bank has reached float so they can use the "extra solar panel energy" that would otherwise be lost once the batteries are full... Pumping water to a cistern, irrigation, clothes washing, running an electric cooker of some time, etc.

-Bill

And just to be clear--PWM (pulse width modulation) is use both in PWM charge controllers and MPPT (maximum power point tracking) charge controllers.

The difference is a MPPT controller has a good size inductor as an energy storage/converter section.

A PWM controller just is a "switch" (on/off) from Array to Battery bank. There is no energy storage inductor/coil--And so a PWM controller cannot "efficiently" take high voltage/low current from the array and "efficiently down convert" to the low voltage/high current needed to charge the battery bank (and run other DC loads).