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towpro's avatar
towpro
Explorer
Oct 18, 2017

reconsidering recomendataions on solar

so after 1 year using my solar my outcome is a little less than expected.

I have 2 100W panels mounted on front slope of roof, and 1 additional panel mounded flat on roof behind the DS panel that is on sloped part of roof.

When I park, I do my best to face south so I get full exposure during the complete day.

Typical summer day goes like this.
Wake up and Bogart meter shows I have 70% of my 180 AH (27 series AGM's) left.
The morning sky the sun is low(er) on horizon, so cells are producing maybe total 12A, but its enough to bring the charge up into the mid 80% by 10 am when sun is getting higher in the sky. Since the charge % is going up, the current required to "finish" the batteries is lower so the solar current shown in Bogart shows even less Amp output power. by 12:00-1:00 I see 95% charge, by afternoon its finished off at 100% (and for those that know bogart, I have my charge finish set on #4 which is harder to reach "full charge".)

Now I make the same trip in October.
sun is much lower in sky all day.
again, morning shows around 70% charged. as morning progresses I am seeing 7-8 amps, mid day I am seeing in the 12a range or so, but without running generator I will not see 100% charged by end of day.
This is all caused by sun being lower in the sky, less output from cells.

now in theory, at 100% efficiency, my three 100w panels should be able to produce 25A @ 12 V. But I never see this because when I need max charge (morning) sun angle is not perfect, and when sun angle is perfect I am already into finish charge period. and nothing is 100% efficient.

Now my thoughts.
Since I am PWM controller I am wired in parallel.
(No need to bring up other charger, If I did it again, I would still buy a Bogart Engineering system with charge controller).
Now all the "work sheets" that talk about 3% wire loss show my input cable should be good for "near" 30A. (actual run from roof is 3' 12G wire, than 3' of 10G wire)

But these "wire loss" charges are for FULL SUN and FULL outputs, which I never see because my charge requirements go down ad sun comes up (as well as low sun angles in spring/fall.

Using this line of thought, I am thinking of adding one more 100W panel onto roof.
sure if I ever saw 100% efficiency charge in middle of day It may cause a little more then 3% voltage drop through wire loss, but the other 99.999% of the time I will be less than 3% voltage loss through wire.

Sound like a good idea? that 4th panel will give me more charge at lower solar angles, when I need it most.

55 Replies

  • towpro wrote:
    I am wired in series.
    And you know that the panel with the least amount of amperage (any one with a larger sun angle) will restrict the array to that amperage.
    hedge wrote:
    I think you must have your panels wired in parallel if you are using a PWM controller.
    I think you're right.
  • I think you must have your panels wired in parallel if you are using a PWM controller. Also based on the observed amp ratings.

    I don't see that you mention what draws you have. I also have 3 100w panels and I'm usually back full by 10-11am in full sun. My uses are small though, mostly just to make coffee in the morning with my inverter/Keurig. In early spring or late fall some furnace use. I've never had a problem getting back to full. I have a Victron 700 monitor but I can't remember what I have the 'full' detection set at.
  • With a PWM controller you'd be better off running the panels in parallel. The higher voltage does you no good, other than cut losses in wiring. The loss across your 6' of wire is going to be minimal (.008 ohms to be precise). The PWM controller is basically a fast switch that pulses the current to limit the average voltage going into the battery. You'd be better off with the panels in parallel giving you more continuous current going into the battery as the PWM controller doesn't have to limit the voltage as much.
  • If you are running your panels in series make sure that they are all completely un-shaded. Personally I have found parallel is better as any shading on one panel doesn't affect production of the others.

    I will be running 10/2 cable from each panel to a combiner box and #2 from there to solar charge controller. Using a voltage drop calculator we will be well under 3% voltage loss which is fine. (system being assembled right now) Wire isn't that expensive in the short runs a TC requires or that heavy.

    Our old camper had (5) 100 watt panels and that would keep us at a daily 100% SOC on days where we got decent sun exposure and didn't use too much power. On cloudy days not so much. Unfortunately with led acid batteries it is hard to get that last 20% or so as the multi-stage charging reduces (and rightly so) the charging rates.

    For some of the reasons you have discovered (like us) I am biting the bullet on LiFePo4 battery bank on this build. They make much more efficient use of the solar charging platform by taking a bulk charge to almost 100% and can be discharged very safely to 80% discharge. (some argue deeper) They also require no need to be fully charged on a regular basis, so you can run along during cloudy days discharged well below 50% day after day with no "memory loss"
  • Is not always reaching 100% daily really an issue? Are you out for long periods during the fall when days are shorter and the sun is lower?
    If you are only down to 70% after overnight use, and the charge brings you back to 85%, after the second night you would theoretically be down to 55%, but when the sun came up that day, your charge rate should be higher since you wont be encountering the "topping off" phase. It may well be able to return you to 85% daily.
    Have you stayed out long enough to see if you reach a point where the panels keep up?