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BFL13's avatar
BFL13
Explorer II
Jul 30, 2014

Heat 1, MPPT 0--More on That

I have been noting that my 230w panel with MPPT is getting fewer amps to the battery lately in this hot weather, with the back of the panel at 51C according to the temp gun pointed at the white backing from a foot away. This with the panel tilted up and lots of fresh air, not flat on a roof.

When it was cooler out, I was getting 15.5 amps (panel aimed at high sun, good insolation conditions) now I am getting about 13.5-14 amps

Some digging finds this article stating a 10% loss can be expected in heat, and the graph at the very bottom of this other link shows how at 50C, Pmax is 90% of normal.

http://www.solar-facts-and-advice.com/solar-panel-temperature.html

http://www.windturbine.ca/sun_panels.html

Going by the amps to the battery as an indication of panel output, it seems to be about what I am seeing. 10% of 15.5 is 1.55 and that from 15.5 is 13.95

I have gone through the business of confirming my wiring is ok and all that, and still can't get "all my amps" so I did suspect it was heat causing the problem. But it is good to see that the amount of loss is "as expected" with some actual measurements, and that there is nothing gone wrong with my solar set-up that needs fixing.

Anyone noting similar results might be able to relax now! :(

BTW in that article where he says to use higher voltage panels to compensate for the heat loss, I think he meant higher wattage (more solar)

4 Replies

  • BFL13's avatar
    BFL13
    Explorer II
    Jul 30, 2014
    If battery acceptance is too low to test the solar, I turn off the solar, and add some load till the total draw from load and battery is more than what the solar can ever do. Then I turn on the solar and see the difference. That diff is the solar max it can do pointing at high sun in blue sky.

    Yes PWM wastes power. Who cares? You get the full amps, which is all you care about. Forget about power.

    In the afternoon with the batts near full, you can use your extra amps to run things for free. Makes no diff 12v or 24v, PWM or MPPT. That extra is there, no advantage to MPPT over PWM for that.

    Your limit there is the max amps you can get. PWM no change with temp. MPPT you lose amps in the heat, gain some in the cold. So it is all about your personal scenario.
  • JiminDenver's avatar
    JiminDenver
    Explorer II
    Jul 30, 2014
    Bfl

    Are you running a load when you get these numbers or is it charging only? I ask because I rarely charge while it's hot, only recovering from a a heavy load like running the microwave. When just charging, I rarely see the 16-17a I know the 245's are good for. Put a big load on the system and I get the max no matter what time of day and it will stay that way until the bank returns to float.

    Now I know nothing of PWM but from what I've read you lose watts early on when the batteries voltage is low. I'd rather have the effect of heat after the batteries are in float at mid day than when I really needed full power in the morning when the batteries are low.

    One way or another it is a compromise.
  • BFL13's avatar
    BFL13
    Explorer II
    Jul 30, 2014
    With the panel tilted up in my contraption and ambient 25C there is panel temp of 51C on the back of the cells in full sun, EVEN WITH the nice sea breeze blowing past the back of the panel at ambient of 25C.

    So IMO the fan would not cool down anything. The sun on the panel top is what heats it up. I was seeing 44C on the back with ambient at 15C and the "full" 15.5amps to the battery.

    The change in ambient is much greater than the change in panel temp by proportion. I have not got this all figured out, but I am not wishing it would go back to 15C this summer instead of 25C just so I can get another amp or two!!!!! :)
  • ktmrfs's avatar
    ktmrfs
    Explorer III
    Jul 30, 2014
    temperature effects are why solar in partly cloudy western oregon can actually be more effective than sunny summer Arizona. 80F vs. 110F outside. And why for panels matched to the battery back voltage (for us RV'rs "12V" panels, actually closer to 18V max output voltage) PWM system and MPPT are similar in performance at high temperature. PWM throws away power to start with and as panel heats up, less gets thrown away as voltage drops. MPPT harvests that extra power, but its gain drops off with temperature.

    I've often wondered if running a small muffin fan with air ducted over the back of a solar panel would drop the temperature enough to more than compensate for the fan current draw. There are lots that draw 1/2A or less at 12V. Keeping the panel at ambient is much better than getting a absorption rise. And unforetunetly, silicon is a very good heat absorber from sunlight.

    Or maybe a portable water mister on the front of the panel.

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