Choosing the Best Batteries
HP Issue date: March 2012
By:
Whether
you need batteries to store energy for your off-grid home, or you want backup
power to keep the lights on when the grid goes down, understanding the
different battery specifications will help you select the ideal batteries for
your application.
To choose the right battery, you first need to know
what you are trying to accomplish. What system type are you working with—off-grid
or grid-tied? Where will the battery bank be located? How much maintenance are
you prepared to do? And how often (or not) do you want to replace your
batteries? The answers to these questions will dictate which batteries make the
most sense for your renewable energy system.
Budget also plays a big role in which batteries you choose.
Buying batteries is a long-term investment, and skimping on these important
components can cripple a system. Getting it right the first time will pay off
in performance and longevity. However, simply buying the most expensive battery
does not ensure you are meeting the needs of your renewable energy system. For
your system to operate and perform well, it is crucial to understand the
various battery specifications and how they relate to RE system design.
Batteries used in an RE system can be broken down into two
basic categories: heavy duty/commercial and industrial. A common heavy
duty/commercial-type battery bank may be comprised of several 6 V, 390 AH (L-16
type) batteries. An industrial battery pack will usually be large 2 V cells
(with thicker lead plates) pre-wired to 12, 24, or 48 V and encased in a large
metal housing. You will pay more for the industrial battery bank than you will
for the equivalent battery pack made of heavy duty/commercial batteries, but
you gain longer battery life and a better warranty.
If you are working with an installing dealer, they often
have preferences about which batteries they will use. For example, some
installers will only work with L-16 type batteries because they are the largest
that they can readily move by themselves—each L-16 battery weighs around 120
pounds, whereas industrial batteries can weigh thousands of pounds, making them
difficult to maneuver without disassembly. If you have no experience with
batteries, shorter-lived, less-expensive batteries may be a better choice to
get you up to speed with battery operation. But some installers will still lean
toward the expensive industrial battery packs because they want to minimize battery
replacement. This can be especially beneficial in an off-grid setting where
just getting to the site may be difficult—much less moving the old batteries
out, getting the new ones in, and having to haul the old ones away for
recycling. However, industrial batteries are only a wise investment if you are
confident in your ability to maintain the battery bank.
Choosing Your Batteries
As with any RE system investment, your best bet will be to
identify your true needs and design a system around them. Grid-tied battery
backup systems generally use low-capacity banks made up of sealed,
non-industrial batteries that will meet your needs for running critical loads
like refrigeration and lighting during power outages. They are generally designed
to stay at float most of the time with only occasional cycling, and are often
made with calcium alloyed with the lead which helps lower battery
self-discharge losses.
To properly size a backup battery bank, compute your
critical load profile to determine daily watt-hour consumption during power
outages. That number can often be your guide for the correct battery size. Most
grid outages are less than one day, and a battery bank sized to be discharged
to 50% of capacity by the critical load profile will meet most needs nicely.
If you’re off grid and rely on your batteries to meet all
your electrical loads, buy a long-lived battery and be prepared to maintain it
well. These systems—which cycle the batteries daily—use batteries with a
lead-antimony alloy, which performs better under conditions of regular
cycling.
Typically, off-grid battery banks are sized by considering
the required “autonomy”—the number of days that the battery will provide for
the loads before reaching 50% depth of discharge (DOD). Off-grid systems
usually size a bank to provide two to four days of autonomy. For example, if
your load profile requires 5,000 WH per day, you’ll want a battery that stores
10,000 WH to achieve one day of autonomy. Four days of autonomy would require a
40,000 WH battery capacity.
Off-grid system designer opinions on maximum DOD vary
widely. Some prefer to keep the depth no greater than 20%, while others have no
fear of going below 50%. The deeper the regular discharge, the fewer cycles a
battery will give you before needing replacement. So if you do not mind
swapping your battery bank more often, go with a deeper discharge—it will save
you money up front. But if swapping batteries into and out of your system is a
royal pain, you might prefer maximizing battery life by buying a
higher-capacity battery. For the design choice that will save you money in the
long run, calculate the savings from buying fewer batteries up front, plus the
cost of more frequent battery replacement (higher DOD)—versus more batteries up
front, with fewer replacements (lower DOD).
...And Don’t Forget
To maximize battery life, batteries need to be properly
maintained by:
• Making sure the batteries get completely recharged at
least once a week by RE generation and/or supplemented with backup generator or
grid charging
·
Monitoring
the electrolyte and adding distilled water when needed if flooded batteries are
used
·
Keeping
the terminals and interconnections clean by removing built-up corrosion and
keeping the battery tops clean and dry
·
Equalizing
the batteries four to six times a year to remove surface sulfation from the
lead plates
Specs Definitions
Manufacturer. Battery manufacturers build batteries for
many different applications. Historically, RE systems used batteries originally
designed for other applications, such as powering electric golf carts. Today,
many battery manufacturers list which of their batteries are appropriate for RE
systems. All battery manufacturer Web sites listed in this guide, with the
exception of FullRiver Battery, list batteries specifically for use in RE
systems.
Model name. These letters and numbers are used
by the battery manufacturer to “name” a group of batteries that have similar
characteristics, and distinguish them from the company’s other battery lines.
It is important to not use batteries with differing model numbers within the
same battery bank, as mixing different battery types can create an imbalance
within the pack which leads to poor system performance and may cause premature
battery bank failure.
Battery type. Flooded lead-acid (FLA) batteries are the most common type
used in RE systems, particularly off grid. They are the least expensive per
capacity and, if well maintained, can have a relatively long life span.
However, they require the most maintenance. Distilled water needs to be added
to the cells on a regular basis, depending upon how often and how deeply the
bank is cycled, and upon battery charging regimens.
Valve-regulated lead-acid batteries (VLRA, a.k.a. sealed
batteries). Two general types of VRLA batteries
are available for RE systems—absorbed glass mat and gel cells. Absorbed
glass mat (AGM) lead-acid batteries are similar in
chemistry to FLA
cells. In their construction, glass mats, placed between the lead plates
(anodes and cathodes), allow the electrolyte to be suspended close to the
plates’ active material. These sealed batteries offer the advantage of not
needing to be watered and greatly reduced gassing during charge cycles. This
type of construction—adding glass mats, sealing the cells, and constructing the
plates to operate with less electrolyte—increases cost while potentially
shortening life span.
Gel cells use a “gel”-type electrolyte—with a
silica additive that causes the liquid to stiffen. Gel-cell batteries are also
sealed, which means no water to add—less maintenance and less gassing. However,
because lost electrolyte cannot be replaced, they also have a shorter life.
They are typically more expensive than FLA
or AGM batteries.
Because AGMs can’t be watered, they have to be charged more
lightly to avoid using up the finite amount of electrolyte they contain. Gel
cells also aren’t watered but need to be charged even more lightly to avoid
drying out the cell, which will kill it.
So why would you ever choose shorter-lived, more expensive
batteries like AGM or gel cells? The reasons vary, but often portability, poor
battery area ventilation, and maintenance are factors. AGM and gel cell
batteries have no liquid electrolyte to spill, so they can be a good choice for
mobile systems. And because they hardly gas, they can work well in places where
adequate ventilation for FLA
batteries isn’t possible. Because they are freeze-resistant, they may be a good
choice in applications where extreme cold is a factor.
AGM batteries are often the best choice for grid-tied
applications with battery backup, since they are designed for float or standby
applications. Because low-capacity battery banks are typical in backup
applications, both decreased cycle life and increased cost can be offset by the
fact that these batteries are rarely cycled. Plus, users with grid-tied systems
are usually less inclined to pay attention to the battery maintenance, since
they are also unaccustomed to “maintaining” their grid power. Finally, VRLA
batteries will outlast FLA
batteries that are not maintained properly (i.e. not watered regularly). If
batteries are to be deeply cycled (50% to 80% DOD), gel-cell batteries may
offer a longer life (more overall cycles) than AGMs.
Nominal Battery Voltage. Lead-acid batteries are built from
individual cells with a “nominal” voltage of 2 V. Battery packs for RE systems
are made up of combinations of cells to achieve nominal battery bank voltages
of 12, 24, or 48. When designing small systems (loads less than 1,000 WH per
day), 12 VDC is often selected as a nominal battery bank voltage if that system
is not projected to grow. So a system for a hunting cabin that isn’t going to
become a vacation home will keep battery costs down by having this low-voltage
design.
For systems with heavier load profiles, larger (and more
electrically efficient) battery voltages of 24 and 48 are commonly used. With
commercial deep-cycle batteries (like golf cart and L16), the basic unit is
often a 6 V battery made up of three, 2 V cells. In the medium-to-large systems,
these 6 V units are typically combined in series (four for a 24 V string; eight
for a 48 V string). To get greater AH capacity at that voltage, additional
strings are then paralleled or higher-capacity batteries are selected.
Amp-Hour Capacity. The sizing of the battery bank
depends on the storage capacity required, the maximum discharge rate at any
time, the maximum charge rate, and the temperatures at which the batteries will
operate.
A battery’s storage capacity—the amount of electrical energy
it can hold—is typically expressed in ampere-hours (amp-hours, or AH) at a
certain discharge rate. One AH represents a flow of electric current of 1 amp
for 1 hour. A battery is like a bucket—the larger your “bucket” is, the more AH
it can hold. Hence, the larger the AH value of a battery, given a particular
discharge rate, the more storage it offers.
Often there’s a choice of selecting a battery with either
higher voltage and lower AH, or lower voltage and higher AH. How do you know
which is most appropriate for your application? In general, limit the number of
battery series strings in parallel to three or less (two are better, and one is
ideal). This reduces imbalances introduced by having multiple paths for the
current to follow and extra electrical resistance created by paralleled battery
cables. In applications where more AH are needed, buy lower-voltage, higher AH
batteries so that several low-voltage batteries can be wired in series and the
number of paralleled battery strings can be minimized.
The denoted AH capacity of a given battery depends on the
rate at which it is being discharged and the amount of time it takes to
discharge it. Large industrial batteries, i.e. for forklifts, are often rated
at the “6-hour” rate, indicating a high current discharge rate, which brings
the battery to its terminal voltage (often at 80% DOD) in 6 hours, about the
length of a forklift’s working shift. For RE systems, a 20-hour rate is
typically used, because that is closely aligned with the more modest discharge
rates that bring the battery to a terminal voltage (again, often at 80% DOD)
over 20 hours—more closely approximating daily home use before recharging.
For converting 6-hour rates to an RE system’s more common
20-hour rate, multiply by 1.24. Using this calculation, a 100 AH, 6-hour rating
offers 124 AH at the 20-hour rate.
Bulk Charge Set Point Voltage. When charging batteries, the goal is
to put as much current as possible into the battery as efficiently as possible.
But charging a battery too quickly can cause heat to build up in the battery,
as well as excessive gassing, and can shorten the battery’s life. To keep from
harming the battery during charging, charge controllers used in RE systems
limit the charge rate based on the batteries’ voltage. As the cell voltage increases,
the charge rate (the number of amps allowed in) is reduced to prevent
overcharging.
The initial phase when all available current is allowed into
the battery is referred to as the “bulk” charge phase. Once the battery has
reached its initial bulk-charge voltage, the charge controller will hold the
voltage there for a programmed period of time (often 2 hours)—the “absorption”
charge phase. This is done to assure full charging throughout the many cells of
the battery. Note that the set points listed in this guide are per cell, so you
will need to multiply it by the number of series-connected cells to determine
the appropriate battery charge set points. For example, if you were to use four
batteries (6 V each, wired in series for a 24 V configuration) and the bulk
charge set point voltage range is 2.4 to 2.49 V for your battery’s cells, the
ideal battery bank bulk-charge voltage set point would be between 28.8 and
29.88 V (3 cells per battery x 4 batteries x 2.4 to 2.49 V).
Float-Charge Set Point Voltage. After the absorption period, the
charge controller ramps down the charging current to achieve the “float” phase,
which is a lower voltage that greatly reduces the batteries’ gassing while
still keeping the battery full. To continue the example, the float-charge set
point voltage range is 2.20 to 2.23 V for each cell. With 12 cells total, the
ideal battery bank float-charge voltage set point for this particular battery
bank would be between 26.4 and 26.76 V.
Both AGM and gel-cell batteries will not tolerate voltages
that are as high as FLAs. The charge controller’s bulk and float set points
must be programmed appropriately to avoid damaging these batteries.
Equalization Charge Set Point Voltage. An equalizing charge cycle is a
controlled overcharging of the battery bank to make sure all cells get charged,
and to remove sulfate ion bonds on the batteries’ plates and to regain battery
capacity—before permanent bonds develop. First, the battery is charged to full
capacity by completing a bulk and absorption charge cycle. Then the battery is
charged for an extended period of time, typically 6 to 12 hours, at a C/20 rate
(charging amps equal to battery’s AH capacity divided by 20). By controlling
the charge rate at C/20, the battery is kept from harm. (Uncontrolled
overcharging can warp the batteries’ plates, causing it to short out and
possibly explode.)
Equalizing an FLA
battery is essential to maintaining battery life, but can be difficult to
achieve with the limited current available from a PV array. In off-grid
applications, a backup engine generator is often used to equalize the batteries
through a charger. Off grid, the use of household loads is generally limited
during equalization to make sure enough current is available. In utility-tied
systems with batteries, the grid substitutes for a generator.
Using the example of the four-battery bank (6 V each, wired
in series for 24 V) and an equalization charge set point voltage range between
2.5 and 2.67 V per cell, the ideal battery bank equalization charge voltage set
point for this particular battery bank would be between 30 and 32.04 V.
It is commonly believed that sealed batteries should never
be equalized, yet some sealed battery manufacturers will provide an
equalization voltage set point for their batteries. It is important to note
that these values are usually the same as the bulk voltage set point for that
battery. Typically, equalizing sealed batteries means merely extending the
absorption period for a longer duration than normal. Additionally, sealed battery
“equalization” is usually done only if the battery is showing signs of
premature capacity loss (i.e., not lasting as long as normal on a charge), and
is not part of routine battery maintenance. Regardless, equalization is very
battery specific, so it is important to find appropriate voltage set points and
charge current ranges for your particular batteries.
Dimensions. When you’re designing your battery
bank, the size of the batteries—their length, width, and height—determines the
size of the containment that you’ll need to buy or build. In addition to
considering the dimensions of the batteries, it’s a good idea to leave
1/2 to 1 inch of space between each battery. This will help keep the individual
batteries operating at the same temperature and allow them to shed heat during
heavy charging regimes.
Weight. Even the smallest batteries used in
RE systems can weigh as much as a Labrador retriever—50 to 60 pounds. The
really big batteries can weigh as much as a small horse. So, adequate trucks,
skids, pallet jacks, and forklifts all become more important in moving
batteries safely as the bank grows in size. You’ll need to make sure your floor
and/or rack is stout enough to support the total weight of the bank.
Warranty. Manufacturers generally guarantee their
products to be free of defects and perform as specified for a set period of
time, and will replace defective units during this time period. Many
manufacturers offer one-year free replacement with additional prorated
warranties for two or three years. During this period, the distributor will
replace the failed unit for a percentage of the replacement cost.
Access
Batteries have enabled Christopher LaForge to live and
work for more than 20 years at his off-grid, sun- and wind–powered homestead,
SunFarm, in Bayfield County,
Wisconsin. He is an
ISP-affiliated PV instructor with the MREA, a NABCEP‑certified PV installer,
and a member of the NABCEP board of directors.
