BFL13 wrote:
Here is how you calculate your "expected amps" to the battery. Use your own set-ups to get your own numbers but it all works the same way. (Panels aimed at the high sun to get full amps in each case)
PWM---you get the total Isc. (Imp has no meaning with PWM)
So my three 100s, each with 6.2Isc, get me 18.6 amps to the battery. Regardless of panel temperature etc etc.
MPPT- Panel temperature say 50C vs ambient 25C means 10% power drop.
So my 300w worth of panels becomes a 270W right off the top.
Wiring loss array to controller (based on Imp for the wire gauge) say it is 2% (which is pretty good) So now your power is 270 - 5.4 = 264.6w
Now the controller itself has an efficiency between input and output- less efficient when input is higher v than output v--eg 24-12 worse than 12-12. Typical 24-12 is say 95%. so now output watts is 264.6 x 95 = 251.4w (which is why I am ok with my 260w Tracer and my "300w" array.
Now you divide the output watts by battery voltage to get your amps to the battery. You get more amps with a lower voltage battery of course. (So the MPPT salesman always picks an example where the batts are low)
251.4/ 13v = 19.34 amps
251.4/ 13.5v = 18.6 amps (same as with the PWM)
251.4/ 14v = 17.96 amps
So there you see that you don't gain much in actual battery charging by spending big bucks on an MPPT controller. Lots of hype vs actual measurements tells the tale.\
Sometimes you do get a touch more with the higher voltage MPPT in low light but it is sort of trivial in the big picture. IE, in low light you don't get much anyway. A little more than not much is still not much!
I do have MPPT and PWM so I don't care. I just don't get any more amps with the MPPT, but I also have a 24v panel I use sometimes and you can't run that with a PWM, so I have the MPPT that can run it OR the 12s either way.
Isc should drop with temperature as well, so that will affect PWM max current vs temperature as well.
But I do agree, a good PWM controller is pretty much on par with a MPPT controller, but at noticeably lower cost. And MPPT controller only buys more amps when the battery can take it and when you have low IR drop in the cableing and when the MPPT controller has good efficiency. once you get near probably 80 percent charge if your panel sizing is reasonable both will start reducing current below what the panels could deliver anyway.
I've got 480 Watts of panels on the roof and 480Watts of portable panels. same panel mfg. same Voc, same Imp and the midnite kids can be set up as a master and slave and the roof controller becomes the master, and when adding in the portable I've got in theory 60A total charging current. Now that's in theory, and today with the roof panels flat and being near 45 degrees latitude the sun angle limits the roof panels to around 20A total. meanwhile one 160W portable panel angled was giving close to 11 A so I could get 30A out of the portable.
In this case the MPPT controller was giving me 10.5 A but Isc on the panels is 8.9A but I had only about a 10ft run of 10ga wire. In this case the MPPT was giving more than a 10 percent improvement over a PWM controller. But the cable run was short which helps a great deal.
so the angle does have a noticeble effect this time of year and in the fall. during the summer not near as much, and of course much less effect the further south one is. One noticeable advantage of portable panels. easy to get angled for max output.
