[KYHAM] KEN Training for Sept. 23: Intro to Auxiliary Power Systems For E-Comm Part 1

[Ron Dodson]

This week, we begin a 8 part series on Auxiliary power
sources and systems.

This is a broad discussion on everything from different
types of batteries, to generators to photovoltiacs.  I hope
you will agree that this is written in a form that is
understandable to all levels of experience from beginners to
old hands.  Again, I thank the author for giving permission
for our use here. 

73, 
KA4MAP
===========================
Introduction to Auxiliary Power Systems For Emergency
Communications
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

Part 1-

RACES operators  can't learn all that they need to know
about auxiliary power for emergency communications from
books or lectures.  Practical experience is needed to apply
theory with common sense.  The following won't make you an
expert, but provides basic knowledge to "keep you out of
trouble," explain the basics of battery-based DC power
systems, portable generators and photovoltaics to plan
auxiliary power for your emergency communications. 

Batteries are only a temporary power source unless you have
a sustainable means of recharging them, independent from AC
mains.  Photovoltaics (PV) provide sustainable DC power at
lower life-cycle cost than generators, when combined with a
properly designed battery bank and charge controller.    

Attention to POLARITY is always important, but especially so
in DC systems!  Whenever using battery power, the correct
connection sequence is important to avoid sparking or damage
to system components.  Just because a battery is lower than
your house current does not mean it is harmless.  An arc
caused by wiring a connection in the wrong order may ignite
hydrogen given off by a battery, causing an explosion!  High
current flows at low voltages can still be lethal.  Always
disconnect all circuits before working on any power system. 

Always follow the correct re-connection sequence:

     1     positive connection to battery
     2     positive connection to load
     3     negative connection to battery
     4     negative connection to load

Typical 12-volt lead acid batteries have a voltage of about
14 volts when fully charged and 11 volts fully discharged. 
Most amateur equipment doesn't operate properly below 11.5
volts, so you can't exceed the depth of discharge at which
battery voltage under load drops to below that figure.
Battery systems are current limited and their capacity is
finite.  Oversized loads or excessive duty cycle cause rapid
depletion of battery capacity.  Battery systems must be
sized to the load, or they cannot supply the current needed.

Cold Cranking Amps (CCA) represent the current a starting
battery provides continuously for 30 seconds at 0 degs. F
before voltage is drops to 1.7 volts per cell (Vpc) which it
is fully discharged.  For MCA or Marine Cranking Amps, the
measurement is taken at 32 degs. F.  Cranking amps tell
nothing about how long a battery can run your transmitter. 
Reserve capacity  is the time a starting battery can sustain
a continuous 25 amp load before cell voltage drops to
1.7vpc. 

The performance measurements used for rating deep cycle
batteries are their amp-hour capacity and depth of discharge 
(DoD).   Amp-hour capacity is total current available over 
time, measured at 80 degs. F.  DoD is the percentage of 
capacity available during a charge- discharge cycle.

Amp-hour ratings of deep cycle batteries are based upon a
discharge rate at 1/20 of a battery's capacity, expressed
as AC over 20.  A marine battery rated 200ah at C20, when
discharged continuously at 10 amps, at 80 degrees F., will
sustain that load for 20 hrs.  Starting batteries are
designed
for  20% DoD, gel cells  25%, "deep cycle" batteries from
50% to 80% and flooded NiCds 100%. 

Starting batteries perform poorly for communications 
because they are designed for short periods of high load. 
Deep cycle batteries are acceptable and flooded NiCds best
for communications because they withstand long periods of
slow discharge.  For a typical 20% transmit duty cycle, a
100w VHF repeater, drawing 20 amps on transmit, requires a
minimum 100ah battery to stay within a C20 discharge rate,
at 80 degrees F. At lower temperatures available capacity
is reduced. Lead-acids lose 50% of their capacity at 32
degrees F!  

More rapid rates of discharge, such as using a marginally
sized battery for the load, further reduce available
capacity and the number of charge-discharge cycles the
battery will provide.  A BCI Group U1 (25 lb., 31ah) gel
cell is well balanced to power a 2-meter mobile at 25% duty
cycle, on medium power transmit, requiring about 6A for 25w
PEP, approximating C20 rate of discharge.  If Tx output is
increased to 50w, current load increases to 10 amps, a
mildly oversized load approximating C10, but enough to
shorten the life cycle of a gel cell!  A deep-cycle, flooded
lead-acid tolerates C10 with some loss of life cycle.  A
portable battery pack designed for a C10 discharge rate
should be a true "deep cycle" type rated for 50% depth of
discharge or greater, such as AGM construction, NOT gel!