mppt rule of thumb

Re: mppt rule of thumb

One rule of thumb should be that efficiency is only one part of maximum power tracking. An important one, but only one parameter of the total harvest.

Re: mppt rule of thumb

As some know from another thread I started about "pushing limits", whereas I used to have two series strings in parallel of 3 Mitsubishi 170w panels I now have only 5 panels to arrange in the best possible way. Because of that thread I'm going to be using only four panels (since five in series slightly exceeds the max Voc for my FM60 and I can't figure any other way to wire five panels).

I have those four panels in series now (still laying on the ground from the storm, I should get them mounted again this weekend).

From THIS thread I see that maybe I should wire them in two series strings of two panels rather than one series string of four? They are 24V panels. The battery bank is 12V.

Is that the correct conclusion?

Phil

Re: mppt rule of thumb

Cariboocoot wrote: »

Phil;

Not necessarily. In your case it would mean some significant re-wiring because more than two parallel panel connections require fuses on each panel string. Plus, the current to the controller will go up; is the wire sized to handle it? And in a similar vein the Voltage will go down, which could lead to a loss in over-all power due to Voltage drop.

No rewiring necessary Coot. I have two wire-pairs from the panel location to the FM60, both fused. Before, there were three 24V panels in series on each for a 72V input.

Now one wire pair has no panels and the fuse is out. Four panels feed the other wire at 96v.

This thread leads me to believe I'd be better off with two panels on each wire-pair, or 48v in to the FM60 feeding the 12V bank instead of the 96V input.

Of course, this thread also indicates I'd be better off with all 4 panels in parallel feeding the FM60 24V but THAT would take a little bit of rewiring.

Saturday's project is getting the panels mounted again. Since they use MC connectors I could try it each way for awhile and maybe see some useful measurements to verify the better arrangement? Once voltage drops are factored in, it may negate any gain from input voltage being closer to battery voltage.

And yes, the fine tuning can be a little more complicated. I just want the maximum power out of the four panels I'll be using. ACTUALLY I want the maximum power out of the five panels I have left but I've given up on that!

Phil

Re: mppt rule of thumb

rabbit_39 wrote: »

Actually, the way I thought it should work is the greater the difference between input voltage (ie array voltage) and charging voltage, the better MPPT works. High voltage, low amp input gets turned into lower voltage high amperage output.

That's why I wired it as it is now, but then I noticed this thread and started re-thinking the way it's connected.

The REAL conclusion I'm coming to is this:

Mount the array with the four panels in series. Get readings.

Cover panels with blanket, change to two panels in series, each set in parallel, uncover panels, get readings.

Cover panels with blanket, change to five panels in series, uncover and get readings.

I'll get the readings as close to the same time as possible which should give me the important reading to compare -- amps to battery -- and I'll post back here and in my "pushing limits" thread.

With the MC connectors it shouldn't take longer than 10 minutes to test each configuration.

The highest amp reading of the first two configurations should be the better choice and may help answer the OP's question.

The third will be higher amps since it'll have 25% more panel, but I'll get an idea of how the FM60 will act using a 120v input, and can multiply the output by 80% to compare with the other methods.

Phil

Re: mppt rule of thumb

baja1 wrote: »

well which way is it?

I certainly don't know "officially" but if you wait until next Monday, I'll let you know the results of my experiment. This test will factor in CC efficiency AND wiring resistance.

Phil

(on edit) The FM60 manual specifically states that a Voc input greater than 150 will damage the unit so I will only be testing two wiring choices this weekend: four 24V panels in series (96V) feeding the 12V bank and those same four panels in two parallel banks of two (48V) feeding the same bank. I will be looking for the best overall efficiency by monitoring the "amps in" to the battery in each configuration. These panels have maybe 25 ft between them and the FM60. The configuration that provides the most "amps in" will be my choice and I'll post the results Monday.

Re: mppt rule of thumb

Ok, I tried wiring my remaining four Mitsubishi 170w panels in two different configurations this weekend. After trying it both ways, I have concluded that the "higher voltage" wins, but not by a lot.

Two parallel strings of two panels (48V) into the FM60 via two 8 awg wire sets with two 30 amp fuses, both wires about 25' from panels to controller yielded 330 watts during early peak sunshine.

Same panels wired all in series (96V) through one of those wire pairs into the FM60 yielded 340watts.

Went back and forth, same results.

I had to use the watts reading (actually a "kilowatt" reading but with the decimal just ahead of the 3rd digit so I just recorded "watts"). On the FM60 the "voltage in" reading jumps all around, same with the "amps in". The only reading that remains steady is the Kw.

The reading on the FM60 Kw is three digits, but the third digit never changes from "0". So the 330w would jump next to the 340w with no intermediate readings. Unlike my two other MX60s that could show "334" or "338", the FM only shows "340" or "330". So the MXs can give a more accurate reading than the FM.

The difference could be less than 5 watts. Or a little more than 10 watts.

But the higher voltage "wins" in my limited experiment and is the way I'm leaving these 4 panels wired.

Phil

Re: mppt rule of thumb

mikeo wrote: »

Probably the slight increase in efficiency of the MX60 with the lower voltage was more than offset by the decrease in voltage drop in the wiring at the higher voltage. So sometimes it doesn't really make that much difference unless you are exceeding the voltage or current ratings of the wiring system or the controller. For longer runs from the panels, I would choose higher voltage normally to decrease voltage drop.

I would think that by DOUBLING the wires, voltage drop may have even been less with the lower voltage since it's only 25' and the difference between 96V and 48V through 8 awg, one pair for the 96V and two pair for the 48V.

Phil

on edit: I suppose I COULD use a formula and figure the voltage drop rather than just "thinking" it'd be less with half the voltage but double the wire, but I didn't... my bad.