Batteries
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Copyright ©2009- 2010 EnerCal Institute Inc. All rights reserved
Batteries
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
1.
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
1.
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. This is more excitement than most of us need in our lives.
2.
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.
Now you've finally got a use for those '70 disco threads.
3.
Stop thinking "None of that will happen to me!" I used to think that too. I'm knowledgeable, professional, and
experienced. Still, I've managed to melt terminals, start fires, blow up batteries, and came near to blinding a
roommate. 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
(ignore at your own peril)
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.
AGM 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.
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
1.
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
1.
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. This is more excitement than most of us need in our lives.
2.
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.
Now you've finally got a use for those '70 disco threads.
3.
Stop thinking "None of that will happen to me!" I used to think that too. I'm knowledgeable, professional, and
experienced. Still, I've managed to melt terminals, start fires, blow up batteries, and came near to blinding a
roommate. 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
(ignore at your own peril)
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.
AGM 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.