CD-ROM Aktief 1995 #3/CDA3.iso/survival/battery.zip/BATTERY.006

Text File | 1991-07-14 | 24KB | 419 lines

Lead-Acid Batteries Richard Perez In 1970, we realized that our
dreams depended on cheap land. The only desirable property we
could afford was in the outback. Everything was many miles down a
rough dirt road and far from civilized conveniencies such as
electricity. The 40 acres we finally bought is 12 miles from the
nearest paved road, telephone, or commercial electrical power. We
were ready to do without. This is not, however, an account of
doing without-- it is a story of having one's cake and eating it
too. We solved the problem of the rough road with a 4WD truck and
countless hours of mechanical maintenance. The electrical power
problem was not so easy to solve. We had to content ourselves
with kerosene lighting and doing all our construction work with
hand tools. The best solution the marketplace could offer was a
motordriven generator. This required constant operation in order
to supply power, in other words expensive. It seemed that in
America one either had power or one didn't. We needed inexpensive
home power. And we needed it to be there 24 hours a day without
constantly running a motor. We decided on a 12 volt battery
system. A lawnmower motor driving a car alternator recharges the
batteries. To this we added a homemade control system. Later, we
installed an inverter. We now have all the power we need, both 12
volts DC and 120 volts AC. This information on batteries is based
on my over 17 years of actual experience with battery based
alternative energy systems. Battery Terms The battery is the
heart of all alternative energy systems. A battery is a
collection of cells which store electrical energy in chemical
reactions. Not all batteries are the same. They have evolved into
different types to meet different needs. We are primarily
interested in the true "Deep Cycle" lead-acid battery. This type
is the most cost effective for home energy storage. In order to
discuss these batteries, we need to agree on certain terms. The
more we know about batteries, the better we can use them, and the
cheaper our power will be. Voltage Voltage is electronic
pressure. A car uses a 12 volt battery for starting. This voltage
is the addition of the six lead-acid cells which make up the
battery. Each individual lead-acid cell has a voltage (or
electronic pressure) of about 2 volts. Commercial household power
has a voltage of 120 volts. Batteries for alternative energy are
usually assembled into packs of 12, 24, 32, or 48 volts. Current
Current is the flow of electrons. The rate of this flow per unit
time is the ampere. A car tail light bulb draws about 1 to 2
amperes. The headlights on a car draw about 8 amperes each. The
starter draws about 200 to 300 amperes. Current comes in two
forms-- direct current (DC) and alternating current (AC). Regular
household power is AC. Batteries store power as direct current
(DC). Power Power is the amount of energy that is being used or
generated. The unit of power is the Watt. A 100 watt lightbulb
consumes 10 times as much energy as a 10 watt lightbulb. The
amounts of power being used and generated determine the capacity
of the battery pack required by the system. The more electricity
we consume the larger the battery must be. The power source must
also be larger to recharge the larger battery pack. Battery
Capacity Battery capacity is the amount of energy a battery
contains. This is usually rated in ampere-hours at a given
voltage. A battery rated at 100 ampere-hours will deliver 100
amperes of current for 1 hour. It can also deliver 10 amperes for




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10 hours, or 1 ampere for 100 hours. The average car battery has
a capacity of about 60 ampere-hours. Alternative energy battery
packs contain from 350 to 4,900 ampere-hours. The specified
capacity of a battery pack is determined by two factors-- how
much energy is needed and how long must the battery supply this
energy. Alternative energy systems work best with between 4 and
21 days of storage potential. A battery is similar to a bucket.
It will only contain so much electrical energy, just as the
bucket will only contain so much water. The amount of capacity a
battery has is roughly determined by its size and weight, just as
a bucket's capacity is determined by its size. It is difficult to
water a very large garden with one small bucket, it is also
difficult to run a homestead on an undersized battery. If a
battery based alternative energy system is to really work, it is
essential that the battery have enough capacity to do the job.
Undersized batteries are one of the major reasons that some folks
are not happy with their alternative energy systems. Battery
capacity is a very important factor in sizing alternative energy
systems. The size of the battery is determined by the amount of
energy you need and how long you wish to go between battery
rechargings. The capacity of the battery then determines the size
of the charge source. Everthing must be balanced if the system is
to be efficient and long-lived. State of Charge A battery's state
of charge is a percentage figure giving the amount of energy
remaining in the battery. A 300 ampere-hour battery at a 90%
state of charge will contain 270 amperes-hours of energy. At a
50% state of charge the same battery will contain 150
ampere-hours. A battery which is dicharged to a 20% or less state
of charge is said to be "deep cycled". Shallow cycle service
withdraws less than 10% of the battery's energy per cycle. State
of Discharge State of discharge is the inverse of state of
charge. A battery at a 90% state of charge is also at a 10% state
of discharge. These terms are important. It is critical for users
to know when the battery is nearly empty and should be charged.
We also need to know when the battery is full and when it is time
to stop charging. We must know the battery's state of charge (or
discharge) in order to properly cycle the battery. Lead-acid
batteries Lead-acid batteries are really the only type to
consider for home energy storage at the present time. Other types
of batteries, such as nickel-cadmium, are being made and sold,
but they are simply too expensive to fit into low budget
electrical schemes. We started out using car batteries.
Automotive Starting Batteries The main thing we learned from
using car batteries in deep cycle service is DON'T. Automotive
starting batteries are not designed for deep cycle service; they
don't last. Although they are cheap to buy, they are much more
expensive to use over a period of several years. They wear out
very quickly. Physical Construction The plates of a car battery
are made from lead sponge. The idea is to expose the maximum
plate surface area for chemical reaction. Using lead sponge makes




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the battery able to deliver high currents and still be as light
and cheap as possible. The sponge type plates do not have the
mechanical ruggedness necessary for repeated deep cycling over a
period of many years. They simply crumble with age. Types of
Service Car batteries are designed to provide up to 300 amperes
of current for very short periods of time (less than 10 seconds).
After the car has started, the battery is then constantly trickle
charged by the car's alternator. In car starting service, the
battery is usually discharged less than 1% of its rated capacity.
The car battery is designed for this very shallow cycle service.
Life Expectancy and Cost Our experience has shown us that
automobile starting batteries last about 200 cycles in deep cycle
service. This is a very short period of time, usually less than 2
years. Due to their short lifespan in home energy systems, they
are more than 3 times as expensive to use as a true deep cycle
battery. Car batteries cost around $60. for 100 ampere-hours at
12 volts. Beware of Ersatz "Deep Cycle" Batteries After the
failure of the