Ni-Cad Battery Charger

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When contributor James E. Bialy sent us a slick design for a AA ni-cad battery charger that works off an automobile's electrical system, we were impressed . . . but we just weren't able to leave well enough alone.
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Chart: Bill of materials for Ni-Cad battery charger.
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Diagram: Circuit board for Ni-Cad battery charger.
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Diagram: Parts layout Ni-Cad battery charger.

You can kiss wasteful disposable batteries goodbye by building your own less-than-$6.00 ni-cad battery charger. (See the ni-cad battery charger diagrams in the image gallery.)

The common battery is a prime example of a product that
usage transforms all too quickly from a valuable device
into a piece of hazardous waste. And much of this
accumulation of dangerous junk is needless, since
rechargeable nickel-cadmium cells can easily be substituted
for conventional batteries in many applications. These
sturdy little power sources may cost twice as much as even
high quality alkaline cells, but they’ll make up the price
difference many times over as they go through hundreds of
charge-discharge cycles.

From a practical standpoint, though, you shouldn’t replace
every battery in your house with a ni-cad. The
rechargeables are most valuable in appliances that are used
regularly and extensively–for example, a desk
calculator that’s operated daily, or a toy that a toddler
just can’t remember to turn off. In a year’s time you could
run through dozens of conventional cells in a heavily used
appliance, while the same use wouldn’t make a
significant dent in a nicad’s life expectancy. On the other
hand, the flashlight that sits idly in the glove
compartment month after month will do better with alkaline
cells. Because nicads self-discharge at a rate of 1% to 3%
of their capacity per day, they’d go flat in an unused
flashlight within a month or two; an alkaline cell,
however, might stay on call for a year or more.

Proper Card and Feeding of Nicad Batteries

To get the most from your investment in nicad cells, you
need to treat them right. Don’t use ni-cads in applications
that drain their power too quickly: A case that’s hot to
the touch is a sure sign of strain. In general,
nickel-cadmium batteries shouldn’t be drawn down from full
charge to flat in less than an hour, though short spells of
high discharge won’t harm them. Also, it’s best to avoid
repeated cycles of equivalent partial discharge and
charge–as with a calculator you use a bit each day
and charge every night. A ni-cad will develop a sort of
electrochemical memory of the partial discharge, and you’ll
lose access to its full capacity. (However, such a cell
can be restored by draining it all the way flat
and then charging it fully.)

Most important, though, a nickel-cadmium battery must be
charged at the correct amperage. A good slow charger
applies a current that’s equal to about one-tenth of the
battery’s amp-hour capacity. Thus a 1.1-amphour ni-cad (a C
or D cell) should be charged at about 0.110 amperes. A
completely discharged battery will be restored in 14 to 16
hours at this charging rate, though it will be usable in 6
to 7 hours. (Ni-cads can be charged at rates high enough to
restore their capacity in an hour, but the charger must
then be shut off to prevent battery damage.)

The Ni-Cad Battery Charger Design

When contributor James E. Bialy sent us a slick design for
a AA ni-cad battery charger that works off an automobile’s
electrical system, we were impressed . . . but we just
weren’t able to leave well enough alone. Our tinkerers
figured that for a minor increase in cost, Bialy’s
brainstorm could be made to charge a wide range of ni-cad
cells. In our adaptation, amperage is controlled to 0.135
amperes by a power resistor (R1), and three switched
resistors (R2, R3, and R4) make the final drops to provide
0.007 amps for 9-volt batteries, 0.045 amps for AAs, and
0.110 amps for C and D cells.

We also added a transistor (TR1) to Bialy’s schematic. The
semiconductor, when biased by resistor R5, limits charging
current to the value selected with the switch. Without it,
the charge rate would change as the internal resistance of
the ni-cad dropped during charging. This addition is an
improvement over most commercial ni-cad chargers. (Note:
Any 2-watt or heat-sinked 1-watt NPN power transistor could
be substituted for the 40-watt Radio Shack item. It’s
simply the lowest-rated suitable device available from the
company.)

You can use the printed circuit design we’ve provided, or a
1-13/16 inch by 2-1/2 inch piece of perforated circuit board, to
secure the capacitors, resistors, diodes, and transistor.
Make sure that the negative sign on the large capacitor is
running toward the negative power connection solder pad
(down, as you look at the printed-circuit side of the
board) and that the diode Dl’s band is toward the powering
end of the board. Diode D2, a 15-volt zener, must have its
black band on the end nearest the positive power-connection
solder pad (up). The capacitors and diode D2 absorb (or
bypass) transient power surges from the car’s alternator,
and diode D1 prevents the ni-cad’s power from back-flowing
when the charger is connected to a switched circuit in the
car’s wiring system. Also, be sure to connect the
transistor so that B, C, and E on the circuit board match
the key on its package. The resistors can be wired either
way, but be sure that resistor R1 has at least a 2-watt
power rating. (Again, Radio Shack’s minimum suitable item
is rated at 10 watts.)

Because the resistors and transistor dissipate power to
limit the charging rate, the circuit board generates heat.
Consequently, you should drill a half-dozen 1/4 inch holes in
the bottom of the Radio Shack project box to allow air to
circulate. The circuit board-with the parts pointing
down-sits on the ridges in the plastic box and should be
glued in place. The rotary switch is secured in a 3/8 inch hole
in the box’s cover. (Note: If you can locate a 4-position
switch, such as the Digi-Key item noted in the Bill of
Materials, it can be substituted for the 12position Radio
Shack switch.) To wire the switch, look for the contact
brush underneath the connectors to determine which post
connects to which circuit. Then double-check your wiring
with a continuity tester-this is an easy place to make a
mistake. And when you mount the switch on the case cover,
mark the positions for 9-volt, AA, and C and D clearly, to
match 82 ohms, 15 ohms, and 5 ohms respectively. Two pairs
of wires pass through holes drilled in the box’s ends: the
power connection and the leads for the 9-volt battery snap
connector. The power connection can be in the form of
insulated wires, the positive of which you’ll connect to
your car’s fuse box. Or, if you’re feeling flush, you can
pay a couple of dollars for a cigarette lighter plug (use a
3-amp fused model), which will allow a more convenient
connection for your charger. In any case, tie a
figure-eight knot in each pair of wires just inside the
case to prevent tension on the circuit board.

If you decide to hard-wire your charger into the car,
you’ll have to choose between a switched and an unswitched
circuit. The former will be turned off when you shut your
engine off, thus preventing drain from your car’s battery;
the latter will allow continuous charging whether or not
the engine is on. Frankly, the amount of power the charger
uses is so small that it would take months to pull down a
healthy car battery.

You can connect a pair of AA batteries (in a Radio Shack
battery holder, No. 272-382+) or a 9volt directly to the
9-volt snap connector. To charge C or D cells, take another
9volt snap connector (you’re the proud owner of five of
these if you bought from Radio Shack), and solder its leads
to a battery holder that will handle two cells. Then you
can connect the two 9-volt snaps. Bear in mind that the
batteries you charge at one time (up to seven will work)
must be wired in series (positive to negative, positive to
negative, etc.). Only one 9-volt cell can be connected at a
time.

One of the most attractive aspects of Mr. Bialy’s use of a
car’s electrical system to charge ni-cad batteries is that
most of us use our vehicles daily. Thus we’ll be reminded
of the cells under charge and avoid wasting power by
leaving the batteries on too long. (The charging rate is
low enough that the batteries can’t be damaged by
overcharging.) Those of you who live in cold climates,
however, please take note: Ni-cads should never be charged
at a temperature lower than freezing; at less than
32 degrees Fahrenheit, the charge rate must be reduced to prevent
damage.

If, for this or other reasons, you’d prefer a charger that
plugs into the wall, you could adapt this circuit with a
12.6-volt transformer (such as Radio Shack’s No. 273-1385).
Use a larger project box (Radio Shack No. 270-222 will
work), eliminate D2 and C2, and use a 470-microfarad
capacitor for C1. Remember that you’ll be dealing with line
current, and shocks from the 120-VAC side of the circuit
can be life threatening.

Whichever way you choose to charge, you’ll be doing your
part to mend our disposable society . . . and you’re going to
save a fair amount of money in the process.

Need Help? Call 1-800-234-3368