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What is a deep
cycle battery?
The essential difference
is in the lead plates. In normal car batteries, these are
sponge type structure or consists of many thin plates,
giving a greater surface area to generate big amps for short
periods; whereas in deep cycle batteries, the lead plates
are thicker and solid.
A deep cycle battery
usually has multiple times the RC (reserve capacity) of a
car battery, but a substantially lesser CCA (Cold Cranking
Amps) rating - the focus is on consistent lower level supply
rather than intermittent short bursts of high supply.
Why not car
batteries?
Car batteries are meant to
give large amounts of juice for a very short period. Once
they've done their work turning over the engine, they are
immediately recharged by the car's alternator.
In a solar power setup,
while the draw mightn't be as brutal in short bursts, it can
continue for a very long time - for example, overnight. Car
batteries just aren't made for this type of application and
will wear out quickly. I found this out the hard way.
Different types of
deep cycle batteries
Flooded
These are very much like
car batteries only with thicker lead plates and have many of
the issues that car batteries have when used in a stand
alone power system - serviceable life, electrolyte
dissipation, stratification, transporting problems and the
dangers of explosive gas.
Gel
Gel deep cycle batteries
contain acid with the addition of silica, which turns the
acid into a jelly. Even when broken, the acid won't spill.
These batteries are ideal for daily use and deep discharge
and will work well in high temperatures. They can be
partially recharged without causing serious battery damage
and readily accept charging due to low internal resistance;
however they shouldn't be recharged using high voltages such
as what a car alternator cranks out.
AGM
AGM stands for Absorbed
Glass Mat. A Boron-Silicate glass mat is placed between the
lead plates, which immobilizes the electrolyte making it
unspillable. This type offers the same sorts of
benefits as gel batteries and AGMs are also tolerant to high
voltage charging.
Another important safety
feature of both AGM and Gel is that either do not off-gas
under normal usage. They are hermetically sealed and
recombine the oxygen and hydrogen produced within the
battery.
The lifespan of a deep
cycle battery will vary considerably with how it is used,
how it is maintained and charged, temperature, and other
factors. In extreme cases, it can vary to extremes - we have
seen L-16's killed in less than a year by severe
overcharging, and we have a large set of surplus telephone
batteries that sees only occasional (5-10 times per year)
heavy service that are now over 25 years old. We have seen
gelled cells destroyed in one day when overcharged with a
large automotive charger. We have seen golf cart batteries
destroyed without ever being used in less than a year
because they were left sitting in a hot garage without being
charged. Even the so-called "dry charged" (where
you add acid when you need them) have a shelf life of 18
months at most. They are not totally dry - they are actually
filled with acid, the plates formed and charged, then the
acid is dumped out.
These are some typical
(minimum - maximum) typical expectations for batteries
if used in deep cycle service. There are so many
variables, such as depth of discharge, maintenance,
temperature, how often and how deep cycled, etc. that it is
almost impossible to give a fixed number.
- Starting: 3-12 months
- Marine: 1-6 years
- Golf cart: 2-7 years
- AGM deep cycle: 4-7
years
- Gelled deep cycle: 2-5
years
- Deep cycle (L-16 type
etc): 4-8 years
- Rolls-Surrette premium
deep cycle: 7-15 years
- Industrial deep cycle
(Crown and Rolls 4KS series): 10-20+ years
- Telephone (float): 2-20
years. These are usually special purpose "float
service", but often appear on the surplus market as
"deep cycle". They can vary considerably,
depending on age, usage, care, and type.
- NiFe (alkaline): 5-35
years
- NiCad: 1-20 years
- Starting
(sometimes called SLI, for starting, lighting, ignition)
batteries are commonly used to start and run engines.
Engine starters need a very large starting current for a
very short time. Starting batteries have a large number
of thin plates for maximum surface area. The plates are
composed of a Lead "sponge", similar in
appearance to a very fine foam sponge. This gives a very
large surface area, but if deep cycled, this sponge will
quickly be consumed and fall to the bottom of the cells.
Automotive batteries will generally fail after 30-150
deep cycles if deep cycled, while they may last for
thousands of cycles in normal starting use (2-5%
discharge).
-
- Deep cycle
batteries are designed to be discharged down as much as
80% time after time, and have much thicker plates. The
major difference between a true deep cycle battery and
others is that the plates are SOLID Lead plates - not
sponge. This gives less surface area, thus less
"instant" power like starting batteries need.
-
- Unfortunately, it is
often impossible to tell what you are really buying in
some of the discount stores or places that specialize in
automotive batteries. The golf car battery is quite
popular for small systems and RV's. The problem is that
"golf car" refers to a size of battery
(commonly called GC-2, or T-105), not the type or
construction - so the quality and construction of a golf
car battery can vary considerably - ranging from the
cheap off brand with thin plates up the true deep cycle
brands, such as Crown, Deka, Trojan, etc. In general,
you get what you pay for.
-
- Marine batteries
are usually a "hybrid", and fall between the
starting and deep-cycle batteries, though a few (Rolls-Surrette
and Concorde, for example) are true deep cycle. In the
hybrid, the plates may be composed of Lead sponge, but
it is coarser and heavier than that used in starting
batteries. It is often hard to tell what you are getting
in a "marine" battery, but most are a hybrid.
Starting batteries are usually rated at "CCA",
or cold cranking amps, or "MCA", Marine
cranking amps - the same as "CA". Any battery
with the capacity shown in CA or MCA may not be a true
deep-cycle battery. It is sometimes hard to tell, as the
term deep cycle is often
overused. CA and MCA ratings are at 32 degrees F, while
CCA is at zero degree F. Unfortunately, the only
positive way to tell with some batteries is to buy one
and cut it open - not much of an option.
Inadequate battery reserve power has long been the Achilles' heel of
RVers who like to get away from the usual trappings of civilization,
including hookups. While an AC generator can be used to supply
auxiliary power, it can't be operated continuously, and RVers who lack
both a generator and campground electrical hookups are very battery-
dependent.
Beyond conventional 12-volt appliances, owners who have discovered the
benefits of power inverters (see "Inverters" - April 1994) to operate
120-volt AC appliances often find their previously adequate auxiliary
batteries lacking. To power all these newly added luxuries, batteries
must provide adequate output and must be kept in excellent condition.
The lead-acid battery types that are most common in successful RV
auxiliary-power applications are all of deep-cycle design. This is
important because a deep-cycle design stands up to repeated heavy
discharge-recharge usage much better than an ordinary automotive
battery. An automotive battery is designed to deliver very large
bursts of current for short periods (when starting an engine) and then
is immediately recharged (by the vehicles' alternator).
Most RV 12-volt DC and inverter power applications require the battery
to provide current for extended lengths of time before receiving any
recharge. An automotive battery will lose a significant percentage of
its full storage capacity after being heavily discharged just one time.
It will typically lose half of its capacity after 50 discharge-recharge
cycles. (A heavy discharge is one that removes all but 20 percent of the
battery's original full charge.)
By contrast, even the lightest-duty deep-cycle battery will typically
toleratre 200 to 300 such discharge-recharge cycles before reaching a
similar state; some of the heavier deep-cycle designs can exceed
10,000 such cycles. In short, no matter how "heavy duty" a battery is
claimed to be, if it isn't a deep-cycle design it won't last very long
in most inverter applications. The only battery in an RV that needn't
be of deep-cycle design is the one that starts the vehicle's engine.
When a battery becomes too old and weak to sustain a usable charge,
sulphation is most frequently the culprit. Every time a battery is
discharged, its sulfuric-acid solution is gradually broken down, leaving
deposits on the battery's lead plates. If the battery is promptly
recharged, most of this sulphation is driven back into solution, leaving
the plates in an essentially unchanged state. Leaving the battery in a
discharged state for extended periods, however, allows the sulphation to
harden into a form that permanently embeds itself within the plates.
Suplhation deposits permanently reduce the battery's storage capacity.
Chronic undercharging or excessive discharge also lead to plate
shedding, in which some of the active solid-plate material flakes off
and accumulates in the bottom of the battery. This accumulation
eventually sorts out the plates, resulting in a dead cell. Consequently,
if full storage capacity over a long service life is to be realized, it
is important to fully recharge a battery promptly and to avoid over-
discharge.
Depth of discharge
- extending battery life
Deep cycle batteries are
designed to be discharged much lower than standard car
batteries and be recharged many more times. The life of a
deep cycle battery under normal conditions is anywhere from
3 - 10 years.
Like any other piece of
equipment, the more you hammer it, the less it will last you
and one of the key strategies for getting the longest life
out of your deep cycle battery relates to depth of
discharge; i.e, how much juice you suck out of it. If a
battery is discharged to 50% (around 12.06v under load or
12.24v with no load - this can be measured with a cheap
multimeter), it will last about twice as long as if it is
taken down to 80% discharge.
A battery
"cycle" is one complete discharge and recharge
cycle. It is usually considered to be discharging from 100%
to 20%, and then back to 100%. However, there are often
ratings for other depth of discharge cycles, the most common
ones are 10%, 20%, and 50%. You have to be careful when
looking at ratings that list how many cycles a battery is
rated for unless it also states how far down it is being
discharged. For example, one of the widely advertised
telephone type (float service) batteries have been
advertised as having a 20-year life. If you look at the fine
print, it has that rating only at 5% DOD - it is much less
when used in an application where they are cycled deeper on
a regular basis. Those same batteries are rated at less than
5 years if cycled to 50%. For example, most golf cart
batteries are rated for about 550 cycles to 50% discharge -
which equates to about 2 years.
 Battery
life is directly related to how deep the
battery is cycled each time. If a battery is discharged to
50% every day, it will last about twice as long as if it is
cycled to 80% DOD. If cycled only 10% DOD, it will last
about 5 times as long as one cycled to 50%. Obviously, there
are some practical limitations on this - you don't usually
want to have a 5 ton pile of batteries sitting there just to
reduce the DOD. The most practical number to use is 50% DOD
on a regular basis. This does NOT mean you cannot go to 80%
once in a while. It's just that when designing a system when
you have some idea of the loads, you should figure on an average
DOD of around 50% for the best storage vs cost factor. Also,
there is an upper limit - a battery that is continually
cycled 5% or less will usually not last as long as one
cycled down 10%. This happens because at very shallow
cycles, the Lead Dioxide tends to build up in clumps on the
the positive plates rather in an even film. The graph above
shows how lifespan is affected by depth of discharge. The
chart is for a Concorde Lifeline battery, but all lead-acid
batteries will be similar in the shape of the curve,
although the number of cycles will vary.
How many/big a
battery to buy?
Really, the bigger/more
the better. For example, I have a 100 AH (amp hour) battery
and under normal weather conditions, even under partial
cloud, that does me fine. One day of total overcast isn't a
problem either, but a couple of grey days back to back and I
start getting around the 50% discharge mark at which point I
grudgingly have to fire up the generator.
As mentioned, given that
my 130 watt panel recharges the battery by midday under
sunny/mostly sunny conditions; that means I have spare
output that is being wasted - so I would have been wiser to
buy the 150 AH battery recommended to me which would help
better carry me through overcast days.
What's with the
AH?
AH, or amp hours, is how
deep cycle batteries are rated. If you have an appliance
that draws two amps; then it could run for 25 hours on a 100
AH battery before you'd hit the 50% DOD (depth of
discharge).
Converting watts
into amps
Most appliances are rated
in watts rather than amps, so how do you make the
conversion?
Use this simple formula:
Amps = Watts / Volts
Tip: Sometimes amps are
noted on appliances, check the manufacturer label on the
back of the item.
Calculating your
deep cycle battery needs
After determining the amp
rating of your appliances, then multiple each by the number
of hours of use per day. Then add up all those figures
and you'll get some idea of the sized battery you'll
need - but bear in mind the 50% depth of discharge
recommendation; i.e. a 100 AH battery really only gives you
50 AH if you wish for the battery to last well.
For example, if you
had a notebook computer running at 3 amps and a light
drawing 1 amp, a fully charged 100 amp hour battery could
run both for 12.5 hours without any input from your solar
panel before you'd hit the 50% discharge mark.
If determining the
amperage of all your appliances doesn't appeal to you; try
this solar
system builder that will give you a full system
recommendation based on your needs.
Figure 1 - Battery State of Charge
Charge Voltage Voltage Specific
Level (12v) (6v) Gravity
------ ------- ------- --------
100% 12.7 6.3 1.265
75% 12.4 6.2 1.225
50% 12.2 6.1 1.190
25% 12.0 6.0 1.155
0% 11.9 6.0 1.120
The maximum storage capacity of a deep-cycle lead-acid battery is
usually rated either in amp-hours, or in minutes of reserve capacity.
The amp-hour value refers to the number of amps a battery will deliver
over a specified period of time (generally implied to be 20 hours if not
specifically stated), before the battery has discharged to a useless
level (10.5 volts for a 12-volt battery).
The reserve capacity value specifies the number of continuous minutes
the battery can last while delivering 25 amps before dropping to this
same 10.5 volts. As a rule of thumb, for the smaller batteries you can
multiply the number of reserve minutes directly by 0.6 to arrive at an
approximate equivalent amp-hour rating for the battery.
Therefore, a 50 amp-hour battery (or a battery with approximately 83
minutes of reserve capacity) can be expected to deliver at least 2.5
amps for 20 continuous hours, or at least 1 amp for 50 continuous hours.
Note that at current drains much higher than those specified at the 20-
hour rate, however, the capacity of the battery starts to decline due
to internal losses and chemical inefficiencies at high currents.
Consequently, this same battery might only be able to deliver 5 amps for
nine hours (45 effective amp-hours), instead of the 10 hours (50 theo-
retical amp-hours) implied by the battery's amp-hour rating. In general,
bigger batteries can deliver higher currents without incurring this
effect.
The life expectancy of a deep-cycle battery, like all lead-acid
batteries, is directly dependent upon how heavily the battery is
routinely discharged before being recharged. Batteries that are
regularly discharged until only 10 percent of their rated capacity
remains have a much short life expectancy than identical batteries that
are rarely discharged below 50 percent. Therefore, you should not buy
a 100 amp-hour battery if you plan on routinely using all 100 amp-hours
between recharges.
A good rule of thumb is that a deep-cycle battery should not be depleted
beyond 80 percent of capacity, with 50 percent being even better. A 50
percent discharge represents a good compromise between battery life and
reasonable battery-bank size. Therefore, you would do well to buy at
least 200 amp-hours worth of batteries to meet an anticipated 100 amp-
hour discharge "budget".
Ambient temperature also has a strong effect on battery performance.
Performance of most batteries is rated at around 80 degrees F. At
higher temperatures, they have greater capacity, but their life span is
shortened, due to the acceleration of detrimental chemical reactions.
At lower temperatures, they last longer than normal (provided the
electrolyte is not allowed to freeze), but their capacity drops.
At 32 degrees F, typical capacity is reducted by 35 percent; at zero
degrees F, it is reduced by 60 percent; and at minus 20 degrees F, it
is reduced by better than 80 percent. A battery's ability to accept a
charge also drops along with the thermometer. In general, the best
trade-off between efficiency and long life occurs when the battery is
maintained at around room temperatures. For RV owners, this means that
batteries in a compartment that is insulated from extreme cold and heat
will last longer and deliver more consistent power.
As a battery is discharged, the sulfuric-acid solution inside each cell
is gradually converted to water. Consequently, the specific gravity of
this solution also drops as the battery discharges. This change can be
easily measured with a hydrometer in order to determine the battery's
state of charge. A good battery hydrometer includes a temperature-
correction scale (specific gravity versus battery charge varies somewhat
with temperature) and will often yield readings that are more precise
than those obtained with a voltmeter. Using a voltmeter is usually more
convenient, however, and is the only accurate method of checking sealed
batteries. Consult Figure 1 when determining the state of charge of a
battery, using either a voltmeter or a hydrometer.
Specific gravity readings should be taken by inserting the hydrometer
suction pipe into the battery cell, squirting the electrolyte into and
out of the hydrometer several times (electrolyte agitation improves
accuracy), and then reading the hydrometer while the suction tube is
still inserted into the cell. Keeping the suction tube in the cell
while taking readings minimizes the chance of spilling the electrolyte,
which could cause burns or destroy clothing. Read the hydrometer scale
at the center of the fluid inside the tube, not at the edges. Note that
any heavy battery charge or discharge currents drawn just prior to
taking specific gravity or voltage measurements will have an adverse
effect on the accuracy of the readings. The greatest accuracy is
obtained after the battery sits idle for at least 24 hours prior to
taking hydrometer or voltmeter readings.
Specific gravity readings are also helpful in determining the overall
health of a battery. For example, differences in specific gravity of
more than 0.050 between any two individual cells in a battery generally
indicate that the battery is headed for problems. By taking specific
gravity readings every month or so, owners can catch battery problems
before they cripple the entire system.
WHAT TO BUY
Regardless of what type of battery is selected, all the house batteries
in an RV should ideally be the same age, size, and brand. This is
because unsimilar batteries tend to charge and discharge at differing
rates, leading to some of the batteries in the group being consistently
undercharged during recharge and overstressed during discharge. Matching
batteries will ensure maximum life for the entire battery bank. If the
bank is diligently maintained, all batteries will wear out at about the
same time, allowing the entire bank to be changed out after a long
service life.
In buying batteries, look for similar date codes stamped on each one.
If the batteries have sat on the dealer's shelf for more than a month,
use a hydrometer or voltmeter to ensure that the state of charge has
been maintained. Don't buy old or partially discharged batteries. If
in doubt, ask the dealer about the date of manufacture and shelf
storage procedure.
Among the deep-cycle variants, the most common type is the RV/marine,
typically sold by hardware and department stores and by RV-parts
counters in automotive package (or group) sizes 24 and 27. Typical
ratings for this class of battery are approximately 80 amp-hours (110
minutes) for size 24 and 105 amp-hours (170 minutes) for size 27. These
batteries represent a reasonable value in smaller systems that are
equipped with inverters, or in installations where space is at a
premium. As deep-cycle designs go, however, they are lightweights, with
relatively short life expectancy in heavy service (typically two to
three years). This deficiency is primarily due to the use of thin lead
plates in their construction and the low antimony content of the plates
themselves.
The next most common deep-cycle version is probably the golf
cart/electric vehicle, typically sold through battery-supply houses,
some wholesale clubs, and occasionally department stores (frequently
by catalog only). These batteries are all of 6-volt design (connection
of two in series produces 12-volt output) and typically cost a tad more
per pair than a single size 27 RV/Marine battery. They provide superior
service in most RV applications (due to thicker plates and higher
antimony content) and probably represent the best value for installations
that can accommodate their large size (10-1/4 inch width, 7-inch depth,
and 11-inch height). Typical ratings are 220 amp-hours, or 400 minutes
of reserve capacity. Expected life is typically three to five years.
Note that connecting two 6-volt batteries in series does not double the
amp-hour or reserve capacity ratings, but connecting two of the resulting
12-volt battery banks in parallel (a total of four golf-cart batteries)
does.
Gelled-electrolyte ("gel-cell") batteries are becoming cheaper and more
popular among Rvers. Available in group 24, 27, 4D, 8D, and 6-volt
golf-cart sizes, they offer very good performance with virtually zero
maintenance. Where ordinary "wet-cell" batteries require monthly checks
of electrolyte levels, the gel-cells are sealed, using an electrolyte
that is jellied with nothing to replenish. They also offer higher
charging efficiency than ordinary batteries and provide slightly higher
output voltage down to complete discharge. Expected life is two to
three years, although some models may better this estimate by a great
margin.
Examples of this class of battery are the Interstate, Dryfit Prevailer,
Sonnenschein, Deka, Johnson Dynasty, and Exide Nautilus Megacycle brands.
Don't confuse these batteries with the "maintenance-free" wet-electrolyte
RV/marine batteries being sold in some department stores under brand
names such as Delco Voyager and GNB Stowaway. Unlike the true gel-cells,
these batteries are basically sealed RV/marine batteries with slightly
altered plate chemistries that reduce battery gassing (and, consequently,
water loss).
To determine how much battery capacity your application requires, add up
the total anticipated amp-hours of all the 12-volt DC appliances you
will be operating between recharges, including the demands of an inverter
if you have one. Select batteries that meet or exceed this amp-hour
value, plus a considerable safety margin. As an example, assume you will
be recharging the batteries every day adnd your appliance use habits
are as shown in Figure 2.
Figure 2 - TYPICAL POWER CONSUMPTIONS
AC Current Daily Total Daily
Appliance Consumption** Use Consumption
------------ -------------- ---------- --------------
TV set 5 Amp-hr 6.0 hours 30.0 Amp-hr
Microwave 85 Amp-hr 0.1 hours 8.5 Amp-hr
Hair Dryer 125 Amp-hr 0.1 hours 12.5 Amp-hr
VCR 3 Amp-hr 3.0 hours 9.0 Amp-hr
120-v Light 1 Amp-hr 3.0 hours 3.0 Amp-hr
120-v Light 1 Amp-hr 4.0 hours 4.0 Amp-hr
Blender 3 Amp-hr 0.1 hours 0.3 Amp-hr
Toaster 90 Amp-hr 0.1 hours 9.0 Amp-hr
-----------------------------------------------------------
Total AC appliance usage: 76.3 Amp-hr
** Measured at the 12-volt input to the inverter.
DC Current Daily Total Daily
Appliance Consumption** Use Consumption
------------ ------------ ---------- -------------
Refrigerator 0.25 Amp-hr 18.0 hours 4.5 Amp-hr
Propane Alarm 0.35 Amp-hr 24.0 hours 8.4 Amp-hr
Water Pump 4.00 Amp-hr 0.2 hours 0.8 Amp-hr
Cassette Player 2.00 Amp-hr 4.0 hours 8.0 Amp-hr
Porch Light 1.80 Amp-hr 3.0 hours 5.4 Amp-hr
Interior Light 1.80 Amp-hr 4.0 hours 7.2 Amp-hr
------------------------------------------------------------
Total DC appliance usage: 34.3 Amp-hr
Total Battery Usage: 76.3 + 34.3 = 110.6 Amp-hr
In this case, figuring a 50 percent safety margin, you would need at
least 221.2 amp-hours worth of batteries. Consequently, installing a
pair of golf-cart batteries would meet your needs, with no power to
spare. likewise, three group-27 batteries would suffice, with some
reserve power.
HOW TO KEEP THEM HAPPY
Although routinely overlooked in battery manufacturers' literature and
in many reference, most deep-cycle batteries (with the excpetion of the
gel-cell and other sealed varieties) are benefited by a periodic
controlled overcharge, which is often referred to as an equalization
charge mode. To equalize a battery, the charging is allowed to continue
well after the point at which the battery is normally considered to be
"full", taking care to avoid excessive battery heating or electrolyte
boil-off.
In a typical equalization cycle, the battery voltage is allowed to rise
to approximately 16 volts, where it is maintained for up to eight hours
by adjustment of the charging current. This process helps to mix up the
electrolyte, which otherwise tends to "stratify" (i.e., separate into
overlappying layers of acid and water), and is also useful in removing
some sulfate deposits. When performed properly, equalization doesn't
make the battery boil over, but does produce fairly vigorous bubbling.
At the end of this cycle, you can expect to add some water.
Most battery manufacturers consider one equalization charge per month
to be appropriate for batteries that are in a continuous state of charge
and discharge; less often is adequate for batteries that see a lot of
standby service. Due to the generation of considerable gas that
accompanies this process, equalization shoud NEVER be performed on a
sealed or gel-cell battery.
Also, most 12-volt DC appliances will not tolerate the 16-plus volts,
so remember to disconnect everything or detach the battery cables before
you equalize. Refer to Figure 3 for the suggest maintenance charge and
equalization voltages for various batteries. Obviously, a charger with
equalization capability is needed; there is no way to alter voltage
output on most RV converters.
Figure 3 - BATTERY VOLTAGES
Charge Cutoff Maintenance Equalization
Voltage Voltage Voltage
Wet-Cell Battery 14.4 13.5 16.3
@ 80 degrees F
Wet-Cell Battery 13.9 13.3 15.8
@ 100 degrees F
Gel-Cell Battery 14.4 13.8 NA
@ 80 degrees F
Gel-Cell Battery 14.1 13.8 NA
@ 100 degrees F
The "charge cutoff voltage" is the battery voltage at which heavy
recharging should cease; the "maintenance voltage" is the voltage at
which the battery can be safely maintained for long periods of time
without excessive water loss.
As a final thought, remember that lead-acid batteries generate highly
explosive gases. The larger the battery bank, the more gas is produced.
Do not mount any battery in an unvented location, and avoid any sparks
or open flame around the battery (particularly during and shortly after
recharging). Making or breaking electrical connections at the battery
terminals is particularly dangerous. Battery explosions often shower
large areas with acid. Wear eye, face, and skin protection, and give
the bank plenty of time to "air out" before attempting any maintenance
or inspection.
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