[KYHAM] KEN Training for Sept. 30: Intro to Auxiliary Power Systems For E-Comm Part 2

[Ron Dodson]

Intro to Auxiliary Power Systems For E-Comm Part 2
Virginia RACES gratefully acknowledges materials provided by
the Battery Council International,   Naval Facilities
Engineering Command, Sandia National Laboratories and the 
U.S. Army Construction Engineering Research Laboratory, used
in this compilation 
Edited by C. Ed Harris, KE4SKY and John Bartone, K4KXK, MSEE
Used here with permission.
--------------------------------

A rule of thumb for sizing a battery systems for a C/20
discharge rate is one amp-hour per watt of transmitter
output.  Estimate the amp-hour capacity required to run your
station for 24 hours by summing all loads: transmit current
times total operating time times duty cycle, plus receive
current with squelch open times standby time and repeat for
each piece of equipment.  Then multiply the total loads by
150% safety factor.  If you are too lazy to actually run the
numbers, use the 1 amp-hour per watt rule for each 24
hours of  SSB operation or 12 hours of CW, FM or digital to
ensure an adequate safety margin.    

Lead-acid batteries consist of lead alloy grid plates coated
with lead oxide paste, immersed in a solution of sulfuric
acid.  In manufacture the plates are subjected to a
"forming" charge which causes the paste on the positive
grid plates to convert to lead dioxide. The paste on the
negative plates converts to "sponge" lead.  Both materials
are highly porous, allowing electrolyte to freely penetrate
the plates.  Plates are alternated in the battery, with
porous, nonconductive separators between them, or with each
positive plate surrounded by an envelope, open at the top. 
A group of negative and positive plates with their
separators makes up an element. When immersed in
electrolyte, an element comprises a battery "cell."   
In lead acid batteries each cell is nominally 2 volts. 
Multiple cells are connected in series to increase voltage. 
Larger or more plates increase amp-hour capacity, but not
voltage. Thicker or fewer plates per cell allow more cycles
and longer life for the battery.  The lower the antimony
content in the plates, the lower the internal resistance
and the less resistant the battery is to charging.  Less
antimony also reduces water consumption through
electrolysis.  

Pure lead plates may break during transportation or service
operations requiring removal of the battery.   More antimony
allows deeper discharge without damage to the plates and
longer service life.  The plates in most automotive
batteries are 2-3% antimony and deep cycle batteries 5-6%
.  Calcium or strontium are used in sealed lead-acid
batteries and offer the same benefits and drawbacks as
antimony, but reduce self discharge when the battery is
stored without being used.  Do not exceed 25% DoD with Pb-Ca
batteries, such as gel cells!

Cells in lead-acid batteries are vented to permit hydrogen
and oxygen to escape during charging and to provide an
opening for replacing water lost due to electrolysis.  Open
caps are common in flooded batteries, but some are flame
arrester type to prevent a flame outside the battery from
entering the cell.  "Recombinant" caps contain a catalyst
which causes hydrogen and oxygen liberated during charging
to recombine into water, reducing the need to replace water
lost from the battery.   These are recommended for
stationary batteries in seasonal equipment which left for
long periods on a maintenance level float charge or to be
used in photovoltaic systems.

The percentage of acid in battery electrolyte is measured by
its specific gravity (Sg).   Only batteries which use acid
electrolyte can use specific gravity as a measurement of the
state of charge.  A hydrometer is used to measure how much
the electrolyte weighs compared to an equal quantity of
water.  The greater the state of charge, the higher the
specific gravity of the electrolyte. The lower the state of
charge, the weaker the acid and the lighter the
electrolyte.  Differences in acid density are measured by
the float in a hydrometer, which rises higher in an
electrolyte sample of high Sg than in one with a lower Sg.  

Measuring Sg of a wet, lead-acid battery during discharge is
a good indicator of the state of charge.  A fully charged
battery has an Sg of 1.265 grams per cubic centimeter, at 
75% charge 1.225, 50% charge 1.19 and fully discharged
1.120.  During charging of a flooded battery Sg lags the
charge state because complete mixing of the electrolyte does
not occur until gassing commences near the end of the charge
cycle.   Because of uncertainty of mixing, this measurement
on a fully charged battery is a better indicator of the
health of a cell.  Therefore, Sg is not the absolute measure
of capacity, but is considered in combination with load
testing and open circuit voltage.  Lead-acid batteries
accept only about 1/10 of the charging current at 30 degs. F
which they will accept at 80 degs. F. 

Lead-acid batteries at normal ambient temperature should be
charged current from 1/10 to 1/20 of capacity. When not in
service, all lead-acid batteries self discharge at rate of
about 5% per month.  The rate of self discharge increases
with the temperature. If  a lead-acid battery is left in a
deeply discharged condition for a long time it becomes
"sulfated" as sulphur in the acid combines with lead from
the plates to form lead sulphate.