A Punch in the Solar Nexus
My thought diversion for today is an update on the Intrepid’s solar system. I mention it because, it now being January, the sun arcs low enough in the sky to starve the flat-mounted 360-watt roof panels. This is not a surprise to me nor too many other people. Since these panels are trying to recharge some 420 amp-hours of battery capacity while a 12VDC compressor fridge is running for unusually long periods of time in hot weather, this power drain and poor panel aiming tends to prevent the pack from reaching and holding its 14.4V peak for its intended 3-hour span. So, the 200-watt ground panels were deployed everywhere I camped, when practical, and re-aimed throughout the day. In a cloudless sky, this combination can get the pack fully charged by 2 PM. Partly cloudy skies slow or prevent that, obviously. The main culprit is the inability to aim the roof panels toward the sun, but a close second is the unusually long runtime of the Dometic compressor fridge. In a marginal charging situation, it pulls enough power and runs long enough to prevent a full charge, and that can be a battery killer. Combine that with clouds, and you have an undesirable situation, even with the ground panels deployed. These were my thoughts as of last fall, anyway.
None of this is a problem over the winter months in Yuma, since the fridge is off and nothing else is pulling power, either. However badly oriented, the roof panels alone can easily recharge the pack then. Were I to have to keep the camper in service over the winter, some changes would be needed because that fridge would be on. The first impulse was of course to increase panel wattage, but this is not a practical option for me. There’s no more space on the roof for additional panels. While I could theoretically add another 100W ground panel or two to pump 300 or 400 watts through the separate Morningstar 200-watt charge controller (a good way to increase charging in poor sun conditions), my storage rack is maxed out and, as far as space in the truck cab and camper goes, there’s no room at the inn. I’m already panel-heavy. Enough with more panels, already! Important note: Don’t try this at home. Don’t exceed your own controller’s maximum input wattage rating if it isn’t a Morningstar-brand MPPT-type controller. For all I know, their much cheaper standard controllers can do this too, but I haven’t checked that. Once the sun rises higher or appears from behind a cloud, your own controller may cease to be and/or enter a different plane of existence. This weird trick is useful only to build a bigger panel array without having to upgrade the expensive controller. It will supply more wattage at the beginning of the day, end of the day, and in overcast. In short, it will be more likely to bring the battery to full charge in adverse conditions. In full sun at noon, there’s no advantage at all, because the controller then will pass no more wattage through than it’s rated for. The rest goes up as heat.
I could instead convert the roof panel mounts to allow tilting for mid-day sun, but I don’t see this as a practical option. The Four Wheel Camper’s thin aluminum roof already has issues with providing enough bite for mounts, and expecting them to hold a raised panel in wind is to tempt fate. Not to mention the extra weight and the need to always park in just one orientation. Nope.
The only other option I see when battery replacement time comes is to decrease the pack’s cell count from 4 to 3 batteries, dropping its rating to 315 amp-hours. Given the Dometic fridge’s hunger for power, however, this is a fine line to dance. In these seasonal conditions, just two days of overcast is about all my 420Ah system can power through, and sometimes three if I’m not personally using any power. Cutting battery capacity by 25% does not have much appeal.
That is, it does not have much appeal unless the basic ratio of panel wattage to battery amp-hour capacity can be pumped up enough that decent charging is possible even in overcast conditions. Back in the days when I sailed in the USS Defiant, it had three small onboard solar systems – all of them tilt-able for mid-day sun. One of those used 390 watts of panels to recharge 420Ah of batteries for the computer-based studio and TV. Another used a 195W panel to recharge two coach batteries totaling 210AH. And a 195W panel recharged a 105Ah battery for running a CPAP device. The first two had basically a 1W/1Ah panel-to-battery ratio, while the third had a 2W/1Ah ratio.
I noticed three things. In Arizona, the first two systems handled heavy overcast poorly, requiring cutbacks in power usage. When one of the “hybrid” (Marine) coach batteries gave out (and changed the system ratio from 1W/1Ah to 2W/1Ah), it recharged better in overcast and posed no particular problem in sustained overcast. Since it had fairly light, intermittent tasks to do, I didn’t need to bother monitoring it. So too, the single-battery CPAP system, with its light draw and 2W/1Ah ratio, also happily charged quickly in everything but black skies with a downpour. A CPAP device doesn’t draw much power, so recharging was not a challenge.
Admittedly, this last one was the “perfect” setup, what with having a tilt-adjustable, oversized panel, a light load only at night, and nothing to interfere with the recharge during the day. Today, the 420Ah battery pack in the Intrepid has plenty to do (everything), has devices that drastically interfere with its recharge, uses the 1W/1Ah ratio on roof panels alone, and the majority of its total available panels cannot be tilted as needed. No wonder it wheezes when the weather is less than perfect.
But here’s where this story gets funky. I’d earlier come across a sentence in Solbian’s Flex Panel installation instructions not to mismatch wattages when grouping together panels that are wired in series, like I have. Wiring in series pumps up the solar array’s output voltage in the same way an old D-cell flashlight does, where the cells are stacked negative to positive. The amperage remains low, and big amperage is what demands big, fat wiring and hearty connectors in order to handle the load. My camper is prewired for a single large panel up there, and goofing with that wiring on a pop-up is a problem-prone idea, as is individually wiring each panel in parallel all the way down to the controller. You can do it on a hardside or on a trailer if you’re industrious enough, but a pop-up, no. So I kept the existing 10Ga wiring intact and wired my panels together such that I could avoid trying to squeeze through any more amperage than any one panel in the group produces. Those wires are rated for 600 volts, so ramping up my voltage instead of amperage poses little problem with wire resistance knocking down that voltage, which would make the system less efficient. However you choose to wire up your solar panels, you don’t want your wiring to absorb any of the power that your controller is supposed to be receiving. Some loss will always occur, but the goal is to minimize it.
But about that mismatched panels thing…you can mismatch panel sizes to a fare-thee-well if you’re going to run each panel’s wires down to the controller separately (parallel). (If you join the wires together in parallel on the roof and run that down to your controller, you’re going to need some pretty stout cable to carry that much amperage.) At the time I came across Solbian’s advice, I was on a tight schedule and the lack of a “why not” made me suspect that it was of minor importance. That was a bad move on my part. With my being locked into series wiring, that made me suspicious after my first true touring season in the camper, when the batteries’ titanic struggle with the refrigerator came to a head in the Fall. Then an email from a reader Mark (of The Landmarks blog fame) mentioned that he was rewiring his hardside per the teachings and doctrine of Handy Bob Solar, which prompted me to do some research. I found a similar sentence in that blog about not mismatching panel wattages in series, again with no explanation. So I emailed him to ask. He promptly replied, saying that he wasn’t dead sure, but thought that the combination would limit the combined output to that of the smallest panel. In my case, that would mean that my setup of three 100W panels plus one 60W panel was yielding a maximum of 240 watts instead of 360 watts. Not the best! Further research on my part confirmed this to be true, as well as the theory that all of the wattage lost is actually pumped into the smallest panel, which has the unhappy task of dissipating it as heat. Good way to lose a panel and collapse all output. Now, Handy Bob hates MPPT controllers with a passion, feeling that they are an overpriced solution in search of a problem, but he did succeed in prompting me to remove the small 60W panel, which should improve my system’s overall output to 300 watts maximum. That’s still not enough for my battery pack, according to my own experiences, but it’s a darn sight better than the 240 watts I’ve been receiving all along. That’s hard on the battery pack, which is “too big” for that little recharging power.
The two 100W ground panels can now pump that wattage total to 500, when needed. This year’s tour should fare better than last year’s, electrically speaking. Given that I’m lazy enough to avoid deploying the ground panels unless absolutely necessary, will this fix be enough to get along solely on the roof panels even in Spring and Fall? Not real likely, but I should have to deploy them less often. Only time will tell!