[KYHAM] KEN Training for Oct. 7: Intro to Auxiliary Power Systems For E-Comm Part 3

[Ron Dodson]

Intro to Auxiliary Power Systems For E-Comm Part 3
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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Auxiliary batteries require a charge controller to provide
regulated, low-level current to compensate for self
discharge and protect against sulfation.  They also require
regular testing, inspection and replacement of lost
electrolyte.  If water is lost during charging and not
replaced, the process of sulfation is accelerated in those
plates which are partially exposed to air.  "Treeing" is a
short circuit occurring between positive and negative
plates.  This may be caused by manufacturing defect or rough
handling resulting in misalignment of the plates and
separators. "Mossing" caused by circulating electrolyte
bringing particulates to plate tops can also cause a short. 

Sealed, flooded (wet) lead-acid batteries are also called
"maintenance free" and experience less self-discharge.  They
contain lead-calcium or lead-strontium plates to reduce
water loss and usually have catalytic recombiners to reduce
water loss and sealed, valve regulated vents. 

Sealed-flooded lead-acids tolerate the same temperatures as
unsealed batteries, but because Sg isn't readily measured,
some sealed-wet batteries are provided with a captive float
hydrometer in the electrolyte.  Sealed-wet batteries are
common for automotive starting, but they should not be
discharged below 25%, or their life is dramatically
shortened. 

Sealed lead-acid (SLA) batteries include gel cells and
absorbed glass matt (AGM), have stabilized or "starved"
electrolyte, are valve-regulated and completely sealed. 
Because there is no free liquid electrolyte to spill, the
battery can be used safely in any position.  SLAs are much
safer than flooded types for indoor use and in sensitive
equipment such as computer uninterruptible power supplies,
which would be damaged by exposure to acid fumes.  Any
sealed battery will vent if overcharged to the point of
excessive gassing, because the valves are designed to purge
extreme pressure building up inside the battery case.

Automotive chargers intended for flooded batteries must not
be used to charge gel cells unless they have voltage
limiting circuitry to preclude their exceeding 14V during
charging.  Self discharge of gel cells is minimized by
storing them in moderately cool areas of 5 to 15 degs. C. 

Gel cells are NOT deep cycle.  A DoD of greater than 25%
significantly reduces their life.  Gel cells must not be
used below -20 degs.C, in engine compartments of vehicles or
in use subjecting them to temperatures above 50 degs. C.  
Absorbed glass matt (AGM) batteries are deep cycle, can be
quickly-recharged with no current limit and provide a broad
operating temperature range.  Their extreme depth of
discharge equals flooded nicads, but with virtually no
maintenance and low life cycle cost.  New aviation AGMs are
substantially more expensive than flooded deep cycle
batteries of equal capacity, but are much less expensive
than flooded NiCds.  Marine or emergency vehicle AGMs such
as Lifeline or Optima are not prohibitively expensive, have
aviation type cell construction and are recommended as
auxiliary power for emergency communications systems.