I’m excited to receive criticism on the off-grid system. All of the things I’ve done on the system so far have been first attempts. Not having experience prior to this endeavor I’ve relied heavily on blog posts, writings, and technical details from others who may or may not have expertise in the topic.
So I’d like to start a list of criticisms and reflect on some of the things I would like to either add, change, or would do differently if I could start over again.
- As was pointed out to me at the Small-Cabin.com forum, using 14x 6V batteries in a 12V system can lead to the expenditure of a good deal of copper and a mess of battery cables. So why did we use 12V instead of 24V? In the beginning we didn’t think that our system would more than double in capacity and that we would eventually add 810 Watts of solar panels. While 12V does not transmit as well over a distance as 24V, we overcame this by using heavy cables (less electrical resistance), and by placing our PV array in close proximity to the batteries. An unexpected benefit of using 12V was the ability to wire in 12V LED lights directly into the battery bank. It should also be noted that sustained loads on the cables rarely reaches above 150o W.
- Hindsight is 20/20 – a 2000W inverter would have suited our needs and may have offered better efficiency at outputs < 500 W.
- I would like to eventually color code the ends of my battery cables (red for positive, white for negative). Admittedly, this is not a high priority – the design/layout of the cables is organized fairly well and I’m the only one who disconnects or connects the cables.
- A battery monitor was pointed out to me at the Northern Arizona Wind & Sun forum as a useful addition to the system given the higher than normal battery to panel ratio. On the forum I was warned that 810 W of panels should ideally be paired with 430 Ah of battery instead of the 1540 Ah in the current set up, and that this mis-match could result in the early death of the battery bank and a greater risk of sulfating (a sort of corrosion that damages the insides of a battery). 430 Ah for a 12V system seems a bit low to me: 2200 AC Watts until 50% SOC. At this point I suspect this warning is based on battery care. The batteries get specific gravity checked yearly and water levels checked at a yet to be determined interval, and a rigid procedure is in place (that uses my SOC charts) to ensure the batteries do not dip below 50% SOC and absolutely do not drop below 30% SOC. Will these measures ensure the longevity of our batteries despite this proposed design mismatch? Time will tell, and I’ll be there to document it.
Since this post was originally put up on 1/19/2011 I’ve had time to address some of the criticisms.
- The batteries are in need of an enclosure of some kind. Exposed terminals are an unnecessary risk. While my folks and I exercise great caution around the battery bank I can’t guarantee that others who visit will do the same. FIXED 1/30/2011
- In addition to an enclosure a way to vent the batteries to the outside would be a good idea. Lead acid batteries produce hydrogen and oxygen gas under heavy charge and equalization (overcharging). This gas production also means that pressure could build up within the battery (the newly formed gas displaces more volume than the electrolyte solution) and explode the battery if the pin-hole vents in the caps are plugged. Battery explosions/cracking is extremely rare in well-maintained batteries. Thinking back to physics and passive diffusion, I doubt that under any circumstances there could ever be a hydrogen buildup significant enough to pose a threat in the garage. However, if/when a battery box is built, a ventilation strategy of some sort will be needed because we will be exponentially decreasing the volume of air surrounding the batteries. FIXED 1/30/2011
- After reading on wikipedia about lead-acid batteries, I would also like to check the battery terminals yearly to look for corrosion. Right now I’m using vaseline to coat the terminals and lugs to deter corrosion; corrosion results from the mismatch of metals between the lug and the terminal. Time will tell if this is adequate or if more expensive dielectric grease is required. SORT OF FIXED after 18 months the copper coated with vaseline shows no signs of corrosion.
I will update this post as I learn more. For anyone from the forum, thanks for the criticism – you’re helping me to build a better and safer system.
offgridcabin 
Sam, (I saw your name was Sam from the other commenter; no stalking intended)
While wanting constructive criticism in the name of learning so we can “do better” is great, we do it too, however, please, please do not let perfectionism stand in the way of your realizing and enjoying your accomplishments. I am so impressed with what you have done and are doing. Your 12V system is a nice system. It meets your needs. Everything looks neat and tidy. When I follow your research path, it looks similar to our research path. Now, this goes back to each individual’s needs. My husband is not retired yet and I’m here at the cabin by myself a lot to take care of things. Being more creative than technical, I have no problem admitting that my mind drifts off and my eyes glaze over at some of the things that my husband says. I don’t know if this a girl thing or what but I pretty much think that everything is going to explode and ultimately end in a bad hair day. With that in mind, my husband was forced to get batteries that are maintenance-free (AGM). However, we cannot get the accurate status readings that you are getting from your type of batteries. It seems there’s always pros and cons to all the different options. He has some very strong opinions on the blah, blah, I think he said something about maximum number of strings, equal length wire connections, bunnies, rainbows…. maybe you might want to email him directly at mike@thesecretcabin.net. Again, I cannot stress how impressed I am with what you’ve done. In fact, I was surprised to see you are not an engineering student. Keep up the good work.
That’s funny you should mention engineering. I originally planned on becoming an engineer (though I never figured out whether it would be mechanical or electrical). In the end I attended UW Oshkosh – mostly because I had a free ride and because they had a good track program. I chose chemistry as my major. Chemistry served as a great base for understanding batteries and electrochemical stuff. I also picked up some good lab skills that come in handy from time to time when measuring specific gravity and maintaining the batteries. But I didn’t want to dwell in a lab somewhere so after three years I applied for, and entered, pharmacy school.
On a side note, my grandpa was an electrician. He also built his own house from the ground up. I may just be wired to understand this stuff. Who knows.
I can understand your husbands frustration with getting an accurate status reading on the batteries. AGMs don’t allow for specific gravity measurements (I’m pretty sure anyway). Perhaps there is a way to track watts used, in and out of the battery bank. I’ve been looking into the Xantrex Battery Monitor (called the LinkPro) but haven’t made up my mind whether or not it will suite our system any better than my charts. I’ve also looked at other meters that record watts used. I’m sure there is a way. Which reminds me, I have a link :: http://stores.mavericksolar.com/-strse-242/2010-Renewable-Energy-Design/Detail.bok
I have the 2008-2009 catalogue, great place to get ideas and learn about obscure off-grid products
I might be a bit of a perfectionist, but I get a great deal of enjoyment out of learning new things and starting new projects (I could start a blog just on that). Our system is not in jeopardy of any major overhauls. I’m only looking for things that will benefit the overall experience of using the system.
Hi Sam I really like the system you have put together. There is a very good battery storage ventilation system which does not require power. We have a PVC pipe fan running off a solar panel to vent a composting toilet. The fan is designed for battery box venting. They are not cheap, and when it gets cold they stop running. I would lean to a non power vent system to keep things simple, save power, and once installed no maintenance required. I forgot the web site but I do have it written down some where. Let me know if you are interested and I will send it to you. If you
Already have something in mind no big deal. Mike
Mike,
I’m trying to research an off the grid system for my parents who are building a cabin. I’m curious how you have made the jump from PV power to run the composting toilet- which I’ve only seen requiring year round electricity. We are also looking into the composting option.
What kind of set up would you suggest- just batteries left in the cabin, or a pv system converted to AC? If you found some literature on this describing the process – it could be a big jump start.
cheers,
-Parker
The toilets can be purchased as 12v or 120v. 12v connected to a battery with a controller/panel charging and protecting works fine.