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I have a 28 foot Sunnybrook Solanta with a relatively sophisticated Victron battery power monitor. A good part of my camping is boondocking, often in forest service campgrounds. I have two 6V golf cart batteries, roughly 200 Amp-hour capacity. I carry a small Honda generator, primarily to charge the batteries. However, even a quiet generator makes noise, and the batteries can only accept so much current. Solar power delivered throughout the day should be a better match to the battery capacity.
I am looking for comments on my solar analysis and calculations.
Past experience gives me an estimated power consumption of 50 amp-hours per day in winter, primarily the furnace. Note that I rarely camp in Colorado during the winter; think Arizona.
My wife recently got a CPAP machine. I used a Kill-a-watt 120 volt AC meter to measure 250 to 300 watt-hours per night used by the CPAP. (Her CPAP is apparently set relatively low, as her apnea is not severe. It is possible that your CPAP machine consumes more or less power, depending on the programming, etc.) I bought a 12 to 24 volt converter for the CPAP, and wired a 12V outlet next to the bed. Based on our recent trip, I would estimate our summer power consumption, including the CPAP, as about 25 amp-hours per day. Using the CPAP and the furnace would be around 75 amp-hours per day, which would run down my battery pretty quickly, and require a lot of generator use. The CPAP has increased our summer consumption to the point where I could profit from solar in the summer, not just in the winter.
At the moment, I am considering two 12 Volt, 100 watt panels, perhaps from Rheogy or Grape solar. I would figure to tilt the panels in winter. I would figure to use a MPPT controller, perhaps a Tracer 2210RN, and wire the panels in series to reduce loss of power in the cable.
Here is how I see the calculations. Two 100 watt panels should produce about 1000 watt-hours of power per day, at least on a sunny day. 1000 watt-hours at 13 volts would be about 75 amp-hours, roughly the total of the winter consumption plus the CPAP consumption. If I only wanted to cover the CPAP, I could get by with one 100 watt panel. I am disregarding MPPT efficiencies, figuring that to be over 90 percent. 75 amp-hours, spread over five hours or more, would mean a charging rate of about 15 amps, which the battery should be able to accept. I think that more solar would be wasteful, as the batteries can only accept so much current, and my power consumption occurs primarily at night. If I needed more charging, I would run the generator after the solar panels have quit producing power.
Do these calculations and assumptions look reasonable? Thanks in advance for your comments.
Richard in Colorado
-- Edited by raferguson on Wednesday 29th of June 2016 02:49:45 PM
Thanks for the comment on a larger battery bank. If the limit is the charge rate on the battery, a larger battery should allow an increased charging rate. Not sure that I have space for a much larger battery, but I could research that.
A friend of mine is an engineer, and an RVer with solar, so I emailed him a copy of the above. His comment was that the charge efficiency is probably 80%, so the amp hours in will be greater than the amp-hours out. That would reduce the amount of available power. Maybe 75 amp-hours in to the battery becomes 60 amp-hours out. (I would note that some sources list 85% as the efficiency of a lead-acid battery).
You could look at a L-16 AGM, they are around 400 amp hour batteries. They are roughly the footprint of a group 27 battery, but are higher. Two of these in series will provide 12v with 400 amp hrs. This would provide 250 amp hr's comfortably.
We have completed our solar install of our 2017 Vilano 325RL. We used the battery bank ( 8-6 volt golf cart batteries) last night for our first time so we can see how our consumption went. Around 7 pm we pulled the plug and went to batteries. Watched TV till 2300, had only one lamp on. Residential frig. was on a while and I played a DVD. Ran my CPAP a total of 8 hrs. Upon waking we had 86% bank on Tri-metric monitor. About what I expected in usage after having the old system in the Cougar and watching its usage in comparison to the Vilano with a residential frig. Have an idea now what to expect on battery bank. 0730 cut heat up and its cycle on and off 3 times in an hr. and a half. Made 3 cup of java and got big lights on in Kitchen. Its now 0900 and tri-metric reads 82%. Not bad I feel.
"His comment was that the charge efficiency is probably 80%"
I'd count on 60 to 70%.
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Winnebago TT 2101DS & 2020 Silverado LTZ Z71. 300 watts WindyNation solar w/MPPT, 2 Trojan T-125s. TALL flag pole. Prefer USFS, COE, BLM, USF&WS, NPS, TVA, state/county camps. 14 year Army vet-11B40 then 11A - old MOS 1542 & 1560.
