by Joel Donaldson
BATTERY BASICS (from Trailer Life, May 1994)
by Joel Donaldson
Inadequate battery reserve power has long been the Achilles' hell 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.
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.
