82 M ODEL AVIATION
tRICkLe-ChaRgIng is a somewhat
controversial subject in the model press and
at the flying field! Being a firm believer in
going to the established experts on a
particular subject, I did exactly that.
Knowing that the real experts are not the
guy with his Mach 3 turbine airplane or the
one with the 1/2-scale Extra that flies all those
impossible maneuvers, I researched the nickel
cadmium (Ni-Cd) battery manufacturers’
literature. Following is some of what I learned.
Sanyo Electric Co., Ltd., probably the
best known Ni-Cd battery supplier to the
Radio Control (RC) hobby, states in its
Engineering Handbook that, “In trickle
charge, the battery is continuously charged at
a very low rate, from C/50 to C/20, and is
kept fully charged and ready for use.”
Let’s review C/X. The rated capacity of
an Ni-Cd cell, or battery, is indicated as “C”
and stated in milliampere-hours (mAh). This
is the maker’s rating, usually printed on the
battery covering, and is generally thought to
be the current in milliamperes that the cell
will produce in a one-hour period.
That is not so; it is the accumulative current at a lower rate
during a period varying with different manufacturers from two to
five hours to a cutoff voltage—usually 1.0 or 1.1 volts per cell.
Sanyo’s cells are rated, “at a 5 hour rate at 0.2C discharge
current.” (That is a direct quote. The text uses C/X in some cases
and .XC in others—a different way of saying the same thing.)
Sanyo’s method of rating its cells for capacity is to load a cell to 0.2
capacity for five hours; the results are the published rating. A 600 mAh cell
is tested at 120 milliamps, and should provide that current for five hours.
With Sanyo’s recommended trickle rate being C/50 to C/20, the
rate for a 600 mAh battery would be 12-30 milliamps.
Saft America Inc. is the battery manufacturer that owns the term
“Nicad,” which is often applied erroneously to cells by any maker.
Saft states, “Trickle charge—a charging technique which maintains
full capacity in a cell or battery, normally at a C/20 or C/30 rate.”
That would be 20-30 milliamps for the 600 mAh battery. Saft
also rates its cells at the 0.2C rate.
The following is according to Power-Sonic Corporation—another
Ni-Cd battery maker: “Trickle charge: In this charge method, charging
is continuous at a low rate—typically at C/20 to C/50—to compensate
for self-discharge and to keep the battery in a fully charged state.”
Power-Sonic also rates its batteries at the five-hour 0.2C rate.
Trickle-charging is a recommended procedure—not by that hotshot
flier at the field, but by the real experts who produce these batteries.
What is the best way to accomplish the task?
There are two basic possibilities: reduce the AC input to the
charger or reduce the DC input from the charger to the battery.
The first method can be accomplished with a number of
commercially available devices, such as the Multi-Trickler described in
the January column. There is also an Auto-Trickle Adapter, available
from TME (Box 340608, Tampa FL 33694; Tel.: [813] 968-9510).
Several accessory chargers available within the Radio Control
(RC) airplane market, such as the ones from Ace R/C, automatically
switch to a trickle rate after the higher full charge rate is completed.
The trickle current is usually published, and should be checked
for compatibility with the battery in use.
I refer to the “RC airplane market” because I know that in some
cases, electric power fliers are adopting chargers primarily intended for
RC cars. Therefore, those manufacturers have reinvented the wheel
when it comes to ratings; they have gone outside the normal parameters
established within the electronics industry or the RC airplane hobby.
Check the ads for RC car electronic speed controls. You will see
current ratings for amperage only possible with a direct connection
to Grand Coulee dam—not from Ni-Cd packs currently available.
Compare those ratings to those for speed control intended for
airplanes, where you will see more logical and useful information.
The same thinking has been applied to RC car-battery chargers,
which have ratings that would be better described as “broil.”
Some of those chargers also switch to a so-called “trickle” rating, but in
that case it is often within the “overnight” parameters set by the battery
makers—most often a C/10 rate (60 mA for our 600 mAh battery).
Although this rate will not be harmful for a few hours, it will
eventually damage most batteries if left on indefinitely; it is
considerably higher than the actual recommended trickle rates.
tinkerers and Do-it-yourselfers: How about those of us who like to
roll our own?
There are a number of ways to skin that cat. It can be done easily
to the output of most chargers by adding a resistor and/or diodes.
I like to use the latter for voltage and current dropping. A silicon
diode causes a .6- to .7-volt drop in any circuit, with a current
reduction depending on the applied voltage and the load—and it is
constant, unlike a resistor.
However, there is a way that is superior to using a resistor or a
diode: a voltage regulator IC (integrated circuit), connected as a
current regulator. Its advantages are many, one of which is a low
parts count—there are only four pieces.
The regulator always furnishes the same current as determined
by the choice of resistor, regardless of the charger output voltage or
the number of cells connected to it.
And since we are dealing with very low currents, all the
components are small; the 100 mA regulator comes in a
Eloy Marez
E l e c t r o n i c s
2626 W. Northwood, Santa Ana CA 92704
Maybe the result of a “broil” charge rate charger! There was too much current too long,
but it couldn’t have been with the charger furnished with an Airtronics VG400.
transistor-size package (TO92), the resistor
can be a small 1⁄4-watter, and the lightemitting
diode (LED) can be the smallest
available size (T1).
All the parts are readily available.
Although the regulator is not a Radio Shack
item, it is available from electronic suppliers
by its basic nomenclature or as an NTE
replacement, No. 1900. There is a Radio
Shack part that can be used: No. 276-
1778—an LM317T with a larger current
capacity and larger (TO220) package.
The regulator can be assembled in a small
plastic box, with no consideration to heat—
there is not a significant amount generated.
One could equip the regulator with the
input and output plugs and plug it in after
the normal charging cycle, or it could be
permanently wired in with a switch to go
from normal to trickle.
To save a buck or two, a dual unit could
be built into the same box to handle the
transmitter and receiver batteries.
Even the decisions are easy on this one!
Determine the trickle current you want. I
have settled for C/30 as an average between
those recommended. That comes out to 20
milliamps for the 600 mA pack.
To determine the value of the one
resistor required, use the formula R
(resistance) = 1.25/I (current in amperes).
That last part can get tricky for those of
us who have been out of school for many
years. Remember that one amp is 1,000
milliamps, and that 20 of them will have to
be expressed as .020!
Working out the formula, we determine
that a 62.5-ohm resistor is required. In this
and in all cases, use the nearest available
standard value resistor, i.e., 62.0 ohms. The
nearest available Radio Shack resistor value
available, a 68-ohm 1/2-watt unit, will result
in an 18-milliamp rate—well within the
recommended values.
To check everything, including how good
your batteries are, check the battery capacity,
recharge, let it trickle for at least a couple of
weeks, then check the capacity again. It
should be very close to the original figure!
Receiver talk: Receivers are probably not
as confusing to us, because they are smaller
and without all those mechanical add-ons
that come on transmitters. They get
installed, connected, and they work—or not!
Yet receivers still hold some mystery. I
was recently asked if a so-and-so receiver
was “Gold Labeled,” referring to the label
that used to be affixed to transmitters to
indicate that they were narrowband units.
Let’s discuss some common, or not so
common, somewhat confusing RC receiver
terminology we may encounter.
Receivers are not subject to the same
federal narrowbanding requirements as
transmitters are. There is a Federal
Communications Commission (FCC) label
on the receiver, but it indicates that the
receiver will not emit a radio signal past a
designated level.
The receiver narrowband requirement is
imposed on us by AMA, and it is
absolutely necessary for us to be able to
use all our assigned frequencies without
shooting each other down. It is a good
rule, which should be enforced by all
contest and club field officials.
If you decide to ignore it and insist on
using that antique you’ve had since the 1960s,
you only have yourself to blame when you
find yourself yelling, “I ain’t got it!”
Some of the terms we use and hear
commonly, such as “frequency” and
“conversion—single and double,” are as
definite as “prop pitch” and “wingspan.”
Others, such as “narrowband,” are not.
Specific measurements can be made to
determine whether or not a receiver’s
performance fits the AMA-established
criteria, but they are highly technical and
require specialized equipment and
experience to obtain.
As applied to our use, “narrowband”
implies that the receiver’s window of
reception is narrower than older equipment
now referred to as “wideband.”
The AMA Membership Manual includes
a simple go-no-go test you can take to
determine the merit of a particular receiver.
Conversion, single and double, refers to
the conversion of the original operation
frequency—that of the companion
transmitter—to a standard one known as the
intermediate frequency (IF), at which all the
signal processing is done.
In a single-conversion receiver, the
operating frequency is “converted” in one
step to 455 kilohertz (kHz)—the standard IF
of all RC receivers and most consumer
home equipment.
The same 455 kHz is reached in a
double-conversion unit, but two steps of
“conversion” are used to get there.
The results are that the doubleconversion
receiver is less susceptible to
some of the type of interference we might
encounter in our flying efforts, and is
easier to make narrowband. It can be done
with single-conversion units, but more
adequate signal filtering has to be done
using ceramic and/or crystal filters to
achieve the same results.
Up to now we have been dealing with
common electronic and receiver
technology and terminology. Remember
that the world in general uses many more
receivers in one form or another than we
RCers do, and much of our technology
comes from there.
We are actually somewhat behind;
many high-quality communications
receivers, such as the AMA-provided Icom
R7000 frequency monitor, are tripleconversion.
I have only skimmed the surface,
because I wanted to discuss some of the
nonstandard terminology we encounter
that can make an already complicated
subject even worse.
JR claims that its receivers, all of
which are basically single-conversion
types, are “ABC&W.”
These receivers work well under
current narrowband requirements, and
they are mechanically well-made. I have
no actual figures, but they seem to be the
system of choice for the radio-critical
serious helicopter fliers—at least those
in my area.
ABC&W is not a receiver-oriented
term; it’s not found anywhere but in JR
literature. And even that is confusing;
earlier it was described as “Automatic
Blocking Circuit and Window,” and now it
is described as “Anti-Blocking Cross-
Modulations and Window.”
I have never seen a technical
description of what this is, although JR
describes it as “ … a very small electronic
window. Any signal that is distorted or
off-frequency won’t fit through the
window, and is immediately rejected. Then
the new signal is cleaned up, amplified,
and sent through the window again.”
The results are described as “unwanted
interference limiting and a higher degree of
signal filtration.” That’s not exactly the
circuit analysis-type of explanation I would
like to see, but it’s what we get.
Whatever ABC&W is, JR receivers work
very well indeed.
Another confusing term is “IPD,” from
the German RC manufacturer Multiplex.
Although not all that well-known in
the US, Multiplex is an extremely
popular, though expensive, brand in
Europe, and is often ahead in design and
technical developments.
Regardless of what you may have read
lately, Multiplex was the first to market a
“digital” servo.
A feature of the Multiplex FM
receivers, IPD stands for “Intelligent
Pulse Decoding.” As with the
aforementioned terminology, it is not
common in the receiver world.
Multiplex describes it as “a (micro-)
processor which analyses the incoming
signal for validity. As does a PCM system,
IPD filters out invalid signals.”
IPD is reported as being: “As fast as
PPM (pulse-position modulation);
Compatible with other (non-Multiplex)
transmitters; Detects invalid signals; No
servo jitter with transmitter switched off;
Hold Function; No servo travel beyond
set limit values.”
So much for the commercial. The basic
difference is that IPD use a specially
programmed computer microprocessor for
the decoding functions, unlike the normal
receivers that use an off-the-shelf IC, often
an LM4017.
This allows the design to include some
features normally found only on PCM (pulse
code modulation), such as the “Hold”
function that keeps the servos in their last
commanded position in the event that the
signal is lost or interfered with.
The IPD receiver reportedly accepts all
FM formats; i.e., negative or positive shift.
I have not had the opportunity to test or
fly one of these units. When I do, I will
share my findings.
What else out there confuses you? Let
me know! MA
May 2001 83
