early days :: research

The batteries were purchased less than a week prior to the installation

Leading up to the purchase of the batteries I looked for resources. In my search I found a few websites I’d like to share and a few excerpts that would be useful to anyone else trying to decide which type of battery to use.

We chose lead acid deep cycle batteries (golf cart batteries) because they were affordable and with a little work should last quite a while in our cabin, which is occupied up to 150 days out of a year.

Northern Arizona Wind & Sun gives a good overview on batteries and their physical properties. While researching, I found the following excerpt particularly useful in judging how much power could be pulled from a battery under varying circumstances:

Internal Resistance

Part – or most – of the loss in charging and discharging batteries is due to internal resistance. This is converted to heat, which is why batteries get warm when being charged up. The lower the internal resistance, the better.

Slower charging and discharging rates are more efficient. A battery rated at 180 amp-hours over 6 hours might be rated at 220 AH at the 20-hour rate, and 260 AH at the 48-hour rate. Much of this loss of efficiency is due to higher internal resistance at higher amperage rates – internal resistance is not a constant – kind of like “the more you push, the more it pushes back”.


A second resource, for both inverters and batteries is www.backwoodshome.com and the writings of Jeffrey Yago, P.E., CEM in his articles titled Solar Power 101. They can be found in issues 87 – 90. An excerpt on inverters that ultimately led to the selection of what was advertised in 2009 as a “true sine wave inverter” for the cabin:

Issue 89 Sep/Oct 2004

This article is the third in a series of our beginner’s course in solar electricity. Simultaneously we have instituted a Home Energy Information (www.homeenergy.info) section on our website where you can ask questions of Jeff Yago, the author of this series. Yago is a licensed professional engineer and certified energy manager who has written many energy articles for BHM. He has extensive solar thermal and solar photovoltaic system design experience, and is author of Achieving Energy Independence — One Step At A Time, which includes many solar system wiring examples.
This third installment of our continuing course on solar electric power system basics will address power inverters. Since all solar photovoltaic modules generate DC electricity, unless all of the lights and appliances being powered are DC, an alternative energy power system will need to include an inverter. This device is used to convert the DC electricity from the solar modules and batteries into AC electricity.

Inverter basics

Early inverters for the consumer market were used mainly for mobile applications like boats and recreation vehicles, and most were designed for 12-volt DC battery ignition systems. Due to an upper capacity limit of approximately 200 amps for the internal power components and heavy welding cables that were being used for connecting these mobile 12-volt systems, 2,400 watts was about the largest capacity inverter that could be made for these applications (12V x 200A = 2,400 W). To keep

inverter costs low and the designs simple, these early inverters generated a “modified” sine wave output to simulate the 60-cycle 120-volt AC line voltage. The more “steps” in the modified waveform, the closer the output voltage will be to a normal AC sine wave.

Until the explosive increase in personal computers and microprocessor controlled appliances and audio/video equipment, most electrical loads that included older technology would work fairly well on a modified sine wave inverter. Incandescent lights and power tools also worked well, although some fluorescent fixtures and light dimmers had problems. An AM radio may produce an objectionable hum in the background, and a microwave oven will take much longer than normal to cook the same food, but most of these devices would still operate on a modified sine wave inverter.

In the early 1990s, quality modified sine wave inverters were being sold by Trace Engineering and Heart Interface. Although still limited to about 2500-watt output using a 12-volt DC input, these became the standard for residential off-grid and back-up power systems. Many of these early models are still in operation and have an excellent reputation for robust design and reliability.

By the mid-90s, lower costs for solar photovoltaic modules and the need to power more sophisticated appliances, computers, and digital audio/video systems created a demand for larger inverter capacities and a smoother 60-cycle voltage waveform. Manufacturers responded with inverter outputs up to 5,500-watts by using higher voltage 24 to 48-volt DC inputs and more sophisticated internal electronics to increase the number of “steps” simulating the 60-cycle sine wave.

Some newer inverter models include a communications “link,” which allows interconnecting multiple inverters to provide synchronized higher wattage and voltage outputs. Since these changes resulted in much higher inverter prices, most manufacturers still offer lower cost modified sine wave models when lower capacity and power quality are not a concern.

The Trace (now Xantrex) SW series 3,600, 4,000 and 5,500-watt inverters and the Outback FX 2000 and 2,500-watt inverters are currently the most popular residential inverters for back-up and off-grid power systems, although there are several other excellent brands just now entering the market. As long as it includes a battery powered inverter, any alternative energy system can continue to power the lights and appliances directly from the batteries after the sun goes down, the wind stops blowing, or the generator is turned off.

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