[KYHAM] KEN Training for Oct. 14: Intro to Auxiliary Power Systems For E-Comm Part 4

[Ron Dodson]

Intro to Auxiliary Power Systems For E-Comm Part 4
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.
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Flooded nickel cadmium (NiCd) batteries have a physical
structure resembling lead-acid batteries, but use nickel
hydroxide for the positive plates, cadmium oxide for the
negative plates and a potassium hydroxide electrolyte.  
Therefore, flooded NiCds are not subject to sulfation.  Cell
voltage of a typical NiCd is 1.2 volt, rather than 2 volts
per cell as for a lead-acid.   Flooded NiCds can survive
freezing and thawing without any affect on performance and
are less affected by high temperatures.  The self-discharge
rate of flooded NiCds ranges from < 3% up to 5% per month. 

Flooded NiCds  can be totally discharged without damage and
their ability to accept charging is less affected by the
ambient temperature than for lead-acids.  Their lower
maintenance cost and longer life cycle makes them a logical
choice for repeater back-up systems in remote or dangerous
locations.  However, flooded NiCds can't be tested as
accu-rately as a "wet" lead-acid battery, because specific
gravity of the electrolyte does not change with different
charge states.  If constant charge monitoring is a
requirement, flooded NiCds are not the best choice.

Dry NiCds used in portable transceivers require care to
avoid deep discharge, which causes cell reversal or
over-charging, which results in irreversible diminished
capacity due to heat damage.  Most amateurs are overly
concerned about their dry NiCds developing "memory" from
being left charged for a long time.  Most dry NiCds do not
fail from "memory," but from prolonged over-charging.  
Always bring a discharged dry NiCd pack back at a slow
controlled rate, but don't charge more than 14 hours.  A
cell which has developed memory or which has been
overcharged can usually be restored by one deep
discharge/recharge cycle as long as it doesn't out-gas.  A
weak NiCd must never be used without recharging, as
irreversible damage occurs inside a discharged dry NiCd when
a load is applied to it.