Solar power under load

[Mike Lewis]

I'm posting this in order to provide more data regarding real-world use of solar power.

A few weeks ago I broke my refrigerator. I had a Coleman PowerChill thermoelectric cooler which runs on 12V or line voltage, so I moved my food into it. The cooler works surprisingly well, but it draws a constant six amps on 12V. At my previous location I had shore power, but I moved down to New Mexico and got a spot without it. I wanted to see how well my solar panels could keep the batteries charged while under this load.

I have three roof-mounted panels totaling 355W of power and the dual six-volt batteries. I moved to a state park near Portales, at around 34.25 degrees latitude. The weather was mostly sunny. When I arrived my batteries were at 100%, due to having shore power the night before and sun on my panels during the trip. I did my usual routine-- using the microwave for a couple of minutes via the inverter, charging my laptop, watching TV for maybe an hour or two. The next morning my batteries were at 62%. During that day they charged up to 80%, not higher.

It became obvious that I couldn't go more than two or three days like this without depleting my batteries, so I eventually moved to a site with hookups. I'll try to get my fridge repaired at an RV shop next week.

[cpaharley2008]

Something does not add up, you should have plenty of reserve with 355 watts while using only 6 amps per hour? Are your dip switches correct?

[Greg A]

I agree with Jim, something not right. The 160w factory unit on ours with twin 6v puts it right back to full within 2-4 hrs from most any depletion point. Who put the solar on? What are the components? Panels wired in series or parallel?

[Mike Lewis]

I have suspected that my system was underperforming, but I didn't know how to check.

I don't see a reference to the dip switches in the Go Power controller manual, though I've seen them mentioned in the forum. Can someone give me a reference for the correct settings, and/or a troubleshooting procedure?

The inital 95W panel and controller were installed by ETI. Two additional panels, controller, and a battery monitor were installed by AM Solar.

[Mike Lewis]

Correction: the GoPower controller was installed by ETI. AM Solar used the initial controller, so it hasn't changed. AM Solar installed two panels and a battery monitor.

I read the GoPower manual and checked to make sure the controller was set to flooded batteries. It was. Other than that, I don't know what to do. In particular, I have no way of detecting a dead solar panel.

[Mike Lewis]

Panels appear to be wired in parallel-- all three go to a rooftop junction box installed by AM Solar. This re-routed the initial wiring by ETI for the 95W panel.

[rubicon327]

Mike: I recall your thread from a few days ago about cleaning the panels and a 1/3 increase in amps. That's significant. Since this issue of optimal recharging appears to have been prior to the cleaning do you have a feel for how you might recharge now?

[Greg A]

Certainly sounds like the install would be correct, you'd want them in parallel. The one thing they have in common is all the panels go into the same GoPower controller and the output seems like it isn't what it should be to the batteries. I'd definitely focus on the controller. Maybe a call into GoPower and see if they can walk you through testing it.

[padlin]

That works out to 144 amps in 24 hrs. The most I've gained over a full July sunny day here in western MA was 50A with my 160w panel. Using those numbers I would have gotten 111 amps if I had had 355w, a loss of 33 amps for the day long cooler use, not counting your other uses.

Your numbers look roughly right to me.

[Mike Lewis]

Originally Posted by rubicon327

Mike: I recall your thread from a few days ago about cleaning the panels and a 1/3 increase in amps. That's significant. Since this issue of optimal recharging appears to have been prior to the cleaning do you have a feel for how you might recharge now?

Yes, I cleaned the panels to make sure I was getting the optimum charging, and because I knew they were filthy. I'm on shore power now, but I could switch that off at the box and let the batteries drain overnight, then check my charging tomorrow. At my new location I'm not in full sun all day, but I am for part of the day. I will check my charging rate then.

[Vermilye]

Originally Posted by padlin

That works out to 144 amps in 24 hrs. The most I've gained over a full July sunny day here in western MA was 50A with my 160w panel. Using those numbers I would have gotten 111 amps if I had had 355w, a loss of 33 amps for the day long cooler use, not counting your other uses.

Your numbers look roughly right to me.

I agree. Along with everything else, you are using more than one can expect from your panels, particularly with the GoPower controller, which is quite conservative.

[Greg A]

Padlin's correct. I missed that 6amp continuous draw when I first glanced at it.

[Mike Lewis]

While perusing the GoPower manual I saw something about manually setting the controller to charge at 14 volts for a short period. It said this could be used during the day when anticipating a high power draw at night, and the 14V setting was to be used only once or twice a day. I've never messed with the controller. Has anyone used this setting?

On my first trip to AM Solar shortly after buying my trailer they suggested replacing the GoPower controller with something else. But as the trailer was new I wanted to minimize swapping stuff out, and I was out of money besides.

[Vermilye]

Originally Posted by Mike Lewis

While perusing the GoPower manual I saw something about manually setting the controller to charge at 14 volts for a short period. It said this could be used during the day when anticipating a high power draw at night, and the 14V setting was to be used only once or twice a day. I've never messed with the controller. Has anyone used this setting?

On my first trip to AM Solar shortly after buying my trailer they suggested replacing the GoPower controller with something else. But as the trailer was new I wanted to minimize swapping stuff out, and I was out of money besides.

Check page 13 of the manual. It is only available on the newer versions of the controller - look for a "Max Boost" button on the face of the controller. It produces 14.4V for 30 minutes...

I prefer the Blue Skies Sun Charger 30. Each stage is fully adjustable (both time & voltage). Of course you need to follow the specifications for your batteries.

[Jeff and Karen]

I'm really outside my area of expertise, but I have read that even very small shadows on solar panels significantly reduce power production. Is it possible that things like vent covers, or even antennas, could be casting shadows across your panels?

[Mike Lewis]

Originally Posted by Jeff and Karen

I'm really outside my area of expertise, but I have read that even very small shadows on solar panels significantly reduce power production. Is it possible that things like vent covers, or even antennas, could be casting shadows across your panels?

Not in the position the trailer was in at the time; they had a clear shot at the sky for much of the day.

From what I understand some panels are immune to the shadow problem due to the way the cells are wired together. I think the 95W panel from ETI is like this, but the other two are affected by shadows. I'm just guessing here.

[tractors1]

Originally Posted by Mike Lewis

Not in the position the trailer was in at the time; they had a clear shot at the sky for much of the day.

From what I understand some panels are immune to the shadow problem due to the way the cells are wired together. I think the 95W panel from ETI is like this, but the other two are affected by shadows. I'm just guessing here.

Our 2014 160W panel did great under the big redwoods at Jeremiah Smith Park in California. We never had a straight shot at sunshine, but with our Group 29 battery down 20-25% when waking up it was always full by 11AM.

[jrb1947]

I have been doing research on solar as well (parallel vs series and pwm vs mppt). Here is a good video on shading.

https://www.youtube.com/watch?v=1qD3mN8VotQ&t=620s

[StarvingHyena]

Thank you Jim for bringing up the video on shading and parallel vs series hookups. I know when I'm dry camping, I need to pay attention as to the direction I park. My roof top AC can shade my solar panels - sometimes forcing me into deciding if I want morning sun or afternoon sun on my 150 watt panel.

Dave Lewis, if your system isn't performing to as high a level as you'd like, I'd look at my batteries. I think likely that your system is 'out of balance'. You have 350 watts of solar but still only twin 6v batteries. Your charging system, batteries and load must be in balance for best performance. To match your panels, I'd say your battery capacity is too small. Admittedly, though, I haven't run the numbers. When I was working on my build sheet, I considered having twin 12 v batteries. But being a trailer newbie and having no real idea of my electrical needs, I folded and followed the crowd and got twin 6v batteries. I'm sorry I did and here is why: with twin 12v batteries if I need more electrical storage capacity, I can add just one more battery and gain something near 30% increase .... but with the twin 6v batteries to do the same thing I'd need to add two more batteries with double the weight but would double my storage capacity with 4 batteries. That would be good but I ordered a 17 and that much extra weight on my rear bumper might produce unwanted handling characteristics. My plan is to swap out the twin 6v and replace with twin12v when the 6's are no longer taking a charge.

Another thing Mike is look at your charge curve. If your batteries are below 80% charge level, they will take all the power you can give them .... but once above 80% charge your charger / controller is shutting down your rate of charge hence your batteries are not getting charged to 100% given the amount of solar exposure. I live off the grid and watch my rate of charge all the time. When I get up in the morning and its cloudy, I'll crank up my generator (nearest neighbor is 1/4 mile away) ... my batteries will charge at 30 amps for about 2 - 3 hours, then my smart charger will cut the charge rate to maybe 5 amps. Your solar panels + controller are doing the same thing hence your batteries are not reaching 100% charge in a "solar day". Perhaps you are running out of daylight before your batteries are all the way up. Handy Man Bob, who has a very informative solar blog, claims our chargers will only charge batteries up to 13.7 volts where he suggests bringing you charge voltage up to 14.3 volts to achieve a 100% charge. (?)

You mentioned the use of your microwave ... many people don't realize how much power a micro uses. They consume a tremendous amount of power while on and take up a lot of space all the time. My micro will be soon be placed in storage until I'm sure that I don't need it. They are a convenience for sure but with a price to pay. I think my batteries will be happier.

Back to your system, it appears to me too, that your charging system is operating up to speed. You stated that you weren't sure how to test it. That excellent video on pv shading showed a clamp amp meter ... now I want on too. A meter like that will give you some hard data to go on.

Thank you for all your posts Mike

Good luck,
Tom

[jrb1947]

Regarding two 6 V in series vs two 12 v in parallel:

What we are really after is watt hours. Watts are calculated by multiplying voltage times current. A six volt 220 amp hour battery holds 1320 watt hours. A twelve volt 110 amp hour battery also hold 1320 watt hours. Both batteries will weigh about the same. The problem is battery degradation. Over time, one battery may output slightly less voltage. In series, this is not an issue. In parallel, the less degraded battery will dump current into the more degraded lower output voltage battery causing heat. If connected in parallel, it is wise to add fuses between the batteries. I prefer two large capacity six volt batteries in series. Note that within the shell of either the 6 or 12 volt battery are lower voltage cells in series.

PS: More battery arcana.

You should never use more than half the specified watt hours of a battery so with two of the above batteries, your maximum usage should be limited to about 1300 amp hours. Using a 1000 watt microwave for 10 minutes with an inverter efficiency of 95% uses about 175 watt hours. Use high current devices sparingly. To get the maximum number of watt hours out, you must pull it from the battery slowly. At high currents, more power is lost as heat within the battery.