Hi 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. 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. 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 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. 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 fairly constant until very near the end. 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 ([EMAIL PROTECTED]) 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." Care and feeding of NiCds 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. Why there will never actually be closure on this topic (From: Mark Kinsler ([EMAIL PROTECTED])). 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. 1 NiCd battery pack will not hold a charge 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. What is this thing in my NiCd battery pack? 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. Zapping NiCds to clear shorted cells 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. I have left out the Part on Zapping for I think it is Dangerous. But if you want me to send it I will send it first to Dave for Approval for first and foremost we must be Safe and Understand this process could go Wrong. To listen to the show archives go to link http://acbradio.org/handyman.html or ftp://ftp.acbradio.org/acbradio-archives/handyman/ The Pod Cast address for the Blind Handy Man Show is. http://www.acbradio.org/news/xml/podcast.php?pgm=saturday The Pod Cast address for the Cooking In The Dark Show is. http://www.gcast.com/u/cookingindark/main.xml Visit The New Blind Handy Man Files Page To Review Contributions From Various List Members At The Following Address: http://www.jaws-users.com/handyman/ Visit the new archives page at the following address http://www.mail-archive.com/[email protected]/ For a complete list of email commands pertaining to the Blind Handy Man list just send a blank message to: [EMAIL PROTECTED] Yahoo! 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