[Milsurplus] Re: [armyradios] BB-490U from Fair Radio

[aGEnuine ham]

John and group:

Let me share a couple of very brief comments about lead acid batteries
(wet or gelled electrolyte) in long term applications.  

First, if the actual chemistry of specific cells is unknown, particularly
the plate alloy contents, it is all guesswork anyway.  Having started
with that caveat, for all lead-acid batteries, constant voltage charging
is the accepted charge regimen.  For most common varieties, at room
temperature, the charge voltage is 2.4 volts per cell.  Various books
show the voltage vs. temperature curves for other environments.  When a
cell which has useful capacity and useful remaining life has been on
charge at that voltage long enough to recharge fully, the current will
drop to a low value, milliamperes for even large cells.  (Sorry, I don't
have my tech manuals here, or I could quote specific values.)  Anyway, if
the charge current is never very high, the cell is probably sulfated or
dry, either constituting failure.  If the current stays relatively high,
the cell has developed internal physical damage resulting in internal
current leakage, and is also failed.  

Now, when the full charge state of a good cell is reached, long-term
constant voltage charging (float) at 2.4 volts will eventually damage the
battery.  In the fully charged state, the float voltage must be dropped,
and here is where it gets very tricky.  The exact voltage is VERY
temperature dependent, and VERY alloy dependent.  I use 13.2 volts for 6
cell batteries as an average, but the actual range may be from 12.8 volts
to 14+ volts, depending.  Now, go back and reread my second sentence
starting with: "First, if..."

The only successful rehabilitation scheme I have ever seen is the pull
out and replace the old plates and separators approach used in telephone
central offices and switching centers back in the early part of the last
century.  But, glass cased batteries are expensive and hard to come by
anymore.

One final comment.  Be aware of the difference between standard
automobile batteries and deep discharge (often called marine or RV)
batteries.  Typical auto batteries are designed to produce the highest
peak (starting) current per unit volume, but will be ruined by one or two
full discharges.  This also applies to many gelled electrolyte cells. 
The deep discharge batteries trade peak current and capacity for the
ability to withstand numerous deep discharges.  Different plate alloy,
and most importantly, stronger separators between the plates.

There are a number of excellent resources in print, both from battery
manufacturers, and in the open literature.  I recommend perusal of some
of these if you want specific numbers or other details.  And, I might
add, I have encountered many designs of battery chargers and battery
operated systems which were designed by people apparently unfamiliar with
even the most rudimentary characteristics of lead-acid batteries.  There
is no fame or glory in being the power supply designer, so that task is
too often shoved down to the lowest person on the team, and he or she may
not have the background or experience to do it right.

73,
George
W5VPQ

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