Hello all
I have a pv panel rated at 250w with the following specs
Voc : 21.6V
Isc : 15.8A
Vmpp : 18v
Impp : 13.9 A
and i already have 10m copper wire with cross section size of 6.0mm² (one way length).
I’ve used the online web calculator for a suitable mppt controller for a 12V system and it says no match.
When I’ve used the spreadsheet version (after inputting the pv specs) and selected the 100/20mppt as an example, it does show that the solar module configuration is accepted.
Your issue is the solar panel voltage predicted at the MPPT terminals at higher ambient temperature with the 10m cable run. If the cable is shorter (7m) or the max temperature lower (40°C) then the SmartSolar MPPT 100/20 is OK. The on line calculator may have slightly different defaults for the current and voltage temperature coefficient, different voltage drop calculation or just different margins for voltage. See if you can find an alternative panel with a higher Vmp if you have not yet purchased the panel.
Thanks for the reply, increasing the number of panels to 2 in series and keeping the 10m wire length wire the online calculator i do get the 100/30 mppt as a suggestion.
Now if i select allow oversizing above 130% , the 100/20 mppt is suggested.
I’m highly considering the 100/20mppt model as i do want the load output feature from the controller which the 100/30 does not have.
Is it ok if the pv power ratio is 172% with the 100/20mppt controller (with a 12V system) while the panel voltage and Isc is within the specs of the controller or should i play it safe and go with the 100/30 mppt model?
Thanks
The 100/20 is max input voltage 100V and max input current 20A, you are inside these with 72% over panelling so you are OK. You will produce max 20A output.
Thanks for the reply. When a mppt controller is getting higher input power ( in this case about 500 watts in peak sunlight at around noon) and is outputting full 20amps ( 14.4volts x 20amps ), is the excess power dissipated as heat by the controller heatsink or does it only use only the amount of power needed to charge the battery.
The MPPT will only draw from the panels what it can output. So 14.4 * 20 = 288W. The MPPT will only collect this amount from the 2 panels. Everything above is clipped. You’ll see something like 36V and 8A being drawn from the panels in the above scenario. The MPPT turns this into 14.4V and 20A max to charge your batteries.
It’s a good idea to have 2 panels in series. The reason is that the MPPT will only start charging the batteries when reaching battery V + 5V. With only 1 panel that is a bit tight with a Vmpp of 18V.
I hear this strange idea again and again. No, PV modules don’t simply release energy; they create a charge separation within the module. A module exposed to sunlight is like a battery with a positive and negative pole, and as long as nothing is connected to the poles, no current flows and no energy is released. And if you connect a load, just like with a battery, only as much current will flow as the load draws due to its resistance.
By the way: A solar module gets hotter when no current flows away. The energy dissipated as current reduces the energy available to heat the module, so the module gets less warm.
