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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 converter 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 cannot 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 fairly 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."
Here are six guidelines to follow which will hopefully avoid voltage depression or the memory effect or whatever: (Portions of the following guidelines are from the NiCd FAQ written by: Ken A. Nishimura (KO6AF)) 1. DON'T deliberately discharge the batteries to avoid memory. You risk reverse charging one or more cell which is a sure way of killing them. 2. DO let the cells discharge to 1.0V/cell on occasion through normal use. 3. DON'T leave the cells on trickle charge for long times, unless voltage depression can be tolerated. 4. DO protect the cells from high temperature both in charging and storage. 5. DON'T overcharge the cells. Use a good charging technique. With most inexpensive equipment, the charging circuits are not intelligent and will not terminate properly - only charge for as long as recommended in the user manual. 6. DO choose cells wisely. Sponge/foam plates will not tolerate high charge/discharge currents as well as sintered plate. Of course, it is rare that this choice exists. Author's note: I refuse to get involved in the flame wars with respect to NiCd battery myths and legends --- sam.
(From: Mark Kinsler (kinsler@froggy.frognet.net)). All of which tends to support my basic operating theory about the charging of nickel-cadmium batteries: 1) Man is born in sin and must somehow arrange for the salvation of his immortal soul. 2) All nickel-cadmium batteries must be recharged. 3) There is no proper method of performing either task (1) or task (2) to the satisfaction of anyone.
This applies if the pack appears to charge normally and the terminal voltage immediately after charging is at least 1.2 x n where n is the number of cells in the pack but after a couple of days, the terminal voltage has dropped drastically. For example, a 12 V pack reads only 6 V 48 hours after charging without being used. What is most likely happening is that several of the NiCd cells have high leakage current and drain themselves quite rapidly. If they are bad enough, then a substantial fraction of the charging current itself is being wasted so that even right after charging, their capacity is less than expected. However, in many cases, the pack will deliver close to rated capacity if used immediately after charging. If the pack is old and unused or abused (especially, it seems, if it is a fast recharge type of pack), this is quite possible. The cause is the growth of fine metallic whiskers called dendrites that partially shorts the cell(s). If severe enough, a dead short is created and no charge at all is possible. Sometimes this can be repaired temporarily at least by 'zapping' using a large charged capacitor to blow out the whiskers or dendrites that are causing the leakage (on a cell-by-cell basis) but my success on these types of larger or high charge rate packs such as used in laptop computers or camcorders has been less than spectacular.
In addition to the NiCd cells, you will often find one or more small parts that are generally unrecognizable. Normally, you won't see these until you have a problem and, ignoring all warnings, open the pack. If it is a little rectangular silver box in series with one of the positive or negative terminals of the pack, it is probably a thermostat and is there to shut down the charging or discharging if the temperature of the pack rises too high. If it tests open at room temperature, it is bad. With care, you can safely substitute a low value resistor or auto tail light bulb and see if the original problem goes away or at least the behavior changes. However, if there is a dead short somewhere, that device may have sacrificed its life to protect your equipment or charger and going beyond this (like shorting it out entirely) should be done with extreme care. These may be either mechanical (bimetal strip/contacts) or solid state (Polyfuse(tm) - increases resistance overcurrent). If it looks like a small diode or resistor, it could be a temperature sensing thermistor which is used by the charger to determine that the cells are heating which in its simple minded way means the cells are being overcharged and it is should quit charging them. You can try using a resistor in place of the thermistor to see if the charger will now cooperate. Try a variety of values while monitoring the current or charge indicators. However, the problem may actually be in the charger controller and not the thermistor. The best approach is to try another pack. It could be any of a number of other possible components but they all serve a protective and/or charge related function. Of course, the part may be bad due to a fault in the charger not shutting down or not properly limiting the current as well.
Nickel-Cadmium batteries that have shorted cells can sometimes be rejuvenated - at least temporarily - by a procedure affectionately called 'zapping'. The cause of these bad NiCd cells is the formation of conductive filaments called whiskers or dendrites that pierce the separator and short the positive and negative electrodes of the cell. The result is either a cell that will not take a charge at all or which self discharges in a very short time. A high current pulse can sometimes vaporize the filament and clear the short. The result may be reliable particularly if the battery is under constant charge (float service) and/or is never discharged fully. Since there are still holes in the separator, repeated shorts are quite likely especially if the battery is discharged fully which seems to promote filament formation, I have used zapping with long term reliability (with the restrictions identified above) on NiCds for shavers, Dustbusters, portable phones, and calculators. WARNING: There is some danger in the following procedures as heat is generated. The cell may explode! Take appropriate precautions and don't overdo it. If the first few attempts do not work, dump the battery pack. ATTEMPT ZAPPING AT YOUR OWN RISK!!!! You will need a DC power supply and a large capacitor - one of those 70,000 uF 40 V types used for filtering in multimegawatt geek type automotive audio systems, for example. A smaller capacitor can be tried as well. Alternatively, a you can use a 50-100 A 5 volt power supply that doesn't mind (or is protected against) being overloaded or shorted. Some people recommend the use of a car battery for NiCd zapping. DO NOT be tempted - there is nearly unlimited current available and you could end with a disaster including the possible destruction of that battery, your NiCd, you, and anything else that is in the vicinity. OK, you have read the warnings: Remove the battery pack from the equipment. Gain access to the shorted cell(s) by removing the outer covering or case of the battery pack and test the individual cells with a multimeter. Since you likely tried charging the pack, the good cells will be around 1.2 V and the shorted cells will be exactly 0 V. You must perform the zapping directly across each shorted cell for best results. Connect a pair of heavy duty clip leads - #12 wire would be fine - directly across the first shorted cell. Clip your multimeter across the cell as well to monitor the operation. Put it on a high enough scale such that the full voltage of your power supply or capacitor won't cause any damage to the multimeter. WEAR YOUR EYE PROTECTION!!! 1. Using the large capacitor: Charge the capacitor from a current limited 12-24 V DC power supply. Momentarily touch the leads connected across the shorted cell to the charged capacitor. There will be sparks. The voltage on the cell may spike to a high value - up to the charged voltage level on the capacitor. The capacitor will discharge almost instantly. 2. Using the high current power supply: Turn on the supply. Momentarily touch the leads connected across the shorted cell to the power supply output. There will be sparks. DO NOT maintain contact for more than a couple of seconds. The NiCd may get warm! While the power supply is connected, the voltage on the cell may rise to anywhere up to the supply voltage. Now check the voltage on the (hopefully previously) shorted cell. If the filaments have blown, the voltage on the cell should have jumped to anywhere from a few hundred millivolts to the normal 1 V of a charged NiCd cell. If there is no change or if the voltage almost immediately decays back to zero, you can try zapping couple more times but beyond this is probably not productive. If the voltage has increased and is relatively stable, immediately continue charging the repaired cell at the maximum SAFE rate specified for the battery pack. Note: if the other cells of the battery pack are fully charged as is likely if you had attempted to charge the pack, don't put the entire pack on high current charge as this will damage the other cells through overcharging. One easy way is to use your power supply with a current limiting resistor connected just to the cell you just zapped. A 1/4 C rate should be safe and effective but avoid overcharging. Then trickle charge at the 1/10 C rate for several hours. (C here is the amp-hour capacity of the cell. Therefore, a 1/10 C rate for a 600 mA NiCd is 50 mA.) This works better on small cells like AAs than on C or D cells since the zapping current requirement is lower. Also, it seems to be more difficult to reliably restore the quick charge type battery packs in portable tools and laptop computers that have developed shorted cells (though there are some success stories). My experience has been that if you then maintain the battery pack in float service (on a trickle charger) and/or make sure it never discharges completely, there is a good chance it will last. However, allow the bad cells to discharge to near 0 volts and those mischievous dendrites will make their may through the separator again and short out the cell(s).
(From: Tom Lamb (tlamb@gwe.net)). * Measuring NiCd capacity - I use a very simple/effective system. Put a 2.5 ohm resistor across the contacts of a cheap travel analog clock, which will time the rundown. It is quite consistent for good cells. A good typical AA NiCd will run one hour. * NiCd zapping - I use a 1 ohm power resistor in series with a car battery, though a series headlight will also work. I charge for about 30 secs or until warm, which will clear the whisker and put in enough charge to see if the cell is salvageable.
Since the nominal (rated) voltages for the common battery technologies differ, it is often possible to identify which type is inside a pack by the total output voltage: NiCd packs will be a multiple of 1.2 V. Lead-acid packs will be a multiple of 2.0 V. Alkaline packs will be a multiple of 1.5. Note that these are open circuit voltages and may be very slightly higher when fully charged or new. Therefore, it is generally easy to tell what kind of technology is inside a pack even if the type is not marked as long as the voltage is. Of course, there are some - like 6 V that will be ambiguous.
For primary batteries like Alkalines, first try a fresh set. For NiCds, test across the battery pack after charging overnight (or as recommended by the manufacturer of the equipment). The voltage should be 1.2 x n V where n is the number of cells in the pack. If it is much lower - off by a multiple of 1.2 V, one or more cells is shorted and will need to be replaced or you can attempt zapping it to restore the shorted cells. See the section: "Zapping NiCds to clear shorted cells". Attempt at your own risk! If the voltage drops when the device is turned on or the batteries are installed - and the batteries are known to be good - then an overload may be pulling the voltage down. Assuming the battery is putting out the proper voltage, then a number of causes are possible: 1. Corroded contacts or bad connections in the battery holder. 2. Bad connections or broken wires inside the device. 3. Faulty regulator in the internal power supply circuits. Test semiconductors and IC regulators. 4. Faulty DC-DC inverter components. Test semiconductors and other components. 5. Defective on/off switch (!!) or logic problem in power control. 6. Other problems in the internal circuitry.
Unless you have just arrived from the other side of the galaxy (where such problems do not exist), you know that so-called 'leak-proof' batteries sometimes leak. This is a lot less common with modern technologies than with the carbon-zinc cells of the good old days, but still can happen. It is always good advice to remove batteries from equipment when it is not being used for an extended period of time. Dead batteries also seem to be more prone to leakage than fresh ones (in some cases because the casing material is depleted in the chemical reaction which generates electricity and thus gets thinner or develops actual holes). In most cases, the actual stuff that leaks from a battery is not 'battery acid' but rather some other chemical. For example, alkaline batteries are so called because their electrolyte is an alkaline material - just the opposite in reactivity from an acid. Usually it is not particularly reactive (but isn't something you would want to eat). The exception is the lead-acid type where the liquid inside is sulfuric acid of varying degrees of strength depending on charge. This is nasty and should be neutralized with an alkaline material like baking soda before being cleaned up. Fortunately, these sealed lead-acid battery packs rarely leak (though I did find one with a scary looking bulging case, probably due to overcharging - got rid of that is a hurry). Scrape dried up battery juice from the battery compartment and contacts with a plastic or wooden stick and/or wipe any liquid up first with a dry paper towel. Then use a damp paper towel to pick up as much residue as possible. Dispose of the dirty towels promptly. If the contacts are corroded, use fine sandpaper or a small file to remove the corrosion and brighten the metal. Do not an emery board or emery paper or steel wool as any of these will leave conductive particles behind which will be difficult to remove. If the contacts are eaten through entirely, you will have to improvise alternate contacts or obtain replacements. Sometimes the corrosion extends to the solder and circuit board traces as well and some additional repairs may be needed - possible requiring disassembly to gain access to the wiring.
While it is tempting to want to use your car's battery as a power source for small portable appliances, audio equipment, and laptop computers, beware: the power available from your car's electrical system is not pretty. The voltage can vary from 9 (0 for a dead battery) to 15 V under normal conditions and much higher spikes or excursions are possible. Unless the equipment is designed specifically for such power, you are taking a serious risk that it will be damaged or blown away. Furthermore, there is essentially unlimited current available from the battery (cigarette lighter) - 20 A or more. This will instantly turn your expensive CD player to toast should you get the connections wrong. No amount of internal protection can protect equipment from fools. My recommendation for laptop computers is to use a commercially available DC-AC inverter with the laptop's normal AC power pack. This is not the most efficient but is the safest and should maintain the laptop's warranty should something go wrong. For CD players and other audio equipment, only use approved automotive adapters.
(From: Greg Raines (ghr@ix.netcom.com)). When I was about 10 years old I was sitting in my dad's driveway in a '65 Plymouth Fury III station wagon while he disconnected the trickle charger from the '67 Fiat in the garage. I heard a pop and saw my dad throw his hands over his face, run to the back door and start kicking it to get someone to open it. Fortunately he wasn't injured. But it was an eye opener. It was probably 30 or below, there was no flame present, and the double garage door was open (this happened in Connecticut). Also in a Fiat 850 sport coupe the battery is in the trunk (front) so there really isn't anything up there that would cause a spark (engine & gas tank in back). So it must have been a spark off of the charger when he pulled it off the terminal (he hadn't unplugged the charger).Go to [Next] segment
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