Using Batteries to Power Your Off-Grid Homestead

The lowdown on energy cells and the battery bank's role in an off-the-grid system.

| February/March 1999


Deep-cycle batteries


A properly sized, well functioning battery bank is the heart of any off-grid electrical system. It absolutely makes the difference between days filled with pleasant convenience and complete, ongoing frustration. Anyone who is considering going off-grid (and those already in the frying pan with inadequate systems) should become a battery aficionado long before buying the first (or next) piece of solar, wind, or hydroelectric equipment. Bad battery advice is easy to come by, and the dangers of misusing battery banks are very real. So let's get down to business.

The types of batteries used for off-grid system storage are called deep-cycle batteries. Don't be conned into trying auto motive batteries for this application (one of the most common bits of bad advice); they just aren't designed for it. If they were, solar catalog companies would sell you batteries rated in cold-cranking amps rather than amp/hours. The two most widely used types of deep-cycle batteries for home power systems are lead-acid (cheaper, more common) and nickel cadmium. These have two very different personalities, which you should consider before choosing what's right for you.

But before talking about each, there is a rule pertaining to both that should be followed: never mix and match batteries.

In fact, never even mix and match same type cells of different ages. The optimum situation is to buy all new cells at one time, use them for their lifetime, and then replace the whole set. If you come across used batteries, always get the exact same cells for the entire bank, and try to get ones that have already been together or, at the very least, have had similar usage. This rule has to do with the voltage of each cell within each battery running consistently as close to all the rest as possible. When an imbalance in voltage occurs, some cells stay topped out or receive too much voltage, while others never fill.

When considering lead-acids, remember that they are temperature-sensitive. A lead-acid cell has half the capacity at 25°F that it has at 80°F The result is that the colder the bank gets, the quicker it will both charge and discharge. You'll want to take this into account to avoid working with half the power when the days are shortest.

The other difficulty with lead-acids is that they must always be kept in the upper tier of their voltage operating range; above 80% is recommended for maximum battery life. Cells drained below a certain voltage can suffer damage and may be ruined. For example, on a 12-volt system, full is generally considered 12.7 volts and, although occasionally dipping into the low twelves is considered acceptable, you should try to keep the voltage from dropping below about 12.5 volts. And the higher the voltage is maintained, the longer the life expectancy of the battery. This makes adequate sizing of a lead-acid battery bank absolutely essential for the preservation of your investment. Installing a volt meter in an obvious place in the house and investing in appliances that turn themselves off when the voltage begins to go critical can help, too.

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