[KYHAM] KEN Training for Oct. 21: Intro to Auxiliary Power Systems For E-Comm Part 5

[Ron Dodson]

This week, we continue our 8 part series on Auxiliary power
sources/systems. 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.  Again, I thank the authors for giving
permission
for our use here. 

73, 
KA4MAP
===========================

The use of properly sized wire and appropriate connections
is important in DC power systems.   DC polarity must be
maintained throughout the system as must color coding
conventions of wire insulation: positive red, negative black
and equipment ground green or bare, following the wire color
conventions used in automobiles. 

Wire gages are much larger than in typical AC systems,
because undersized wiring causes excessive voltage drops
which result in loss of available power, which causes some
loads to work poorly, or not at all.  

For instance, if too small a wire gage is used between a
charge controller and battery, the voltage drop measured
during full charging rates reduces the steeping the battery
is recharged to, reducing operating capacity and life cycle.

To minimize the effects of voltage drop, keep cable runs as
short as possible.   For instance, in a 12-volt system with
a 10 amp load, such as a 2-meter mobile transceiver, the AWG
#14 wire normally provided results in a 5% voltage drop over
11 ft.  AWG #10 has a 2% drop over the same distance and 5%
drop over 18 ft.  If you must extend the battery leads to
reach a trunk-mounted transceiver, use larger gauge wire for
as much of the distance as possible.  

All splice connections must be secure and able to withstand
vibration, moisture and corrosion.  Splices of wire up to
AWG#8 should be overlapped for a length not less than five
times their diameter, spiral wrapped at least three turns,
soldered, covered with shrink wrap or electrical tape and
then waterproofed.  Larger gauge wires should be overlapped
and connected with split bolts, soldered, covered with
electrical tape and then waterproofed.  

Batteries are connected in series to increase voltage or in
parallel to increase their amp-hour capacity.  These
interconnected groups of batteries are called "battery
banks."  To determine the size of the battery bank needed,
determine the daily power requirement in watt-hours times
the number of "days of autonomy" that the equipment must
operate the loads without re-charging, plus a 30% safety
factor (to ensure the batteries are not damaged from
exces-sive discharge).  To convert watt-hours to amp-hours,
divide by the voltage.  

Always be careful around battery banks!  Even when partially
charged, an interconnected battery bank can deliver
sufficient voltage and current to arc weld!   Keep sparks
and other ignition sources away at all times.  Never allow
tools to fall onto terminals or connections.  Never permit
construction or use of shelves above the batteries.  Battery
banks must always be adequately vented.

When paralleling batteries, reduce the effects of voltage
drops which cause unequal resistances between parallel 
branches, so that all batteries in the system operate at an
equal current and voltage level.  Use the same length of
cable from each battery terminal to a central junction
point.  Positive and negative do not necessarily have to be
of the same length.  This eliminates uneven voltage drop
between batteries.  Cable size is calculated based upon the
peak charging and load current demands of the system
multi-plied by the resistance of the wire.  Marine grade or
multi strand welding cable of AWG #8 or larger is
recommended.