How Sizing Our Battery Bank Led to Uncomfortable Truths About Our Lifestyle

I get so tired of hearing people condemn alternative energy by saying the power is only available when the wind blows or the sun shines. This belief, like so much of public opinion in the U.S. is fueled—as ever—by poor information and uninformed assumptions. Electricity created with an off the grid power system is stored in batteries for use. It’s as simple as that.

The size of one’s battery bank sets the true limit of one’s home power system. We have a small battery bank that meets the demands of a small home with few power needs. We use eight 6-Volt lead acid batteries, tied in series and parallel.

When we bought the homestead, it had a bank this size, located in the outhouse! This was very inconvenient, both in terms of power usage and monitoring, and the outhouse’s intended purpose. Within a year we realized that half the batteries had frozen one winter. Replacing the whole set doubled our capacity. We built an insulated, vented battery box inside the cabin.

The indoor battery bank. Wish you could have seen it in the outhouse!

Our battery bank. Much better than in the outhouse!

Another factor that limits our capacity is the set of restrictions we enforce to ensure maximum battery life. We avoid drawing the battery bank down below 50-60% charge. This restriction should double or better the expected battery life (about five years). The longer the batteries last, the more value we realize, for their cost and the effort of bringing them home. Operating on the top 40% of the bank’s capacity gives us about a day and a half of “heavy” electrical use without wind or sun to recharge the bank. If we’re careful, we can make it longer. Occasionally we do have low power days. When those happen, we prioritize our usage, doing without when necessary. It’s not a big deal.

What makes this conservation tricky is the nature of the charge. Lead acid batteries “draw down” during periods of heavy use, but the actual charge evens out once the draw stops. We’ll see our apparent levels drop from 100% to 60% or lower, but after we “power down,” the charge slowly returns to the true power level, generally 80-90% or more.

I taped a voltage chart to our voltage meter, matching percentage of charge to number of volts. The chart’s more detailed than our display, which only shows the nearest 10th of a volt, so we always err on the side of caution.

We learned a lot from David Smead’s Living on 12 Volts With Ample Power. I’d love to have two separate battery banks tied into the electrical system, as he suggests, using each bank on alternating days, but we lack the space and money. More batteries would be great, but they just won’t fit into our 600 square foot cabin! (Some people arrange their homes according to Feng Shui. We arrange ours according to Feng Shoehorn.)

When we moved, we slashed and burned, cutting possessions, especially electrical appliances, to the bone. Anything with a heating element, which uses (and wastes!) more electricity than other devices, got sold. The plug-in coffee maker? Out. Hotplate? No! Clothes iron? We’ll learn to live with wrinkles. We kept a hot glue gun (rarely and briefly used) but little else.

Here are our daily/frequent power needs:

  • Lights
  • Internet
  • Computers
  • Recharging phones and small batteries as needed
  • Radio/CD
  • Entertainment

Our lights and radio/CD player, a car deck, are DC (direct current, from batteries). The rest are AC (alternating current) power, and require an inverter to convert DC to AC for home appliances.

Of these, “entertainment” represents our greatest power draw.

That’s because the Zeiger family—so pioneering, so rustic, so deprived, so primitive—loves movies! Upstairs in our tiny cabin, we have a home theater: flat screen TV, amplifier, DVD player, VHS deck, and surround sound speakers.

This system is relatively efficient. The LCD television draws far less power than our previous set, and was far lighter to haul to the property. One strip powers all components. We only turn it on at show time. After we power down the system, we turn off the power strip—no phantom loads for us!

Even so, watching movies accounts for the bulk of our power usage. Our battery bank is sufficient to handle it, even the epic productions we prefer, with 2-3 hour running times.

This lends a certain irony to our power use strategy, not the least because of Aly’s homeschooling. When Aly attended public school, we didn’t allow movies on a “school night.” Once she began unschooling, we reached a paradox. Unschooling holds that humans learn naturally every waking moment. Ergo, every night is a school night. According to our rules, we can never watch movies! Instead, any night became a potential movie night, provided we had the time, inclination, and adequate power. We err on the side of caution—100% battery charge doesn’t hack it—we’ll only start a movie at 12.7 volts or, preferably, higher.

Aly soon became what Bill Cosby would call “the Power Sheriff of the House.” Her zeal for conservation is truly inspiring, until you realize she’s hoping that at the end of the day, we’ll have enough power to watch a movie! We don’t watch movies constantly, but occasionally it seems like it.

Sizing our system properly required acknowledging that we wanted to watch movies fairly often. It’s a luxury, but when we make our own electricity, it’s affordable. Besides, life’s too short to make sacrifices!

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2 Responses to How Sizing Our Battery Bank Led to Uncomfortable Truths About Our Lifestyle

  1. Survival Skvez says:

    I’d love to hear some details about the size of your system.
    What is the combined Ah of your battery bank?
    What is the charging system (wind and presumably solar) what are these rated at and typically what do you actually see (usually a far cry from the units rating)?
    Are you aware that Voltage of lead acid batteries varies not only with charge but also with temperature so you need separate columns on your Voltage to charge chart for a few temperatures? Although one of the advantages of keeping them indoors is that your home probably doesn’t vary in temperature too much but I’d still expect to see a winter/summer difference.

    Forgive me if you’ve covered these elsewhere, I’ve just found your blog have a lot of catching up on older articles to do!

  2. Mark Zeiger says:

    Hi Skvez,

    I’ve never been able to answer questions like yours satisfactorily. Others can rattle off the specs on their system, but not me! I think my batteries are rated at about 225 amps. I have 8 6 Volt T-105s tied in series and parallel. I charge it with a Whisper H40 or 100 wind generator, (currently using H40, 100 is “in the wings” if needed) rated at 460 watts and a bank of 4 64-volt solar panels.

    We have ways of figuring out our daily charging, but as long as we charge to 100% every 5 days, I don’t worry about that.

    We moved the batteries inside into an insulated box to limit temperature swings. Our monitor, a Trimetric 2025, has a thermometer that extends down into the battery area, and gives readings based on the current temperature.

    We’re in our 7th or 8th year on these batteries, and I’m beginning to work on replacing them. If we can swing the space, we’ll set up a new bank in the house and alternate days between the new and old until the old ones die completely.

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