Battery Basics

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-
 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-
 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
 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.
 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
 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)
 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
 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.
   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.
 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
 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.
                      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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