Here are pictures of our electric panel w/ 12V outlets, inverter w/ 120V outlets, and battery charger: For those who may be interested in the nuts and bolts of this project, it's about to get technical:
Our 6 golf cart batteries (about $100 each at Costco) are hooked up to a small fuse panel and power a dual 12V car outlet (like cigarette lighter outlets in your vehicle) and our 1000W Xantrex pure sine inverter for 120V power (which we generally use only after dark for the lightbulbs). At first, I intended to run lighting (CFLs) on a 2 battery bank and run everything else from the 4 remaining batteries. As I studied the problems with CFL in general, I changed my mind and hooked the whole bank up together. I am running our 14W CFLs from the inverter at 120V rather than what I intended originally with 12V CFLs directly from the batteries. This seems to be working fine, as it should, but it is only as efficient as the inverter, which is about 80% or a little better. What that means is that I lose some power to the inverter for the price of increasing the voltage on a nice sine wave. This is where one can easily go mad with the variables trying to stay on budget. The cost of 12V CFLs is about 5X that of 120V thanks to the economy of scale in the latter due largely to the partially aborted attempt to mandate them. Whatever you may think of this government manipulation, this is the retail reality in which I'm trying to plan. Longevity is another serious concern with the 12V, as well as some rumor and innuendo that the lumen output may be inferior. It's true that they aren't available in larger outputs like their 120V counterparts. It's also true that there is no reason to expect the price to drop significantly, since most of the world doesn't live on 12V. That said, the cost will always be higher for these bulbs, which pits the theoretical efficiency gain (arguably lost due to lower lumen outputs for equal wattage) against the cost of battery storage and power production. My decision was largely based on the fact that for the time being and forseeable future, we won't be using a lot of power. I also like to be able to pick up a bulb or twelve in town for cheap. I think I probably over estimated our power needs, though the non-linear math of actual battery capacities and discharge will take up what should have been a surplus. As I will discuss further on, the battery bank may be slightly undersized in the final analysis. In planning, I expected to have a microwave, but I'm not so sure anymore. There are a few tasks we miss, but since we all eat together most, if not all the time, most of the basic reheating we used to do doesn't need done. Living in one room, we use only one 14W CFL (60 or 75W equivalent) bulb at a time and get along very well. I hung two pull-string fixtures, so we have the choice of two locations: center of the house over the kitchen or above the dining table (and farther from the kids' beds). We can plug the computer into a car charging adapter in one of the 12V outlets along with a cell phone charger. Our news, weather, and entertainment come in on a nice little wind-up radio that takes an audio input cable from our MP3 player. Some evenings, we watch a little Rin Tin-Tin (DVD) on the laptop before bed. All of us pile onto our loft bed and enjoy a pretty comfortable in-home theatre. Who'd have thought a full size mattress and 15” screen would seem so huge? I had some buyer's remorse for getting such a large bed until all 7 of us fit comfortably across it for movie nights. Elyssa and I were pretty used to a twin together in the tent, so I get disoriented with all the space at night. The photovoltaics aren't up yet because I don't have the roof or the sunlight to really mount them. In the meantime, we use my generator which is pretty miserly on gas. It runs the large jack hammer all day on half a tank (2 gal). I've tried three battery chargers, since I can't afford a fancy charge controller with a 120V generator input. Maybe it isn't that I can't afford, but I refuse to buy at this time. I've settled on a Shumacher 60A manual charger that I babysit to get the charge right. I designed the system to only discharge 10% over about 3 days of use. So far, the battery capacity is a little underwhelming, but I'm still trying to figure out how to really top off the charge correctly. I should theoretically have 345 AH @ the 20 hour rate, as indicated by the battery label. That means I should have about 34 AH for three days of limited use at a time without exceeding the 10% threshold. I may have made an error in calculating based on the 20-hr rate, as is the conventional wisdom in battery planning. The truth is, most of our use really comes in about 6 hrs over a three day period, so the 5 hr rate on the battery may have been more accurate, though appreciably lower capacity than it seemed. All that to say, to really get my 10% drain formula right may take two more batteries. The industry standard is to design systems around a 50% discharge, but it doesn't make sense to me to tax the batteries like that when you potentially get 4-10 times the life if you don't discharge more than 10% (this is true of lead-acid batteries, though some lithium and other technologies may differ slightly). It's cheaper to buy more batteries to match your capacity to a 10% load than to replace the bank because you discharged too deeply too often. It's also cheaper to buy more capacity in less expensive batteries than to buy fewer high dollar industrial batteries only to run them down on a regular basis (80% discharge is the lower threshold in “professional” designed systems, but a few of these events will kill an entire bank). With a bank of batteries, charging is a bit different than for a single car battery. I sat for about 4 hours staring at the multimeter as I charged to try to figure out the pattern and gauge the battery reaction. What I discovered, which some of you may already know, is that a large capacity bank is like a big body of water and the state of charge is a bit like a tide, or storm swell. Watching the voltage numbers roll by on the meter, I counted the duration between changes in hundredths of a volt (thanks to digital meters). I could see how the microprocessor on the charger worked and finally concluded after 10 hours of charging on the generator that this was not a viable plan. Inexpensive, but large, automatic chargers claim to have circuitry to keep from over-charging the battery if left unattended. The problem is that the processor is too conservative to charge a system with approximately 10X the capacity of a car battery. I switched to a manual charger and can now top off in about 1.5 hours or less instead of 10+hours on the automatic. Since I'm only using the first 10% of discharge, my hope is that a higher amp charging rate for shorter duration will suffice. Even at the nominal 30A charge rate (about 20A actual) that is put into the bank by a linear manual charger (linear in that the wattage input to the battery remains pretty constant, but the amperage will drop while voltage increases) I'm at less than 10% of capacity for the charge rate, which is still, to my understanding, a “trickle” charge appropriate to the task, with little risk of damaging the cells. If all goes well in my charging experiments, I can keep the batteries full while I run the generator to do building projects intermittantly. In this case, the extra fuel costs to keep charged might only be a few dollars a month (right now it looks like about $6 more than I use ordinarily). I'm no accountant, but I'd say that's a pretty good utility bill for temporary power with no tap fees (up to $4K around here for grid power plus you have to have a building permit which can't be issued on our road since there is no water but rain . . . ha!). The photovoltaics are more powerful and will charge even faster than the generator-based charger, but maybe you can see why I'm not in a rush to install them, since I'm using only about 2 gallons of unleaded gas and minimal extra wear on the generator per month. Much of this is due to the fact that I have to run the generator anyway for large power tools. Also, the generator is a pretty large one (4000W continuous), so I don't lose much by plugging an extra appliance like the charger in while I run a table saw or air compressor, with idling between cuts or cycles. Some portion of the power would be wasted, otherwise. Efficiency-nazis might croak at a few of our practices, but in reality, the low consumption so far out-paces the losses to inefficiency that it hardly seems practical to spend any more time perfecting for now.
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LanceLance doesn't like to publish his writing, as he reserves the right to change his mind. =P Archives
April 2013
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