Notes on the Troubleshooting and Repair of Audio Equipment and Other Miscellaneous Stuff


  11.5) About fuses, IC protectors, and circuit breakers

The purpose of fuses and circuit breakers is to protect both the wiring
from heating and possible fire due to a short circuit or severe overload
and to prevent damage to the equipment due to excess current resulting
from a failed component or improper use (i.e., excess volume to loudspeakers).

Fuses use a fine wire or strip (called the element) made from a metal which
has enough resistance (more than for copper usually) to be heated by current
flow and which melts at a relatively low well defined temperature.  When the
rated current is exceeded, this element heats up enough to melt (or vaporize).
How quickly this happens depends on the extent of the overload and the type
of fuse.

Fuses found in consumer electronic equipment are usually cartridge type
consisting of a glass (or sometimes ceramic) body and metal end caps.  The
most common sizes are 1-1/4" mm x 1/4" or 20 mm x 5 mm.  Some of these have
wire leads to the end caps and are directly soldered to the circuit board but
most snap into a fuse holder or fuse clips.  Miniature types include: Pico(tm)
fuses that look like green 1/4 W resistors or other miniature cylindrical or
square varieties, little clear plastic buttons, etc.  Typical circuit board
markings are F or PR.

IC protectors are just miniature fuses specifically designed to have a
very rapid response to prevent damage to sensitive solid state components
including intergrated circuits and transistors.  These usually are often
in TO92 plastic cases but with only 2 leads or little rectangular cases
about .1" W x .3" L x .2" H.  Test just like a fuse.  These may be designated
ICP, PR, or F.

Circuit breakers may be thermal, magnetic, or a combination of the two.
Small (push button) circuit breakers for electronic equipment are most
often thermal - metal heats up due to current flow and breaks the circuit
when its temperature exceeds a set value.  The mechanism is often the
bending action of a bimetal strip or disc - similar to the operation of
a thermostat.  Flip type circuit breakers are normally magnetic.  An electro-
magnet pulls on a lever held from tripping by a calibrated spring.  These
are not usually common in consumer equipment (but are used at the electrical
service panel).

At just over the rated current, it may take minutes to break the circuit.
At 10 times rated current, the fuse may blow or circuit breaker may open
in milliseconds.

The response time of a 'normal' or 'rapid action' fuse or circuit breaker
depends on the instantaneous value of the overcurrent.

A 'slow blow' or 'delayed action' fuse or circuit breaker allows instantaneous
overload (such as normal motor starting) but will interrupt the circuit
quickly for significant extended overloads or short circuits.  A large thermal
mass delays the temperature rise so that momentary overloads are ignored.  The
magnetic type breaker adds a viscous damping fluid to slow down the movement
of the tripping mechanism.

  11.6) Fuse post mortems

Quite a bit can be inferred from the appearance of a blown fuse if the
inside is visible as is the case with a glass cartridge type.  One
advantage to the use of fuses is that this diagnostic information is
often available!

A fuse which has an element that looks intact but tests open may have just
become tired with age.  Even if the fuse does not blow, continuous cycling
at currents approaching its rating or instantaneous overloads results in
repeated heating and cooling of the fuse element.  It is quite common for
the fuse to eventually fail when no actual fault is present.

A fuse where the element is broken in a single or multiple locations blew
due to an overload.  The current was probably more than twice the fuse's
rating but not a dead short.

A fuse with a blackened or silvered discoloration on the glass where the
entire element is likely vaporized blew due to a short circuit.

This information can be of use in directly further troubleshooting.

  11.7) Fuse or circuit breaker replacement

As noted, sometimes a fuse will blow for no good reason.  Replace fuse,
end of story.  In this situation, or after the problem is found, what are
the rules of safe fuse replacement?  It is inconvenient, to say the least,
to have to wait a week until the proper fuse arrives or to venture out to
Radio Shack in the middle of the night.

Even with circuit breakers, a short circuit may so damage the contacts or
totally melt the device that replacement will be needed.

Four parameters characterizes a fuse or circuit breaker:

1. Current rating - this should not be exceeded (you have heard about not
   putting pennies in fuse boxes, right?)  (The one exception to this rule
   is if all other testing fails to reveal which component caused the fuse to
   blow in the first place.  Then, and only then, putting a larger fuse in
   or jumpering across the fuse  **just for testing** will allow the faulty
   component to identify itself by smoking or blowing its top!)  A smaller
   current rating can safely be used but depending on how close the original
   rating was to the actual current, this may blow immediately.

2. Voltage rating - this is the maximum safe working voltage of the circuit
   (including any inductive spikes) which the device will safety interrupt.
   It is safe to use a replacement with equal or high voltage rating.

3, Type - normal, fast blow, slow blow, etc.  It is safe to substitute
   a fuse or circuit breaker with a faster response characteristic but
   there may be consistent or occasional failure mostly during power-on.
   The opposite should be avoided as it risks damage to the equipment
   as semiconductors tend to die quite quickly.

4. Mounting - it is usually quite easy to obtain an identical replacement.

   However, as long as the other specifications are met, soldering a normal
   1-1/4" (3AG) fuse across a 20 mm fuse is perfectly fine, for example.
   Sometimes a fuse will have wire leads and be soldered directly onto the
   circuit board.  However, your own wires can be carefully soldered to the
   much more common cartridge type to create a suitable replacement.

  11.8) Testing a power transformer

Here are some simple tests to perform where you want to determine if a used
(or new) power transformer with known specifications is actually good:

0. Look for obvious signs of distress.  Smell it to determine if there is any
   indication of previous overheating, burning, etc.

1. Plug it in and check for output voltages to be reasonably close (probably
   somewhat high) to what you expect.

2. Leave it on for awhile.  It may get anywhere from just detectable to
   moderately warm but not to hot to touch and it shouldn't melt down, smoke,
   or blow up.  Needless to say, if it does any of the latter, the tests are

3. Find a suitable load based on: R = V/I from the specifications and make
   sure it can supply the current without overheating.  The voltage should
   also not drop excessively between no and full load (but this depends on the
   design, quality of constructions, whether you got it at Radio Shack :-),

  11.9) Identifying the connections on an unknown power transformer

Start with a good multimeter - DMM on the lowest ohms scale or VOM on the
X1 resistance range.  (You will need to be able to measure down to .1 ohms
for many of these.)  This will permit you to map the windings.

First, identify all connections that have continuity between them.  Except for
the possible case of a water soaked transformer with excessive leakage, any
reading less than infinity on the meter is an indication of a connection.  The
typical values will be between something very close to 0 ohms and 100 ohms.

Each group of connected terminals represents one winding.  The highest reading
for each group will be between the ends of the winding; others will be lower.
With a few measurements and some logical thinking, you will be able to
label the arrangement ends and taps of each winding.

Once you do this, applying a low voltage AC input (from another power
transformer driven by a Variac) will enable you to determine voltage ratios.
Then, you may be able to make some educated guesses as to the primary and
secondary.  Often, primary and secondary windings will exit from opposite
sides of the transformer.

For typical power transformers, there will be two primary wires but
international power transformers may have multiple taps as well as a
pair or primary windings (possibly with multiple taps) for switching
between 110/115/120 VAC and 220/230/240 VAC operation.  Typical color
codes for the primary winding(s) will be black or black with various
color stripes.  Almost any colors can be used for secondary windings.
Stripes may indicate center tap connections but not always.

Note: for safety, use the Variac and another isolated transformer for this.

  11.10) Determining unknown connection on international power transformers

Most likely, you can figure this out if you can identify the input connections.

There will be two primary windings (resistance between the two will be
infinite).  Each of these may also have additional taps to accommodate
various slight variations in input voltage.  For example, there may
be taps for 110/220, 115/230, 120/240, etc.

For the U.S. (110 VAC), the two primary windings will be wired in parallel.
For overseas (220 VAC) operation, they will be wired in series.  When
switching from one to the other make sure you get the phases of the two
windings correct - otherwise you will have a short circuit!  You can test
for this when you apply power - leave one end of one winding disconnected
and measure between these two points - there should be close to zero voltage
present if the phase is correct.  If the voltage is significant, reverse
one of the windings and then confirm.

A multimeter on the lowest resistance scale should permit you to determine
the internal arrangement of any taps on the primaries and which sets of
secondary terminals are connected to each winding.  This will probably
need to be a DMM as many VOMs do not have low enough resistance ranges.

It is best to test with a Variac so you can bring up the voltage gradually
and catch your mistakes before anything smokes. 

You can then power it from a low voltage AC source, say 10 VAC from your
Variac or even an AC wall adapter, to be safe and make your secondary
measurements.  Then scale all these voltage readings appropriately.

  11.11) Determining the ratings of a fried power transformer

A power transformer can die in a number of ways.  The following are the most

* Primary open.  This usually is the result of a power surge but could also
  be a short on the output leading to overheating.

  Since the primary is open, the transformer is totally lifeless.

  First, confirm that the transformer is indeed beyond redemption.  Some have
  thermal or normal fuses under the outer layer of insulating tape or paper.

* Short in primary or secondary.  This may have been the result of overheating
  or just due to poor manufacturing but for whatever reason, two wires are
  touching.  One or more outputs may be dead and even those that provide some
  voltage may be low.

  The transformer may now blow the equipment fuse and even if it does not,
  probably overheats very quickly.

  First, make sure that it isn't a problem in the equipment being powered.
  Disconnect all outputs of the transformer and confirm that it still has
  nearly the same symptoms.

There are several approaches to analyzing the blown transformer and/or
identifying what is needed as a replacement:

* If you have the time and patience and the transformer is not totally sealed
  in Epoxy or varnish, disassembling it and counting the number of turns of
  wire for each of the windings may be the surest approach.  This isn't as bad
  as it sounds.  The total time required from start to dumping the remains in
  the trash will likely be less than 20 minutes for a small power transformer.

  Remove the case and frame (if any) and separate and discard the (iron) core.
  The insulating tape or paper can then be pealed off revealing each of the 
  windings.  The secondaries will be the outer ones.  The primary will be the
  last - closest to the center.  As you unwind the wires, count the number of
  full turns around the form or bobbin.

  By counting turns, you will know the precise (open circuit) voltages of each
  of the outputs.  Even if the primary is a melted charred mass, enough of the
  wire will likely be intact to permit a fairly accurate count.  Don't worry,
  an error of a few turns between friends won't matter.

  Measuring the wire size will help to determine the relative amount of
  current each of the outputs was able to supply.  The overall ratings of
  the transformer are probably more reliably found from the wattage listed
  on the equipment nameplate.

If you cannot do this for whatever reason, some educated guesswork will be
required.  Each of the outputs will likely drive either a half wave (one
diode), full wave (2 diodes if it has a centertap), or bridge (module or
4 diodes).  For the bridge, there might be a centertap as well to provide
both a positive and negative output.

* You can sometimes estimate the voltage needed by looking at the components
  in the power supply - filter cap voltage ratings and regulators.

* The capacitor voltage ratings will give you an upper bound - they are
  probably going to be at least 25 to 50 percent above the PEAK of the input

* Where there are regulators, their type and ratings and/or the circuit itself
  may reveal what the expected output will be and thus the required input
  voltage to the regulators.  For example, if there is a 7805 regulator chip,
  you will know that its input must be greater than about 7.5 V (valleys of
  the ripple) to produce a solid 5 V output.

* If there are no regulators, then the ICs, relays, motors, whatever, that
  are powered may have voltage and current ratings indicating what power
  supply is expected (min-max).

  11.12) Determining power (VA) ratings of unknown transformers

For a transformer with a single output winding, measuring temperature rise
isn't a bad way to go.  Since you don't know what an acceptable temperature
is for the transformer, a conservative approach is to load it - increase
the current gradually - until it runs warm to the touch after an extended
period (say an hour) of time.

Where multiple output windings are involved, this is more difficult since
the safe currents from each are unknown.

(From: Greg Szekeres (szekeres@pitt.edu)).
Generally, the VA rating of individual secondary taps can be measured.  While
measuring the no load voltage, start to load the winding until the voltage
drops 10%, stop measure the voltage and measure or compute the current. 10%
would be a very safe value. A cheap transformer may compute the VA rating with
a 20% drop. 15% is considered good. You will have to play around with it to
make sure everything is ok with no overheating, etc.

(From: James Meyer (jimbob@acpub.duke.edu)).

With the open circuit voltage of the individual windings, and their DC
resistance, you can make a very reasonable assumption as to the relative
amounts of power available at each winding.

Set up something like a spread-sheet model and adjust the output current to
make the losses equal in each secondary.  The major factor in any winding's
safe power capability is wire size since the volts per turn and therefore the
winding's length is fixed for any particular output voltage.  

For the advanced course:

(From Winfield Hill (hill@rowland.org)).

We know there are many things you can learn about a fully potted transformer.
Certainly the turns ratio (from ratio of ac input-output voltage), and the
magnetizing and leakage inductances can be easily measured.  With some
assumptions about the core material (which may actually be visible at some
spot), we can move on to an estimation of the number of turns, which with the
DC resistance tells us the likely wire size...  calculating winding fill area
as a double check, we can hone in on possible core gaps.  Next, measuring
primary core saturation further illuminates the earlier guesses about the core
material, gaps and windings.  By the time one is finished with this process it
may be possible to have a rather complete description of the transformer,
allowing not only for more accurate engineering with it, but also its
replication or improvement for your task.

  11.13) Transformer repair

Some power transformers include a thermal fuse under the outer layers of
insulation.  In many cases, an overload will result in a thermal fuse opening
and if you can get at it, replacement will restore the transformer to health.

Where an open thermal fuse is not the problem, aside from bad solder or crimp
connections where the wire leads or terminals connect to the transformer
windings, anything else will require unwrapping one or more of the windings to
locate an open or short.  Where a total melt-down has occurred and the result
is a charred hunk of copper and iron, even more drastic measures would be

In principle, it would be possible to totally rebuild a faulty transformer.
All that is needed is to determine the number of turns, direction, layer
distribution and order for each winding.  Suitable magnet (sometimes called
motor wire) is readily available.

However, unless you really know what you are doing and obtain the proper
insulating material and varnish, long term reliability and safety are unknown.
Therefore, I would definitely recommend obtaining a proper commercial
replacement if at all possible.

However, DIY transformer construction is nothing new:

(From: Robert Blum (rfblum@worldnet.att.net)).

I have a book from the Government Printing Office . The title is: "Information
for the Amateur Designer of Transformers for 25 to 60 cycle circuits" by
Herbert B. Brooks. It was issued June 14, 1935 so I do not know if it is still
in print.  At the time I got it it cost $.10.

(From: Mark Zenier (mzenier@netcom.com)).

"Practical Transformer Design Handbook" by Eric Lowdon.  Trouble is, last I
checked it's out of print.  Published by both Sams and Tab Professional Books.

(From: Paul Giancaterino (PAULYGS@prodigy.net)).

I found a decent article on the subject in Radio Electronics,
May 1983. The article explains the basics, including how to figure what
amps your transformer can handle and how to size the wiring.

  11.14) Grounding of computer equipment

While electronic equipment with 3 prong plugs will generally operate properly
without an earth ground (you know, using those 3-2 prong adapters without
attaching the ground wire/lug), there are 3 reasons why this is a bad idea:

1. Safety.  The metal cases of computer equipment should be grounded so that
   it will trip a breaker or GFCI should an internal power supply short occur.

   The result can be a serious risk of shock that will go undetected until
   the wrong set of circumstances occur.

2. Line noise suppression.  There are RLC filters in the power supplies of
   computer and peripheral equipment which bypass power supply noise to
   ground.  Without a proper ground, these are largely ineffective.

   The result may be an increased number of crashes and lockups or just plain
   erratic wierd behavior.

3. Effectiveness of surge suppressors.  There are surge suppression components
   inside PC power supplies and surge suppression outlet strips.  Without a
   proper ground, H-G and N-G surge protection devices are not effective.

   The result may be increased hard failures due to line spikes and overvoltage

Chapter 12) Batteries

  12.1) Battery technology

The desire for portable power seems to be increasing exponentially with
the proliferation of notebook and palmtop computers, electronic organizers,
PDAs, cellular phones and faxes, pagers, pocket cameras, camcorders and
audio cassette recorders, boomboxes - the list is endless.

Two of the hottest areas in engineering these days are in developing
higher capacity battery technologies (electrochemical systems) for
rechargeable equipment and in the implementation of smart power management
(optimal charging and high efficiency power conversion) for portable devices.
Lithium and Nickel Metal Hydride are among the more recent additions to
the inventory of popular battery technologies.  A variety of ICs are now
available to implement rapid charging techniques while preserving battery
life.  Low cost DC-DC convertor designs are capable of generating whatever
voltages are required by the equipment at over 90% efficiency

However, most of the devices you are likely to encounter still use pretty
basic battery technologies - most commonly throwaway Alkaline and Lithium
followed by rechargeable Nickel Cadmium or Lead-Acid.  The charging circuits
are often very simple and don't really do the best job but it is adequate
for many applications.

For more detailed information on all aspects of battery technology, see
the articles at:


There is more on batteries than you ever dreamed of ever needing.  The
sections below represent just a brief introduction.

  12.2) Battery basics

A battery is, strictly speaking, made up of a number of individual cells
(most often wired in series to provide multiples of the basic cell voltage
for the battery technology - 1.2, 1.5, 2.0, or 3.0 V are most common).
However, the term is popularly used even for single cells.

Four types of batteries are typically used in consumer electronic equipment:

1. Alkaline - consisting of one or more primary cells with a nominal terminal
   voltage of 1.5 V.  Examples are AAA, AA, C, D, N, 9V ('transistor'),
   lantern batteries (6V or more), etc.  There are many other available
   sizes including miniature button cells for specialty applications like
   clocks, watches, calculators, and cameras.  In general recharging of
   alkaline batteries is not practical due to their chemistry and
   construction.  Exceptions which work (if not entirely consistently as of
   this writing) are the rechargeable Alkalines (e.g., 'Renewals').
   Advantages of alkalines are high capacity and long shelf life.  These now
   dominate the primary battery marketplace largely replacing the original
   carbon-zinc and heavy duty types.  Note that under most conditions, it
   not necessary to store alkaline batteries in the 'fridge to obtain maximum
   shelf life.

2. Lithium - these primary cells have a much higher capacity than alkalines.
   The terminal voltage is around 3 volts per cell.  These are often used
   in cameras where their high cost is offset by the convenience of long life
   and compact size.  Lithium batteries in common sizes like 9V are beginning
   to appear.  In general, I would not recommend the use of lithiums for use
   in applications where a device can be accidentally left on - particularly
   with kids' toys.  Your batteries will be drained overnight whether
   a cheap carbon zinc or a costly lithium.  However, for smoke alarms,
   the lithium 9V battery (assuming they hold up to their longevity claims)
   is ideal as a 5-10 year service life without attention can be expected.

3. Nickel Cadmium (NiCd) - these are the most common type of rechargeable 
   battery technology use in small electronic devices.  They are available in
   all the poplar sizes.  However, their terminal voltage is only 1.2 V per
   cell compared to 1.5 V per cell for alkalines (unloaded).  This is not the 
   whole story, however, as NiCds terminal voltage holds up better under load
   and as they are discharged.  Manufacturers claim 500-1000 charge-discharge
   cycles but expect to achieve these optimistic ratings only under certain
   types of applications.  In particular it is usually recommended that
   NiCds should not be discharged below about 1 V per cell and should not
   be left in a discharged state for too long.  Overcharging is also an enemy
   of NiCds and will reduce their ultimate life.  An electric shaver is an
   example of a device that will approach this cycle life as it is used
   until the battery starts to poop out and then immediately put on charge.
   If a device is used and then neglected (like a seldom used printing
   calculator), don't be surprised to find that the NiCd battery will not
   charge or will not hold a charge next time the calculator is used.

4. Lead Acid - similar to the type used in your automobile but generally
   specially designed in a sealed package which cannot leak acid under most
   conditions.  These come in a wide variety of capacities but not in standard
   sizes like AA or D.  They are used in some camcorders, flashlights, CD
   players, security systems, emergency lighting, and many other applications.
   Nominal terminal voltage is 2.0 V per cell.  These batteries definitely do
   not like to be left in a discharged condition (even more so than NiCds) and
   will quickly become unusable if left that way for any length of time.

  12.3) Battery chargers

The (energy storage) capacity, C, of a battery is measured in ampere hours
denoted a A-h (or mA-h for smaller types).  The charging rate is normally
expressed as a fraction of C - e.g., .5 C or C/2.

In most cases, trickle charging at a slow rate - C/100 to C/20 - is easier on
batteries.  Where this is convenient, you will likely see better performance
and longer life.  Such an approach should be less expensive in the long run
even if it means having extra cells or packs on hand to pop in when the others
are being charged.  Fast charging is hard on batteries - it generates heat and
gasses and the chemical reactions may be less uniform.

Each type of battery requires a different type of charging technique.

1. NiCd batteries are charged with a controlled (usually constant) current.
   Fast charge may be performed at as high as a .5-1C rate for the types of
   batteries in portable tools and laptop computers.  (C here is the amp-hour
   capacity of the battery.  A .5C charge rate for a 2 amp hour battery
   pack would use a current equal to 1 A, for example.)  Trickle charge at a
   1/20-1/10C rate.  Sophisticated charges will use a variety of techniques
   to sense end-of-charge.  Inexpensive chargers (and the type in many cheap
   consumer electronics devices) simply trickle charge at a constant current.
   Rapid chargers for portable tools, laptop computers, and camcorders, do at
   least sense the temperature rise which is one indication of having reached
   full charge but this is far from totally reliable and some damage is
   probably unavoidable as some cells reach full charge before others due
   to slight unavoidable differences in capacity.  Better charging techniques
   depend on sensing the slight voltage drop that occurs when full charge
   is reached but even this can be deceptive.  The best power management
   techniques use a combination of sensing and precise control of charge
   to each cell, knowledge about the battery's characteristics, and state
   of charge.

   While slow charging is better for NiCds, long term trickle charging is
   generally not recommended.

   Problems with simple NiCd battery chargers are usually pretty easy to
   find - bad transformer, rectifiers, capacitors, possibly a regulator.
   Where temperature sensing is used, the sensor in the battery pack may
   be defective and there may be problems in the control circuits as well.
   However, more sophisticated power management systems controlled by
   microprocessors or custom ICs and may be impossible to troubleshoot for
   anything beyond obviously bad parts or bad connections.

2. Lead acid batteries are charged with a current limited but voltage
   cutoff technique.  Although the terminal voltage of a lead-acid battery is
   2.00 V per cell nominal, it may actually reach more than 2.5 V per
   cell while charging.  For an automotive battery, 15 V is still within
   the normal range of voltages to be found on the battery terminals when
   the engine (and alternator) are running.

   A simple charger for a lead-acid battery is simply a stepped down rectified
   AC source with some resistance to provide current limiting.  The current
   will naturally taper off as the battery voltage approaches the peaks
   of the charging waveform.  This is how inexpensive automotive battery
   chargers are constructed.  For small sealed lead-acid batteries, an IC
   regulator may be used to provide current limited constant voltage charging.
   A 1 A (max) charger for a 12 V battery may use an LM317, 3 resistors,
   and two capacitors, running off of a 15 V or greater input supply.

   Trickle chargers for lead-acid batteries are usually constant voltage and
   current tapers off as the battery reaches full charge.  Therefore, leaving
   the battery under constant charge is acceptable and will maintain it at the
   desired state of full charge.

   Problems with lead-acid battery chargers are usually pretty easy to
   diagnose due to the simplicity of most designs.

  12.4) Substituting NiCds for Alkalines

First note that rechargeable batteries are NOT suitable for safety critical
applications like smoke detectors unless they are used only as emergency
power fail backup (the smoke detector is also plugged into the AC line) and
are on continuous trickle charge).  NiCds self discharge (with no load) at
a rate which will cause them to go dead in a month or two.

For many toys and games, portable phones, tape players and CD players, and
boomboxes, TVs, palmtop computers, and other battery gobbling gadgets, it
may be possible to substitute rechargeable batteries for disposable primary
batteries.  However, NiCds have a lower terminal voltage - 1.2V vs. 1.5V - and
some devices will just not be happy.  In particular, tape players may not
work well due to this reduced voltage not being able to power the motor
at a constant correct speed.  Manufacturers may specifically warn
against their use.  Flashlights will not be as bright unless the light
bulb is also replaced with a lower voltage type.  Other equipment may
perform poorly or fail to operate entirely on NiCds.  When in doubt, check
your instruction manual.

  12.5) Can a large electrolytic capacitor be substituted for a NiCd?

The quick answer is: probably not.  The charger very likely assumes that
the NiCds will limit voltage.  The circuits found in many common appliances
just use a voltage source significantly higher than the terminal voltage
of the battery pack through a current limiting resistor.  If you replace
the NiCd with a capacitor and the voltage will end up much higher than
expected with unknown consequences.  For more sophisticated chargers, the
results might be even more unpredictable.

Furthermore, even a SuperCap connot begin to compare to a small NiCd for
capacity.  A 5.5 V 1 F (that's Farad) capacitor holds about 15 W-s of energy
which is roughly equivalent to a 5 V battery of 3 A-s capacity - less than
1 mA-h.  A very tiny NiCd pack is 100 mA-h or two orders of magnitude larger.

  12.6) Determining the actual capacity of a NiCd battery

When a battery pack is not performing up to expectations or is not marked
in terms of capacity, here are some comments on experimentally determining
the A-h rating.

When laying eggs, start with a chicken :-).  Actually, you have to estimate
the capacity so that charge and discharge rates can be approximated.  However,
this is usually easy to do with a factor of 2 either way just be size:

        Size of cells       Capacity range, A-h
             AAA               .2 - .4
             AA                .4 - 1
             C                  1 - 2
             D                  1 - 5
        Cordless phone         .1 - .3
          Camcorder             1 - 3+
       Laptop computer          1 - 5+

First, you must charge the battery fully.  For a battery that does not
appear to have full capacity, this may be the only problem.  Your charger
may be cutting off prematurely due to a fault in the charger and not the
battery.  This could be due to dirty or corroded contacts on the charger
or battery, bad connections, faulty temperature sensor or other end-of-charge
control circuitry. Monitoring the current during charge to determine if the
battery is getting roughly the correct A-h to charge it fully would be a
desirable first step.  Figure about 1.2 to 1.5 times the A-h of the battery
capacity to bring it to full charge.

Then discharge at approximately a C/20 - C/10 rate until the cell voltages
drops to about 1 V (don't discharge until flat or damage may occur).  Capacity
is calculated as average current x elapsed time since the current for a NiCd
will be farily constant until very near the end.

  12.7) NiCd batteries and the infamous 'memory effect'

Whether the NiCd 'memory effect' is fact or fiction seems to depend on
one's point of view and anecdotal evidence.  What most people think is
due to the memory effect is more accurately described as voltage
depression - reduced voltage (and therefore, reduced power and capacity)
during use.

(The next section is from: Bob Myers (myers@fc.hp.com) and are based on a
 GE technical note on NiCd batteries.)

The following are the most common causes of application problems wrongly
attributed to 'memory':

1. Cutoff voltage too high - basically, since NiCds have such a flat
   voltage vs. discharge characteristic, using voltage sensing to determine
   when the battery is nearly empty can be tricky; an improper setting coupled
   with a slight voltage depression can cause many products to call a battery
   "dead" even when nearly the full capacity remains usable (albeit at a 
   slightly reduced voltage).

2. High temperature conditions - NiCds suffer under high-temp conditions; such
   environments reduce both the charge that will be accepted by the cells
   when charging, and the voltage across the battery when charged (and the
   latter, of course, ties back into the above problem).

3. Voltage depression due to long-term overcharge - Self-explanatory.  NiCds
   can drop 0.1-0.15 V/cell if exposed to a long-term (i.e., a period of
   months) overcharge.  Such an overcharge is not unheard-of in consumer gear,
   especially if the user gets in the habit of leaving the unit in a charger
   of simplistic design (but which was intended to provide enough current for
   a relatively rapid charge).  As a precaution, I do NOT leave any of my
   NiCd gear on a charger longer than the recommended time UNLESS the charger
   is specifically designed for long-term "trickle charging", and explicitly
   identified as such by the manufacturer.

4. There are a number of other possible causes listed in a "miscellaneous"
   category; these include -

  * Operation below 0 degrees C.
  * High discharge rates (above 5C) if not specifically designed for such use.
  * Inadequate charging time or a defective charger.
  * One or more defective or worn-out cells.  They do not last forever.

To close with a quote from the GE note:

"To recap, we can say that true 'memory' is exceedingly rare.  When we see
poor battery performance attributed to 'memory', it is almost always certain
to be a correctable application problem.  Of the...problems noted above,
Voltage Depression is the one most often mistaken for 'memory'.....

This information should dispel many of the myths that exaggerate the idea of
a 'memory' phenomenon."

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Written by Samuel M. Goldwasser. | [mailto]. The most recent version is available on the WWW server http://www.repairfaq.org/ [Copyright] [Disclaimer]