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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!)

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  • BFL13's avatar
    BFL13
    Explorer II
    Mar 20, 2015
    gatorcq wrote:
    how about this. Are you using an external amp meter with shunt? or are relying on the digital display of the bluesky.
    Of course you need 2, in / out


    I don't have a BlueSky, I have an Eco-Worthy MPPT and also a Solar30 PWM. I use the Trimetric (with its shunt) as an ammeter, the controllers' ammeters on their own outputs, and my multimeter to cross-check those readings.

    I have an IR thermometer you aim from about a foot away. It gets the same ambient temps as others so it seems accurate. To measure panel temp, I aim it up under the tilted panel.

    At 16C ambient I get 44C panel (diff 28) and with 25C ambient I get 51C (diff 26) panel. Some blurbs say your panel temp is about 25C or higher than ambient, so that works.

    That suggests that with the 25C panel temp they use for STC, ambient must be freezing out, at 0C.

    The problem is the changes in amps to the battery don't match the change in temps according to the graph above. At 44C I got 15.5a (13.2 Vbat) and at 51C I got 13.5a.(13.5 Vbat) Power out went from 205w to 182w.

    In the graph, there is not much change in power between 44C and 51C panel temp. At 51C I should have a panel power loss of 10%. 230-23 is 207. I lose another 1% with wire loss. With 205 in and 182 out that is a 23w loss from 205 = 11.2% which is way too high for controller efficiency loss.

    The output at 44c of 205w from 230w panel says the controller is ok. At 44C panel should be down say 8% on that graph? So 230- 18.4= 211.6 and 211-205 is 6 for 3% so controller efficiency would be 97% which is within its specs.

    So how could things go so wrong between 44C and 51C? Just one of the mysteries I need to solve this summer.
  • gatorcq's avatar
    gatorcq
    Explorer
    Mar 20, 2015
    how about this. Are you using an external amp meter with shunt? or are relying on the digital display of the bluesky.
    Of course you need 2, in / out
  • BFL13's avatar
    BFL13
    Explorer II
    Mar 20, 2015
    liborko wrote:
    Lots of errors in your post, BFL13:

    -first of all, all abbreviations are written with capitol letters, W, V, A etc.

    -Isc is a SHORT CIRCUIT CURRENT. You get it only when panel terminals are SHORTED. You don't get it when panel is connected to the controller or directly to the batteries.

    In your example with 230W panel,29.3V and 7.27A your boosted output current is 15.32A @ 13.4V, your actual boost is (15.32A-7.27):7.27.100=110%, not 8%. Your PWM + 8% would be in ballpark with 12V nominal panel volts(36 cell panel). Your 60 cell panel produces much higher boost in charging amps and that is the beauty of MPPT controller (something Salvo does not understand).

    And finally, you should invest in an ammeter for a real scientific work.


    At least I spelt my name right!

    When I say I "get the Isc to the battery" what I mean is I get the "same current in amps as the Isc". I will continue to express it that way for convenience. :)

    On the IV curve, the I is what Isc would be. I take the Isc with the panel shorted and then hook up and then see the same amount of amps at the battery with the Trimetric (allowing for other 12v things), the solar controller's output on its display, and if it is below 10amps also using my multimeter --lots of cross-checking.

    In the second part, I need more coffee. But note that the panel does not boost any charging amps. The buck converter in the controller does. Is it correct to say "boost" the amps with a "buck" converter?

    Seriously, I would like to understand what is really going on with my solar and I do need to get an ammeter on the panel- to- controller path. That is my missing number. With that, I could see my controller's input watts as well as its output watts I get now, and then I would have my controller's efficiency at the time.
  • liborko's avatar
    liborko
    Explorer
    Mar 20, 2015
    Lots of errors in your post, BFL13:

    -first of all, all abbreviations are written with capitol letters, W, V, A etc.

    -Isc is a SHORT CIRCUIT CURRENT. You get it only when panel terminals are SHORTED. You don't get it when panel is connected to the controller or directly to the batteries.

    In your example with 230W panel,29.3V and 7.27A your boosted output current is 15.32A @ 13.4V, your actual boost is (15.32A-7.27):7.27.100=110%, not 8%. Your PWM + 8% would be in ballpark with 12V nominal panel volts(36 cell panel). Your 60 cell panel produces much higher boost in charging amps and that is the beauty of MPPT controller (something Salvo does not understand).

    And finally, you should invest in an ammeter for a real scientific work.
  • JiminDenver's avatar
    JiminDenver
    Explorer II
    Mar 19, 2015
    Instead of wasting the money for a respectable controller, the same power and a bit more info, BFL could spend the same money on a second 230/ eco-w set up, have twice the power and not worry. Worked for me. :)
  • Almot's avatar
    Almot
    Explorer III
    Mar 19, 2015
    BFL13 wrote:
    Right now at Rathtrevor again (Jan, Feb, and now Mar) wondering why anyone would go to Mexico to get away from this (seals, herring, eagles, killer whales, pilot whales, geese, gulls).

    Sun. BC and PNW in winter is pretty much grey, and sometimes I've heard it rains.
  • BFL13's avatar
    BFL13
    Explorer II
    Mar 19, 2015
    Almot wrote:
    Yeah, well... I like reading test results, but sometimes reports from the BFL Research Center are not easy to understand. Or - why this is even important ;)...

    May I suggest getting a proper MPPT controller with a proper display that shows panel amps In, controller amps Out, and watts for those who need them.


    Sorry Almot, it is all about Science. :) I will test in future, my MPPT Eco-W controller in 24v vs 12v and that against the PWM Solar30 in 24v mode. Of course I have to do that twice, when panel temp is 44C and when it is 51C so it may not be until July when I get that all solved.

    Right now at Rathtrevor again (Jan, Feb, and now Mar) wondering why anyone would go to Mexico to get away from this (seals, herring, eagles, killer whales, pilot whales, geese, gulls). Oh well, to each his own!
  • Almot's avatar
    Almot
    Explorer III
    Mar 18, 2015
    Yeah, well... I like reading test results, but sometimes reports from the BFL Research Center are not easy to understand. Or - why this is even important ;)...

    May I suggest getting a proper MPPT controller with a proper display that shows panel amps In, controller amps Out, and watts for those who need them.
  • ken_white's avatar
    ken_white
    Explorer
    Mar 17, 2015
    BFL13 wrote:
    "... 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."...


    LOL, you still at this?

    This rule of thumb is just another way of restating power in = power out when losses are ignored...

    Take care,

    Ken

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