[HomeBrew] NIMH Battery charger re-visited

[Philip Atchley]

Hi all,
First I'd like to thank all that sent me various ideas for a battery charger
for my Kenwood HT (battery pack reloaded with six 600mA NIMH cells).  Above
all it HAD to be extremely cheap to build.  A number of folks suggested using
the LM-317 set as a constant current source.  That is what I ultimately did,
along with a "second" Voltage limited trickle charger, switched between the
two.

The Specifics:  The LM-3127 was set up with a 22 Ohm resistor as a 56mA
constant current source which makes for a slow charge rate of .09C (C in this
case being 600mA).  From information gleaned from the Internet it "should"
fully charge a dead battery in roughly 15-16 hours and a .1C charger could be
left on indefinitely without damage to the battery.

However, I felt that the Constant Current source "might" eventually overcharge
a nominal 8 Volt battery pack as it's running off of a 13.6 Volt station
battery.  So a second "keep alive" charger was built on the same board using a
78M09 3 pin regulator and an 8 Volt 50mA lamp between it and the battery for
current limiting (and protection in case the battery shorts out).  I manually
switch between chargers.  The "keep alive trickler" seems to run just a few mA
into a charged battery (as indicated by the very faint glow of the 50mA bulb)
and should overcome any "self discharge" that would drop the battery Voltage
over time.

Time will tell how well this unit works.  But, the best part of the deal is
that there was ABSOLUTELY NO COST in building this thing, all parts having
been pulled from junked consumer electronics stuff in the past.  Even the 3
LEDs used as status indicators came from old VCR's.

73 de Phil KO6BB
REAL Radios "Glow In The Dark".

RX 1:  Farnsworth BC-342N Military Surplus (circa 1943).
RX 2:  Hallicrafters SX-71 (circa 1955).
RX 3:  Grundig YB-300 digital (doesn't glow, spotting set ONLY).
Antennas: 65' sloper, 1/2 size G5RV Inverted Vee,
                 15M Inverted Vee, Hustler 6BTV Vertical.