Forum Discussion

BFL13's avatar
BFL13
Explorer II
Mar 16, 2015

Expected Amps with MPPT

BlueSky 3000 manual says for a rule of thumb,

"... Ignoring conversion losses the conversion process produces an
output current roughly equal to PV current times the ratio of PV voltage to battery voltage. If a 60 cell module is operating at 25V with 5 amps of output, and battery
voltage was 13V, output charge current from the 3000i would be about 5 amps times 25V ÷ 13V or about 9.6 amps."

I have figures for my 230w panel last summer, where it was operating at 29.3v for Vmp, and output current (calculated using 96% efficiency) was 7.27a and battery voltage was 13.4v (amps to the battery was 15.32 at the time)

So how does that come out using the BlueSky rule of thumb?

7.27 times 29.3/13.4 = 15.9a (without conversion losses) and I got 15.32.

So that seems to work! It does not tell you what happens with higher panel temps when power drops off.

Another set of numbers from real life with hot panel.

MPPT is still holding at 29.3vmp but now power was less so output current was 6.47 (calculated) battery 13.2v

6.47 times 29.3/13.2 = 14.36a I got 13.88a to the battery

So my conversion loss was 15.9-15.32 =0.58 from 15.9 = 3.6 and
14.36-13.88 = 0.48 of 14.36 = 3.3 so in ball park for in-the-field measuring. My est of 96% leaves 4 so 3.6 is reasonable.

EDIT--Maybe I counted the loss twice? I already used 96% controller efficiency to calculate the panel amps output. (Estimating controller output watts as seen on display is 96%, get input watts, divide by Vmp to get calculated amps)

With higher panel temps Isc goes up, but with MPPT buck conversion, Isc means nothing. Amps is only from watts/volts. So panel output amps drop with panel power while Vmp stays the same (it appears!)

Didn't know that. Maybe it's wrong. The trouble is I have no ammeter on that panel output. Only know the controller output watts and panel volts with an assumed efficiency percentage. Got to get an ammeter on that line to confirm calculation, but it does seem to come out fairly close as is.

without knowing any of that stuff, my other way to get "expected amps" is just to take what PWM should get and add say 8% for MPPT but that only works for a cold panel.

eg that 230w with MPPT above with 15.3a actual to the battery.

PWM means 130w Isc 8.2a so 230/130 x 8.2= 14.5a Add 8% (1.16a) for MPPT = 15.66. So you can get somewhere close that way, but only for a cold panel (and panels get hot!)

20 Replies

  • JiminDenver's avatar
    JiminDenver
    Explorer II
    Mar 21, 2015
    Awww that was just one of those days when I was sick of looking at controller specs. The thing is that we both know what the 230/Eco-w can do tracking and if I never wanted a mounted system, it would probably be enough to do most of what we did last year. Well ok maybe not run the little window shaker but I could easily have my pot of joe in the morning.
  • BFL13's avatar
    BFL13
    Explorer II
    Mar 20, 2015
    Glad to hear JiminDenver is still in the game!

    Was worried when he said he would leave all that "stuff" at home when going camping next time, as though all that "stuff" were not the whole point of going camping-- to see if the "stuff" works!
  • JiminDenver's avatar
    JiminDenver
    Explorer II
    Mar 20, 2015
    When the last system sells I have $1500 to play with. The panels are $525 so I'll be able to afford a "respectable" controller. ;)

    I'll try my HF Clamp on meter while I demonstrate a system tomorrow. I've never been thrilled with it but haven't needed one bad enough to replace it so far.

    What I look forward to is comparing the 245w mono side by side to the 250w poly. The 230w poly comes within a amp of the 245w mono so it should be interesting.
  • BFL13's avatar
    BFL13
    Explorer II
    Mar 20, 2015
    JiminDenver wrote:
    It's been 77 F here (25c) but not today. In any event my results are off due to altitude.


    Unless you secretly got that fancy controller once planned, you don't have the input readings anyway. Unless you have a clamp-on ammeter.

    What is of interest ( :) ) is the many Forum members here who have the extremely expensive MPPT controllers and all the read-outs imaginable, who won't post figures to show they are not having the same problem I am having.

    They love snickering about the "Chinese junk" Eco-W and how that is no doubt the problem, but never post their own figures to prove it!

    I suspect they are afraid to check their own set-ups in case they find the same thing happening with their fancy controllers, and would be too embarrassed to post their results. :)

    I am not complaining about the Eco-Worthy as such. I suspect there is something else going on--but that could still be only with my set-up--no way to know in this vacuum of comparable info. :(
  • JiminDenver's avatar
    JiminDenver
    Explorer II
    Mar 20, 2015
    It's been 77 F here (25c) but not today. In any event my results are off due to altitude.
  • MEXICOWANDERER's avatar
    MEXICOWANDERER
    Explorer
    Mar 20, 2015
    Clarity of the atmosphere, presence and type of cloud edging effect, ambient temp, panel temp, degrees latitude, date and inclination of the earth to the sun. I think you made your hoop .005" bigger than the basketball. Learning is always fun. Just keep in mind to not become too severe a taskmaster.
  • BFL13's avatar
    BFL13
    Explorer II
    Mar 20, 2015
    The temperature is just a possible factor in what I am seeing. My object is to understand and maybe fix why my max amps to the battery in the summer is so much lower than in the spring.

    My Eco-Worthy controller only displays output watts, battery volts and amps to the battery. it also shows panel Isc when it does its "demo" trick, but that is not the amps I need, which is Imp.

    I get the "panel voltage" from either end of the panel-to-controller path with the panel connected. I take this as being the Vmp the controller has selected for the moment. Typically, 29.3 at the panel vs 29.0 at the controller, so I have a 1% loss on that path.

    Last summer, JiminDenver suggested it might be the fault of the controller's temp comp. The controller is in the shade in a breezeway, so not in the sun. I put ice bags around the controller and got its ambient down quiet a bit, confirmed with my IR gizmo, and it didn't help any with the amps. Anyway, I will explore this whole thing some more this summer.

    The blurbs all say my PWM should only come up close to the MPPT when the panel is hot, but at 25C ambient I get more amps with the same watts of PWM than with MPPT, so that seems peculiar if you believe the blurbs. I do know when I see 13.5 amps MPPT and I could get 14.5 with PWM that I get annoyed! I will explore using PWM with the 230w panel, but that means making the batteries into a 24v bank, which is not convenient. I will do it just for the testing, but not to live with. While I have the bank at 24v I will also try the MPPT so I will have a comparison between 12v and 24v with that too.

    If anyone is already at 25C ambient, on solar with MPPT, and has input and output readings to share, that would be very interesting. maybe it is just my set-up! Or not?
  • liborko's avatar
    liborko
    Explorer
    Mar 20, 2015
    I don't see much point worrying about panel temperature because there is not much you can do about it other than install the panel with high enough gap above the roof for good air circulation. If you want to know what the panel is doing in relation to the temperature, you have to measure voltage and current from panel, not the output from the controller. I don't know what your controller displays, mine shows output voltage and output current from the panel as well as from the controller.
  • Salvo's avatar
    Salvo
    Explorer
    Mar 20, 2015
    I've found measuring panel temperature with IR probe is not very accurate. I use a K-type thermocouple that comes with this Harbor Freight meter.

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