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Fig. 3. Circuit fo r complete conditioner/c harger.
cell. Al l of the se problems
mentioned will p revent t he
battery from reaching the
se nsor cutoff, so that highrate
charg ing can continue,
eventually damaging the
battery.
At t hi s point. let m e
d igress. Som e experimenters
and suppliers of chargers
believe that the cha rge
sho uld be term inated aft er
replacing 130% t01 50% of
the battery ca pacity. This is
to prevent lo sing elect rolyte
within the cells. O thers
prefer the battery to remain
on a trickl e charge wh ich
may be as low as .01C of the
battery capac ity. Ce ne ra l
Electric Co m pany's Nickel
Cadm ium Battery Handbook
recommends that
nicad batteries be c ha rged
at a fairly high rate, but that
at completion of the normal
c ha rging period they be
kept on a .j Ccra te topping
cha rge . As all cells in the
battery may no t h a ve
identical c haracteris tics,
the topping c ha rge will perm
it the weaker cells to get a
fu ll charge without harming
the othe r cells . C .E . goes on
to say that mo st nlcads may
be left on the .1C cha rge
rate for extended periods of
time without harm.
Another item worth men-
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An other adva n tage
ga ined by this cond itioning
is more o perati ng time per
charge. In Fig. 1, the re lay
used should operat e on a
low current and ha ve a coi l
rating somewha t lower than
the batte ry voltage. Re sistor
R is chosen so that the
total current dra in on the
battery, includ ing the pilot
light and rel ay, sho uld total
the Ampere-ho ur rating of
your battery.
To describe t he operation
o f the condit ioner:
When a battery whi ch is no t
fully discharged and has
greater tha n 1 volt per cel l
is connected, current will
flow through t he ze ner
diode. This will place a positive
bias on t he base of the
transistor, causing it to canc
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boning here is the memory
effect o f nicads, but since
we are all probably aware
o f this cond itio n by now, no
disc ussion should be necessary.
Evolution of the
Condi tioner/Charger
Ve ry few chargers described
in magazine articles
o r that are available from
the HT manufacturers take
into considerat ion the state
of charge remaining in the
nicad battery. If the battery
is no t depleted when put into
t he charger, this can lead
towa rd d e ve lopin g t he
memory effect just mentioned.
To prevent thi s from
happening, the cond itioner
shown in Fig. 1 was developed.
This can be built as a
stand-alone unit or may be
inc orpora ted into a complete
charger system. Before
the battery is pl aced
on cha rge, it is put into the
conditioner. It will immediately
go into disc harge at
the 1C rate. When the battery
voltage d ro ps to the
t-volt-per-cel l cutoff point
the relay will drop o ut,
thereby termina ting a ny
fu rt her d ischarge . This procedure
will no t harm the
battery and it will erase any
memory effects.
Parts list lor Fig. 1
0 1- Zener diode (1 V per cells in battery)
02-Silicon diode
Rl-1ooo Ohms, % Wall
R2-75 Ohms, 10 Walls
AY-Coil rated at less than battery voltage
01-Motorola HEP 50038
Lamp is type 387 in sui table holder.
Parts List IOf Fig. 2
IC-HEP C4058P programmable timer
Q'-HEP SOO38 transistor
D-Sllicon diode
RY-12·volt relay with dc coil resistance greater than 250 Ohm s
R1-27oo Ohms, V. W
R2-6200 Ohms, V. W
R3-1 megohm, V. W
R4-10k, 'I. W
S1-Momentary contact push-button switch
C-2.2 uF, 35 V
Parts List lor Fig. 3
T- Stancor P 6469, 25.2 ve t A
F-1-A fuse
nt-c-zener diode (1 V per celts in battery)
02, 3-Sillcon diodes
D4-1-A bridge rectifier
R1-1ooo Ohms, '12 W
R2-75 Ohms, 10 W
R3-1ooo Ohms, V~ W
R4-25O Ohms, 10 W
R5-430 Ohms, '!J W
R6-620 Ohms, V~ W
RY1 - 4POT tz-vou coil (Allied Control TF 154C-C) (P and B KHU
1701 1)
AY2-SPST 12-..,011 coil (greater than 250 OhmS)
S1 - OPST toggle switch
S2-Momentary contact push-button-type switch
L1 , 2. 3-Type 387 lamps in appropriate holders
Cl - 10 uF, 25 V
C2-4_7 uF, 35 V
C3 - .47 uF, 35 V
C4-50 uF, 50 V
01 , 2- Motorola HEP SOO38
IC1- 7812 regulator
duct. The relay is pulled in
and the discharge cycle
starts . When the battery
vo ltage drops to the t -voltper-eel!
level, the zener
diode sto ps co nd uc ting,
cutting o ff the transistor.
This causes the re lay to
d rop o ut. terminating a ny
fur ther di sch arge .
The next considera tion in
the design of the charger
was the method to be used.
I wanted a constant current
to cha rge at a fai rly high
rate . As it is necessary to
replace about 130 % of the
battery capacity for a full
charge, I decided to cha rge
at the .sC rate for approx i­
mately two ho urs to rep
lace 100% o f the capacity.
At the end of this time
the charge r sho uld switc h
73 Magazine· Aprll ,1981 107