While waiting for Midnite Solar's "Brat" PWM solar charge controller to be released for sale, I came across this succinct statement regarding the concerns which started this thread (see item 2, below):
"Frequently Asked Questions
• How do I size my charge controller?
Take the total wattage of your array (number of panels x watts per panel) and divide it by the voltage of your battery bank to get amps, then round up. The amp rating of your charge controller must exceed this number. If you need any assistance sizing your charge controller, please don't hesitate to call us at 888-899-3509 ext 1.
• Should I get an MPPT or PWM charge controller?
Simply put, if the voltage of your panels is close to the voltage of your battery bank, a PWM charge controller is an affordable choice. However, if your panel voltage is much higher than your battery bank, you should use an MPPT controller because it is designed to convert that higher voltage into amps, which will charge your batteries faster and provide you with a more efficient system."
from:
http://www.ecodirect.com/Midnite-Solar- ... r-brat.htm
I know this has been beaten to death in this thread, but...
Given my previous unsatisfactory experience with an MPPT controller for which I had to wire my 17.6Vmp polycristaline panels in series to increase the voltage high enough for the MPPT controller to have excess volts to convert into amps*), but then losing all or some of my panels effectiveness due to tree and other obstacle shading which is an unfortunate effect of wiring in series, I will likely try a PWM controller for my 17.6Vmp panels which I will wire in parallel because when parallel-wired panels are shaded by a tree limb, etc. the panel is not "turned off" by the diodes in the panel like when the sae panels are wired in series. **
* Because when wired in parallel (i.e., although when wired in series volts are additive, in parallel the volts remain the same) my 17.6Vmp panels would only provide a couple of volts over the 14.2v? necessary to charge a 12v battery for the MPPT to convert to amps.
** I was told over at the Midniteforum.com that because my panels are older they probably only have one or two diodes per panel, which means that if a portion of a panel is shaded the diode will turn off a full one-half of that panel. More diodes per panel are better if you are wiring in series because the function of the diode is to turn off the portion of the panel which is shaded. If my panels have two diodes, one diode turns off one-half of the panel. If the shadow of the limb crosses the areas protected by both diodes, each diode then turns off its one-half of the panel for a whopping 100% loss of charging coming from that panel!
*** My goal eventially would be to purchase monocrystaline panels with sufficiently high Vmp voltage to run an MPPT controller.
**** Since I am consolidating my thoughts here, now that I understand Scalff77's statement it bears repeating here because it adds the variable of the affects of ambient temperature on real life panel voltage output:
With a Vmp of 17.6 you are close to getting something out of a MPPT controller, but a lot will depend on the installation, voltage loss from panel to controller and the temperature and location that you are in. The voltage of a panel will drop in higher temp, so as the voltage goes down there is little room for a MPPT controller to squeak and extra power out.
It would be interesting to see if on a 100F day if my panels' voltage would drop below the 14.2v needed to charge a 12v system...
***** I think what all this really means is that: 1. I should wire my current and any future panels in parallel; 2. For cooler Winter temps, I should run a 30amp MPPT controller; 3. in the 95F to 100F Sacramento Summers I should run a 30amp PWM controller.