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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.
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.
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.
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 concluded! 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 :-), etc.
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.
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.
A power transformer can die in a number of ways. The following are the most common: * 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 voltage. * 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).
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.
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 required. 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.
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 events.
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: http://www.repairfaq.org/filipg/HTML/FAQ/BODY/F_Battery.html There is more on batteries than you ever dreamed of ever needing. The sections below represent just a brief introduction.
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.
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.
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.
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.
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.
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."Go to [Next] segment
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