laptop batteries

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 379 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. Comments on Building Charger for Small Lead-Acid Batteries The following applies to the sort of lead-acid batteries found in some camcorders and other portable equipment: The simple way is to build a power supply that outputs 13.8 volts regulated, with a current limit of 0.5 A. 13.8 V can be left connected to the battery forever without damage - this is called a float charge. The 0.5 A current limit protects the battery from drawing too much current and overheating if it's been deeply discharged. This sort of charger will get the battery back up to 80% charge within a few hours, so it's fine for most uses. 380 However, when designing it, make sure the charger doesn't self-destruct if the input voltage goes away (due to AC power failure) while still connected to the battery. With a standard series regulator, when the input power fails the whole battery voltage gets applied to the base- emitter junction of the output transistor in reverse. Many transistors are only specified to withstand about 6 V reverse base-emitter voltage, so with this design your charger will be toast at the first power failure. If you want higher-performance charging, there are special charge controller chips that provide 3 or more charge phases. They are:  Constant current charge at maximum safe current (see battery spec sheet) until the voltage rises to about 14.5 V.  Constant voltage charge at 14.5 V until the current drops to a fraction of the initial current limit.  Float charge at 13.65 V after that. By using the 14.5 V instead of 13.8 V for the initial charge voltage, this type of charger gets the battery back up to 90% charged in considerably less time. But if you only care about charging overnight, you don't need the extra complexity. On the other hand, NiCd batteries can safely be charged in less than an hour with suitable electronics. Lead-acid simply can't be recharged that fast. 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. And, there is a slight, but nonzero chance that some equipment may actually be damaged. This might occur if its design assumed something about the internal resistance of the batteris; the resistance is much lower for NiCds than Alkalines. 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 381 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 Pack 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. 382 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 - o Operation below 0 degrees C. o High discharge rates (above 5C) if not specifically designed for such use. o Inadequate charging time or a defective charger. o 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." Memory Effect in NiMH Batteries? The party line is that Nickel-Metal-Hydride batteries do not have any memory effect. Perhaps, perhaps not. Care and Feeding of NiCds Here are six guidelines to follow which will hopefully avoid voltage depression or the memory effect or whatever: 383 (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. All of which tends to support my basic operating theory about the charging of nickelcadmium 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. Nickel Cadmium Versus Nickel-Metal-Hydride in a Nutshell NiCds are inexpensive, reliable, and easy to charge, but may suffer from voltage depression (what people call the memory effect) from repeated shallow discharge cycles. NiMHs have slightly higher capacity and no memory effect but have higher initial cost and are more sensitive to overcharging. Must be used with compatible charger. First Aid for NiCd Battery Packs CAUTION: Opening these battery packs will of course void any warranty but you knew that. Also, make notes of exactly how the cells and anything else inside is arranged. Improper reassembly can result in damage to equipment and/or risk of overheating should cells short inside the pack due to lack of or misplaced insulation. Under no circumstances should all thermal switches be removed - not only are they a safety device to prevent excessive temperatures but may also be part of the charging circuit. So, if they are removed, your next charge may be your last! I'd highly recommend that all of them be replaced (from another pack as a last resort) and installed in exactly the same positions they were originally. Many "name brand" camcorder and other similar battery packs contain two or even 3 thermal switches (those rectangular, un-identifiable, wired between the cells). They contain a bimetal strip operating a set of contacts which open at a preset temperature. 384 Often only one of these will fail, resulting in a $40 NiCad that won't charge. Since these little suckers are pricey if ya kind find them, a safe and cheap fix, is to test the thermal switches for continuity (they should be closed at room temp) and remove the defective one. If needed move the other, or at least one, to the mid-point of the cells series. If a battery pack has 8 separate cells, (i.e.: a 9.6 V VHS-C camera pack) the thermal switch should be wired between the 4th and 5th, and as far away from the charging contacts as possible. The extra switches were added as a safety factor but since the average one is designed to open at 87°C, there is no fire hazard so long as the pack is re-sealed after working on it. A quick fix for a NiCad pack left on the dashboard. Since good ol' solar power can heat a battery pack to the point where the thermal protection can open (and even warp a case) you can be stuck at the soccer game with what seems like dead batteries. The trick is to drop the temp below 87°C. Wrap the battery in plastic so the contacts won't get wet, and stick it in the cooler with the kids lunch and your six-pack. A few minutes and the thermistor should close. letting the batter work normally. Also, if the cord is long enough, never recharge a NiCad inside the car. Place the battery and charger under the car, in the shade, so it doesn't heat quickly and will get a full charge. Identifying Technology of Unmarked Battery Packs 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 marked. Of course, there are some - like 6 V that will be ambiguous. Powering LEDs with Batteries LEDs look like diodes with a high forward voltage drop. Above the that voltage, the incremental resistance is very low and without current limiting, the current would be critically dependent on the exact voltage of the power source. Most of the time, they are spec'd at a particular maximum current and need some means to limit the current to that value based on the input voltage. Some devices may depend on the internal resistance of the batteries to provide the current limiting - this is a poor approach and depends greatly on the type and capacity of the batteries being used. Most common is just a resistor but this provides no regulation and poor efficiency. Better designs (used in LED flashlights) will use a DC to pulse inverter with regulation achieving constant light output regardless of battery state-of-charge and high efficiency. LEDs can usually withstand short high current pulses and this allows the circuit to be designed with low losses. 385 The specifications for LEDs you see in electronics distributor's catalogs may look the same as those for incandescent lamps but they are not. Incandescent lamps provide their own current limiting; LEDs do not. It's possible to luck out and happen to have a given LED work without current limiting with a particular set of batteries but it hardly an acceptable design approach. Slight variations in battery parameters will result in gross changes in light intensity and possible shortening of life or outright destruction of the LED. Battery Problem Troubleshooting and Repair Problems with Battery Operated Equipment For primary batteries like Alkalines, first try a fresh set. For NiCds, test across the battery