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.