Date: Sun, 17 Aug 97 22:32:12 CDT
Sender: Vanagon Mailing List <vanagon@vanagon.com>
From: Joel Walker <JWALKER@ua1vm.ua.edu>
Subject: How to Select the Right Battery (long)
(you might want to pick up a copy of this issue: the Figures and charts
are much better in the magazine. and some nice pictures of Alberta and
Saskatchewan :)
Motorhome magazine, September 1997
Batteries Demystified!
Knowing how to select and maintain batteries can pay big dividends.
Joel R. Donalson
There's always a sense of the super-natural associated with lead-acid
batteries. Old wives' tales prescribe aspin tablets, Rolaids or acid
transfusions as cures for any number of battery illnesses, the cause
of which is invariably attributed to several days of storage on a
concrete floor. Mysterious mail-order potions hold forth the promise
of bringing long-since-expired cells back from beyond the grave.
Shades of Frankenstein!!
Even without all the myths, there's much to learn about batteries.
Knowing how to select, install and maintain your batteries can pay
big dividends in peace of mind ... not to mention saving you some
money over the long run.
Types Defined
Deep-cycle batteries are designed to deliver moderate amperage over
long periods of time between recharges. They are designed to be cycled
(discharged and charged) hundreds of times, with plates that are more
robust than engine-starting batteries to withstand the frequent deep
discharging they must endure.
Engine-starting batteries, on the other hand, must deliver high amperage
for relatively short bursts during cranking. Their design includes more
numerous but thinner and more porous plates (compared to deep-cycle
units) to present more surface area for higher current output over short
periods. Starting batteries used in deep-cycle service will typically
last only 50 or fewer cycles.
There are several major types of lead-acid storage batteries:
flooded, "low maintenance", sealed non-gel and gel cell. The oldest and
most commonly used design is the conventional flooded cell. Flooded-
cell batteries contain blends of lead-antimoney plates with liquid
sulphuric acid as an elecrolyte. Flooded-type batteries are the least
expensive for a given storage capacity, but require regular addition of
water, and they give off corrosive and potentially explosive vapors.
Thermoil "low maintenance" batteries might be considered a subgroup of
flooded-cell batteries. They containe an oil that floats on the
electrolyte which reduces acid mist and "out-gassing" that corrodes
battery terminals. This design makes them better suited to withstand
overcharging and also decreases the need for adding water. Thermoils
solve some of the problems of conventional flooded cells without the
cost penalty of more exotic batteries.
Sealed non-gel batteries, which are flooded-cell calcium batteries (also
known as "maintenance-free"), contain a special lead-calcium plate
material with liquid acid electrolyte. These batteries can be sealed
because they don't "off-gas" as much and use less water. However, the
main advantage of flooded-cell calcium batteries is that they require
no maintenance. They typically don't withstand deep-cycling quite as
well and don't recharge as easily as conventional wet-cell batteries.
Gel-cell batteries also use lead-calcium plates, but the electrolyte
is a thick, pasty gel-type acid that doesn't splash and flow like
conventional electrolytes. Gel cells are more expensive than
conventional cells, but offer several advantages. They're maintenance-
free, can be tipped over without leakage, cause very little corrosion,
self-discharge more slowly during storage, and are less susceptible to
deterioration from sulphation if left discharged for a while. However,
their reserve capacity ratings usually are slightly lower thanb those
of equilvant-size flooded batteries.
The newest type of deep-cycle battery, the "absorbed electrolyte" design
by Optima, uses recombinant technology that allows the cells to be
completely sealed. These batteries are presently the most expensive
type based on rated output. They are completely sealed and maintenance-
free and offer excellent resistance to vibration and leakage. Optima
has offered engine-starting batteries for several years and recently
introduced deep-cycle versions.
Rate Batteries by the Numbers
Among engine-starting battiers, the Cold-Crankning Amp (CCA) is probably
the most common specification for comparing different products. This
rating describes how many amps the battery will deliver for 30 seconds
at 0 degrees F. Sometimes a Marine Cranking Amp specification is used
instead, in which the test temperature is increased to 32 degrees F.
Both ratings are intended to convey some idea of how well the battery
will spin the starter on a cold engine; the higher the number, the
better the cold-weather performance.
A good yardstick for characterizing deep-cycle performance is the
amp-hour (AH) rating, which describes how much current a battery can
continuously deliver over a specific time period (usually 20 hours)
before the voltage drops to 10.5 (which is considered to be fully
discharged). The battery's AH rating is determined by multiplying the
load current by the length of time it lasts. For example, a battery
capable of deliving 11 amps for 20 hours would earn a 220-AH rating.
Reserve Capacity is an similar, more modern specification that describes
how long the battery will deliver 25 or 75 amps before going dead. The
idea behind reserve capacity is to provide an indication of how long a
vehicle can be driven with a "dead" alternator. To convert 25-amp
reserve capacity to a roughly equivalent AH rating, multiply it by 0.6.
Larger deep-cycle batteries are rated at 75 amps; golf-cart batteries
are usually rated in this manner. A 105-minute battery is rated to
sustain a 75-amp load for 105 minutes at 80 degrees F. A high-minute
rating indicates high reserve capacity. To convert 75-amp reserve
capacity to a roughly equivalent AH rating, multiply the minutes by 2.0.
------------------------------------------------------------------------
Figure 1 - Approximate Lifetime Ownership Costs
For Some Typical Batteries
------------------------------------------------------------------------
Battery CCA AH Cycle Price Lifetime Cost
Type Rating Rating Life Range per 1000 AH
............. ....... ...... ....... ......... .............
******* Group 24 520-550 85 100-350 $50-$90 $2 - $11
Wet Group 27 550-600 105 100-350 $60-$100 $2 - $10
Cell Group 29/30 665-675 130 100-350 $80-$120 $2 - $9
Deep- GC-2 1025 220 500-750 $80-$200 $0.50 - $2
Cycle (Golf Cart)
******* Group 8D 1200 220 200-350 $225-$275 $3 - $6
******* Group 24 400 70 200-325 $135-$170 $6 - $12
Gel Group 27 490 85 200-325 $175-$200 $6 - $12
Cell Group 30 550 95 200-325 $215-$300 $7 - $16
Deep- GC-2 5850 180 350-500 $400-$470 $4 - $7
Cycle (Golf Cart)
Group 4D 1100 180 200-325 $375-$400 $6 - $11
******* Group 8D 1250 225 200-325 $450-$490 $6 - $11
******* Group 24 165-625 50 10-25 $30-$50 $24 - $100
Engine Group 27 270-700 110 10-25 $40-$80 $14 - $73
Starting Group 30 380-685 130 10-25 $75-$120 $23 - $92
Battery Group 4D 490-950 160 20-50 $120-$175 $15 - $55
******* Group 8D 850-1250 200 20-50 $150-$190 $15 - $48
------------------------------------------------------------------------
Figure 2 - Prolonging Battery Life
How far you cdrain your house batteries before recharging them has a
dramatic effect on how long they'll last. Run them completely down on
a regular basis, and you'll also be buying replacements on a regular
basis ... Up to six times as often as for a 25 percent discharge/
recharge regimen.
------------------------------------------------------------------------
Frequently Frequently Frequently Frequently
Discharged Discharged Discharged Discharged
25 percent 50 percent 75 percent 100 percent
or less
RESULT: RESULT: RESULT: RESULT:
Maximum Battery Life Battery Life Battery Life
Battery Life Reduced Reduced Reduced
50 to 70 90 to 200 200 to 600
percent percent percent
------------------------------------------------------------------------
Figure 3 - Battery State of Charge
------------------------------------------------------------------------
Battery Specific Depth of
Voltage** Gravity** Discharge
12.66 - 12.75 1.265 - 1.280 0 percent **NOTE:
12.45 - 12.51 1.225 - 1.240 25 percent Valid only if battery
12.25 - 12.27 1.190 - 1.200 50 percent has been idle for
12.03 - 12.05 1.145 - 1.160 75 percent several hours.
11.79 - 11.90 1.100 - 1.120 100 percent
------------------------------------------------------------------------
Battery Discharge & Charge Chemistry
------------------------------------------------------------------------
During discharging of a lead-acid battery, lead (Pb) from the plates
combines with sulphate (SO4) from the sulphuric acid (H2SO4) electrolyte
to form lead sulphate (PbSO4) in the plates. Water (H2O) is given off
during discharge and this lowers the specific gravity of the electrolyte.
During charging, the reactions are basically the reverse of discharging.
Sulphate (PbSO4) in both plates splits into hydrogen (H) and oxygen (O).
As the sulphate leaves the plates, it combines with hydrogen and becomes
sulphuric acid (H2SO4) again. Meanwhile, the oxygen combines with the
lead of the positive plates to form lead dioxide (PbO2).
The specific gravity of the electrolyte increases during charging
because sulphuric acid is being formed and is replacing water in the
electrolyte.
A battery will give off gas as it's being charged. Excessive water use
occurs when the battery is charged to a higher rate than it can accept.
Selecting A Starting Battery
As a rough rule of thumb for gasoline engines, the CCA rating of a
replacement battery should at the very least match the cubic-inch
displacement (cid) of the engine it will be starting; e.g., at least
350 CCAs for a 350-cid engine. However, more CCAs are better.
For diesels, the CCA rating is typically three to four times the engine
displacement, and is ually spread among two identical batteries.
Normally, both batteries should be replaced at the same time.
A top-quality starting battery will generally last at least four or
five years. Some vehicles go through batteries much sooner, usually due
to high engine-compartment temperatures and water loss or sulfation from
of use. Since a location next to an engine exhaust or in a poorly
ventilation corner can quickly kill a new battery, it's sometimes
worthwhile to either move the battery to a cooler location or install a
heat shield and/or air ducting.
Comparing Coach Batteries
The Chart in Figure 1 includes the approximate cycle lives for a variety
of battery types and sizes, along with long-term costs of ownership; in
other words, each battery's purchase cost divided by the approximate
number of amp-hours it can be expected to deliver over its lifetime.
>From this chart, it becomes apparent that golf-cart batteries generally
provide the lowest ownersip cost. Since these are 6-volt batteries, they
must be used in series-connected pairs to obtain 12 volts. This is an
advantage, since series-connected batteries share currents equally,
thereby distributing the wear and tear equally among all batteries.
Note that unlike connections in parallel, wiring two batteries in series
doesn't increase the AH capacity.
The same chart shows that starting batteries are hideously expensive to
use in deep-cycle battery service, even though their up-front cost is
sometimes less. Consequently, it's best to restrict their use to the
application for which they are optimized ... that of starting an engine.
How Much Battery Is Enough?
Estimating 12-volt electrical requirements in your motorhome is easy;
simply multiply the current consumption of each 12-volt light or
appliance by the anticipated number of hours you'll be using it
between battery recharges to find the AH requirements for that light or
appliance. For example, if a reading lamp draws 2 amps and you use it
for three hours, that's 6 amp-hours that will be drained out of your
batteries. Do the same thing for all the other lights and appliances
you'll be using, add them all together, and you'll know what your total
AH requirement is. You could buy a battery rated for that same number
of amp-hours, but it's wise to add a considerable safety margin.
Here's why:
* Depth of discharge: Battery life expectancy is seriously shortened by
deep discharges (see Figure 2). For a reasonable lifetime, no more
than 50 to 60 percent of a battery's rated AH capacity should be
drained on a regular basis.
* Aging: Battery capacity tends to drop with age, sometimes long before
the end of the battery's useful life. Furthermore, most new batteries
require a break-in period (typcially 10-15 cycles).
* Temperature: The effective capacity of any battery drops considerably
at low temperatures (typically 35 percent at 32 degrees F). Also,
extensive furnace operation places high demands on batteries.
* Internal resistance: At high discharge currents, the effective AH
capacity of any battery falls off. For example, a 220-AH battery that
would last 20 hours with an 11-amp load might last only two hours
with a 75-amp load, a cant 150 AH. This effect is particularly
important for users of inverters and other high-current 12-volt
appliances.
Since any one of these factors can leave you with fewer amp-hours than
you thought you were buying, it pays to "go large" when shopping for a
battery.
Upgrading House Battery Capacity
If your 12-volt electrical demands exceed the present capacity of your
motorhome's batteries, it's time to consider upgrading your battery
bank. If a second battery (or one or more larger batteries) will fit
inside your existing battery compartment, then you're home free.
In this vein, note that Group 29, 30, or 31 deep-cycle batteries can
sometimes be substituted for the more common Group 27 size. They're
only a tad larger, but pack considerable extra punch.
either to enlarge the existing battery compartment or to add a second
one. Remember that any compartment alterations or additions must provide
adequate ventilation, corrosion resistance, ease of maintenance, and
structural support for up to several hundred pounds of additional weight.
Note that installing sealed batteries can greatly simplify every one of
these requirements except weight.
Ideally, all house batteries should be the same size, brand, and age.
This ensures that each will share the load and recharging current
equally. For this reason, it may be advantageous to delay upgrading
your battery capacity until your present batteries are worn out.
Full or Ready to Recharge?
The chart in Figure 3 shows the approximate state of battery charge,
based upon measurements with both a voltmeter and a hydrometer (or
specific gravity meter). These readings will vary somewhat among
various brands and types of batteries. Note that all voltage readings
should be taken with a digital voltmeter, since most analog-type meters
lack enough precision to be useful.
Several recently developed products make it possible to directly display
state of charge, using a microcomputer to continuously measure the amp-
hours flowing into or out of the batteries. By taking into account such
factors as battery-bank size, age, electrolyte type, ambient temperature
and previous discharging/recharge history, these instruments can provide
quite accurate readings, considerably better than traditional voltage or
specific gravity measurements. With prices currently starting at around
$200, these products aren't for everyone, but nonetheless may, in some
cases, be a worthwhile investment for properly maintaining an expensive
bank of batteries.
Recharging Them
For non-sealed coach batteries, a good recharging technique limits
maximum charger current to no more than 10 to 25 percent of the battery
bank's total AH rating (e.g., up to 22 to 55 amps for a 220-AH bank),
keeping maximum battery voltage under approximately 14.4 to 14.8 volts.
This allows the batteries to absorb a charge as rapidly as possible,
without excessive heating or plate stress. As the batteries reach a full
state of charge at this voltage, the current will gradually decrease to
around 2 to 4 percent of the bank's total AH rating (e.g., 4 to 9 amps
for a 220-AH bank). At this point, it is desirable to either shut off
the charger, or apply a long-term maintenance voltage of approximately
13.3 to 13.5 volts.
For gel cells, maximum charging current should be limited to 20 percent
of total AH capacity, with charging voltage limited to 13.8 to 14.4.
Any long-term maintenance voltage should be in the range of 13.5 to
13.9 volts.
Note that in the interest of simplicity and economy, the battery-charger
circuits in many DC power converters use a single voltage setting
(approximately 13.8 to 14.0 volts) for both recharging and long-term
maintenance purposes. This can result in considerably longer recharge
times and greate water loss. If gel cells are installed, it may be
necessary to reduce this setting to prevent eventual battery damage.
Maintaining the performance of most non-sealed deep-cycle batteries
is aided by applying an equalization charge at least once a month. In
this process, low-amperage charging voltage is allowed to rise to
approximately 15.5 - 16.0 volts for a short period of time. This
overcharge serves to remix the electrolyte and helps in removing
sulphate deposits. During this procedure, care should be taken to
disconnect any sensitive 12-volt loads, ensure adequate ventilation
and maintain battery water levels. Gel cells and other sealed batteries
should never be equalized due to the amount of gas generated and
becaue their electrolyte does not need remixing.
Note that all the above recharging and maintenance voltages are valid
for batteries at 80 degrees F. At considerably warmer or cooler
temperatures, some adjustments to these values usually will be necessary.
Maintaining Them
Prompt, full recharging is vital to battery health. Most batteries
shouldn't be left in a partially discharged state for more than several
days; to do otherwise permanently reduces battery capacity and life
expectancy. Periodically checking water levels and cleaning terminal
connections on non-sealed versions is also essential. Only distilled
or de-ionized water should be added.
Now, regarding those potions and old wives' tales:
Few battery additives provide enough benefit to justify their cost, and
adding anything else to a battery (besides distilled water) generally
does consideralbly more harm than good. Finally, self-discharge due to
storing batteries on concrete hasn't been a problem for years ... not
since back when battery cases were made out of tar and rubber.
NOTE: Be sure to use special care when working around batteries.
Flooded-cell versions produce explosive hydrogen gas, so avoid sparks
and provide adequate ventilation. Also, battery electrolyte can be
extremely corrosive; always wear protective clothing and face gear.
Finally, be careful when using hand tools, wearing jewelry and handling
other metal objects around battery wiring. An accidental short circuit
can easily generate temperatures high enough to melt metal and cause
severe burns.
------------------------------------------------------------------------
Battery Manufacturers
------------------------------------------------------------------------
(Note: Some manufacturers supply products under several different brand
names.)
AC Delco (800) AC-DELCO
660 W. Bristol Road, #1740
Flint, MI 48554
Advanced Energy Sources, Inc. (800) 995-8305
1031 S. Santa Fe Avenue
Compton, CA 90221
East Penn Manufacturing Co., Inc. (610) 682-6361
P.O.Box 147 maker of Deka wet-cells
Lyon Station, PA 19536
Exide Corporation (610) 378-0500
645 Penn Street
Reading, PA 19601
GNB Technologies, Inc. (800) 289-4627
375 Northridge Road, Suite 100
Atlanta, GA 30350
Interstate Battery System of America (800) CRANK-IT
12770 Merit Drive, Suite 400 conventional and gell types
Dallas, TX 75251
Johnson Controls, Inc. (800) 365-7777
Specialty Battery Division
900 E. Keefe Avenue
Milwaukee, WI 53212
Optima Batteries, Inc. (888) 8-OPTIMA
17500 E. 22nd Avenue deep-cycle and engine-starting
Aurora, CO 80011
Thermoil (561) 694-9505
4362 Northlake Boulevard, Suite 213
Palm Beach Gardens, FL 33410
Trojan Battery Company (800) 423-6569
12380 Clark Street variety of heavy-duty batteries
Santa Fe Springs, CA 90670
WestCo Battery Systems, Inc. (800) 214-8040
1640 S. Stadium View
Anaheim, CA 92806
