Batteries & Controllers

A battery is a storage container for electrons, or electricity in other words. Does your system need a battery? Not necessarily. Most solar electric systems that feed power to the grid (your friendly local utility company), do so without batteries. But if you're beyond the power lines, or want some emergency or backup power, then you'll need batteries to allow the storage of some energy for later use.

Batteries can be made with a wide variety of chemistries, but we're going to limit our discussion to common lead-acid batteries. This is the same type of battery your car uses for starting. In terms of storage capacity per dollar, nothing else comes close. So when we work with batteries large enough to run an entire household for several days, lead-acid is the only practical choice.

Batteries designed for long-term storage and deep discharges are constructed differently from batteries designed to start a vehicle. Starting batteries will suffer short, ugly lives if put into renewable energy systems…don't go there! Using readily available car batteries on your solar system is a quick route to aggravation and trouble. Use the right tool for the job. In this case, that tool is called a "deep-cycle" battery. Golf-cart type batteries are the smallest true deep-cycle, with an average life expectancy of 3 to 5 years. There are a wide variety of choices building up to large forklift type batteries with 15 to 20 year life expectancies.

In addition, batteries designed for emergency backup are quite different from batteries designed for daily use. For emergency backup we strongly recommend the slightly more expensive sealed type batteries for two good reasons. One, backup batteries, despite all the best intentions in the world, tend to get neglected, and when the cells run low on water it does irreversible harm. Two, sealed batteries have their chemistry tweaked slightly to better tolerate long periods of inactivity. Lead-acid batteries are much like the muscles of your body, they need regular exercise, not too much, not too little, to stay in really good condition. Without exercise they get stiff and weak. Sealed batteries can tolerate long periods of inactivity a lot better than conventional wet-cell batteries. And, you'll never need to kick yourself for forgetting to water your batteries.

- Safety Advice
- Wet Cell Batteries
- Battery Sizing Advice
- Sealed Batteries
- AGM vs. Gel
- Solar Terms (Glossary)



Safety Advice

  • Protect your eyes and skin. Battery acid is a slightly dilute sulfuric acid. It will burn your eyes almost immediately, and your skin after a few minutes of exposure. Goggles or safety glasses are a must, rubber gloves are optional. Keep a box or two of baking soda and at least a quart of clean water in the battery area. Flush any battery acid contact with plenty of water. If you get acid in your eyes, flush with clear water for 15 minutes and then seek medical attention
  • Tools must be plastic-coated. Even small batteries are capable of awesome energy discharges when short circuited. Use electrician's tools, or Plastic-Dip yours so they can't possibly short out between terminals. The larger batteries we commonly use can easily turn a 10-inch crescent wrench red-hot in seconds while melting the battery terminal into a useless puddle, and for the grand finale possibly explode and start a fire at the same time.
  • Wear old clothes or polyester. No matter how careful you are around batteries, you'll probably still end up with holes in your jeans. Wear something you can afford to lose, or at least have holes in. Polyester fiber is immune.

  • Don't think, "None of that will happen to me!" The safety stuff is easy, and the potential harm is permanent.


Wet Cell Batteries

These are conventional lead-acid cells with removable caps for adding distilled water. Wet cells are your best choice for off-grid homesteads, or other stationary battery packs that get used regularly. This type of lead-acid battery is the least expensive, but needs topping up with water every 3-4 months, and appreciates being exercised regularly. Wet cells also produce hydrogen and some smelly byproducts during final charging, so they need to be enclosed and vented to outside both for safety and nasty odors avoidance.

If your batteries are going to be moved or handled a lot, are going to live in a vehicle, or are primarily for emergency backup service, then you should be looking at sealed batteries.



Battery Sizing Advice

For an off-grid household, the battery should be sized to deliver about 3 to 5 days of power while being discharged to around 50% to 60% of capacity. Less than 3 days capacity means you'll be cycling the battery heavily on a day to day basis, which isn't great for life expectancy. More than 5 days capacity usually starts getting so seriously expensive that a backup generator or other backup power source is a better investment.

Why only take 50% to 60% of the battery capacity? The more deeply you cycle a battery, the fewer charge/discharge cycles you'll get out of it. You can cycle your battery to 100% of capacity if you want to, but after a handful of cycles, you'll be buying a new one. True deep-cycle batteries like we sell are often rated for how many 80% depth of discharge cycles they'll tolerate. (This isn't a written guarantee, it's an average based on destructive testing.) For instance, the top-of-the-line Hawker Industrial Batteries are rated for 2100 cycles. In comparison, the typical Golf Cart type battery does about 225 cycles. If your cycle depth is shallower, say something around 10% to 20% like we usually aim for on a daily basis, then your Golf Cart type will give you close to 3,000 cycles, and the Hawker Industrial will give in excess of 30,000 cycles.

Battery sizing is about the capacity for storing electrons, expressed as amp-hours, not about the physical size of the battery.

All lead-acid battery cells deliver approximately 2.0 volts. Build a bigger cell, you can store more electrons, which equals more amp-hours, but it's still going to be at 2.0 volts. To raise the voltage you connect cells in series. To raise the amp-hours you use bigger cells or you connect in parallel.

Large household-sized battery packs can consist of many small batteries connected in series and parallel to deliver the voltage and amp-hour capacity needed, or it can consist of a few large cells in series. As a general rule, battery packs with a few large cells are going to last longer, be easier and less time-consuming to maintain, have less problems, but cost more initially. Cell interconnect cables have to be tightened yearly, and are a potential source of corrosion. The fewer the better.

Batteries must all be the same age, same size, and same brand within a pack. Mismatches will cause smaller batteries to work extra hard, and larger batteries to loaf and sulfate. (Hard sulfur crystals form on the lead surface reducing the available lead area, and the amount of sulfur ions to react with the lead.)



Sealed Batteries

Sealed batteries are the choice for emergency backup power systems, and they're a good choice if batteries are going to be moved regularly, or if regular maintenance isn't practical. You'll find sealed batteries in computer UPS power supplies, in all those portable battery pack gizmos to start your car or inflate your flat tire, in many self-powered medical equipment pieces, in most automatic gate openers, in cordless electric trimmers and lawnmowers, and in a lot other surprising places. They're clean, spillproof, and easy to live with. But in return they're more expensive, and they're a little fussier about their charging.

Sealed batteries should not be subjected to charging voltages above 2.35 volts per cell (14.2v for a 12-volt battery), for anything more than a few minutes. In a more or less fully charged battery, higher charging voltages would produce gassing. This creates pressure in the battery. Since exploding batteries are an unpleasant concept, manufacturers install one-way pressure relief valves. But that venting gas is water that you've lost from the battery with no way to replace it. A charge controller setup for perfectly acceptable wet cell battery charging at 14.8 volts, will kill a sealed battery within a few months. So make sure your charging sources are setup properly when you install your expensive sealed batteries.



AVM vs. Gel

There's two ways to build a sealed battery, and both claim that their way is superior. We sell both kinds, so here's the honest scoop. Absorbed Glass Mat uses a fiberglass-like material between the cells to hold the liquid electrolyte. AGM batteries are by far the most common sealed battery type, are easier to make, generally cost a bit less than gels, and because their electrolyte is more liquid, can tolerate higher charge and discharge rates. But because they're almost starved for electrolyte, AGM cells are more easily damaged by overcharging. They can't afford to lose any water. Gel cells use a jellied electrolyte with more liquid content, and are more tolerant of the occasional overcharge. But they absolutely must be assembled without any air bubbles or voids, so they cost more to build. The extra cost may be offset by the extended life expectancy gels typically offer -- 20% to 30% longer regardless of the cycle depth.