|
Arabic
Bengali
Chinese
English
French
German
Hebrew
Hindi
Italian
Japanese
Korean
Malay
Polish
Portuguese
Spanish
Turkish
Ukrainian
Vietnamese|
ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Operation of sealed Ni-Cd batteries
Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells The widespread use of sealed Ni-Cd (disk and cylindrical) batteries has also led to great interest in their operation, methods and devices for charging them. Many articles have been published on these topics, including in the Radio magazine. In recent years, due to the emergence of new household appliances powered by rechargeable batteries (AB), interest in this topic has increased significantly. However, there are not so many articles on the operation of batteries. The reason for this situation is quite objective: conducting research on the operation of AB is a very long and laborious task. And in full it is unbearable for radio amateurs. This, of course, does not mean that radio amateurs should not engage in this kind of work - just that the results obtained should be taken critically and not generalized based on single results. A typical example is the well-known method of charging batteries with an asymmetric current [1, 2]. Everyone was well aware of its merits, only one trifle remained unclear - where did it come from, what was the primary source. But such a "trifle" obviously did not bother anyone, because after two or three publications based on this method of chargers, one could safely write: "... as is well known, charging batteries with an asymmetric current allows ..." and further in the text . Another example is the Woodbridge method, which is so often referred to. It was developed in those years when the mass production of batteries began for the needs of the developing automotive industry and the issues of their operation became so relevant that they required the involvement of science. This methodology was created for specific (acid) batteries, and the justification for expanding its scope is unknown. In other words, the use of this technique for other batteries is not justified. As a result, the situation today has become so confusing that it has become simply unrealistic to understand it. This is confirmed by conscientiously conducted reviews on the topic by some authors and attempts to draw practical conclusions on its basis - the authors do not even notice the contradictions in the sources they refer to. Really serious publications are much rarer, and one of them is [3]. The article sets a more modest, and therefore quite real task - to present the experience accumulated by the author on this topic. Let us remind you once again that the article is devoted only to sealed Ni-Cd batteries of domestic production, therefore, when applying all its provisions to other batteries, criticality and caution should be exercised. The main characteristic of electric batteries is the amount of energy stored in them, for the measurement of which an off-system unit of measure is usually used - kWh or multiples of it. In practice, it is more convenient to use another characteristic of batteries - the charge stored in them. It is commonly referred to as capacity. In the SI system, the charge is measured in coulombs (1 C = 1A x 1 s), but more often they also use an off-system unit of measurement - Ah, and for small-capacity batteries - mAh. They are so accustomed to this parameter that they often forget (or don’t know at all) that the main indicator of a battery is the amount of stored energy, not capacity. The relationship between the energy E of the battery and the capacity C is determined by the simplest formula: E \uXNUMXd C x Ucp, where Ucp is the average battery voltage. This expression provides sufficient accuracy for practice. More precisely, the energy is calculated through the integral. Rated capacity is a typical value given in the characteristics of batteries. It is primarily determined by the design of the battery and manufacturing technology. It is the latter reason (more precisely, the technological variation in manufacturing) that leads to the fact that the capacity of the batteries, even in one production batch, has a variation that reaches two or more times. In the literature, it is sometimes indicated that ABs are assembled from batteries with similar capacities, but in conditions of mass production, this, of course, is simply unrealistic. In the USSR, the nominal capacity was often determined on the basis of the "less than less" principle, which provided a margin that made it possible over time to "increase" the capacity of AB 7D-0,1 and other batteries simply by changing the numbers on the label. Now 7D-0,1 has turned into 7D-0,125. It is important to note that the capacity is a multifactorial value, since even for a specific instance it depends on a number of parameters: ambient temperature, charging and discharging modes, etc. Therefore, when it comes to battery capacity, a methodology for its definitions, since only by changing the methodology it is easy to "change" the capacity several times. But usually it is the methodology that is not given. During operation, the battery voltage decreases from maximum to minimum. The minimum voltage is the voltage at which the remaining energy (charge) of the battery is insignificant and further operation is impractical, since the voltage also sharply decreases (when completely discharged, it is equal to zero). For Ni-Cd batteries, the minimum voltage is about 1 V, and this value is a clear criterion for the completion of a discharge. Thus, the working area for the battery is the voltage range from maximum to minimum. In the working area, the remaining energy (charge) can be approximately determined by the voltage on the battery. Rated voltage is the average between the maximum and minimum; it is he who is usually given in the reference data for the battery. For Ni-Cd batteries, this voltage is approximately 1,2 V. The rated voltage of the battery, like any other galvanic cell, is determined only by its electrochemical system, i.e., a galvanic couple and an electrolyte. It is structurally or technologically impossible to change this value. At the end of charging and turning off the charger, the battery voltage (UM3) is maximum and is approximately 1,43 ... 1,45 V. It quickly decreases and after 10 ... 25 minutes reaches a stable value of UMP equal to 1,37 ... 1,39 V. The spread of these values is mainly due to the measurement error, but greater accuracy is not required. The main problem in the operation of batteries is related to their charging and is due to the lack of a reliable criterion for its completion. Using the battery voltage for this is ineffective, since it can be reached even before it is fully charged. This criterion has often been used in amateur designs. Recent publications indicate that one criterion is not enough, additional ones are needed, and as one of them they suggest measuring the temperature of the battery. Temperature is an important parameter, since it allows you to determine where the electricity "goes" - for charging or for heating, that is, it allows you to determine the state of the battery, but by no means the degree of its charge. It can also be added that, other things being equal, the influence of the ambient temperature will be manifested to a large extent. From the foregoing, one can draw a not too comforting conclusion - today there are no reliable criteria for the end of charging. More precisely, there is one such criterion, and it will be discussed below, but for all its outward simplicity, its implementation is very problematic. The lack of reliable end-of-charge criteria is, of course, disappointing, as it does not allow for a full charge of the battery. But after all, batteries have been successfully used for decades. And the first question that arises is how much is really needed, really, a full charge? In real conditions, the difference in capacity up to 15% is practically imperceptible, and this is much less than the variation in capacity for different specimens. Sealed accumulators are designed in such a way that sealing is ensured by gas pressure inside the case. When charging, this pressure increases, and if it reaches the yield strength of the case material, the battery will swell. In this case, the contacts are broken, which leads to a complete failure of the battery. For disk batteries, it is sometimes possible to restore working capacity - they must be compressed in a vice (through an insulating gasket) to their previous dimensions. In more severe cases, the batteries are opened (silent explosion), and it is impossible to restore them. Gas pressure can serve as a reliable criterion for the end of charging, in any case, it allows you to determine the limit beyond which further charging becomes dangerous. But the practical implementation of this method is problematic even for high-capacity batteries, and for a small one it is simply unrealistic. During discharging, the pressure drops, and if the voltage is below the minimum, it can drop to a level that does not provide tightness, resulting in leakage of electrolyte. Among other troubles, the leaked electrolyte shunts the battery electrodes, after which, due to surface leaks, the self-discharge current increases. Long-term storage of a discharged battery will damage it. It is known that batteries that have not worked for a long time lose capacity and performance. You can restore them in several charge-discharge cycles. It does not matter how exactly to do it - "revival" will happen in any case. As time goes by, natural aging processes occur and battery performance deteriorates. Batteries typically have a service life of 3-5 years, but with normal use they work reliably for 10 years or more. In practice, the so-called standard charging mode is most common - 150% of the rated capacity is "pumped" into the battery, charging it for 15 hours with a current of 0,1 C. The efficiency of batteries, i.e., the ratio of energy output to energy received, is very difficult to determine for a number of reasons, so this indicator is usually not given. For small batteries, it is generally insignificant, since the losses in the charger are obviously greater. It can be determined purely approximately based on the above standard charging mode - 0,65 (65%). The standard mode has proven itself in practice, and it can be considered a reference. The charger that implements it can be extremely simple and contain a rectifier diode and a quenching resistor. The advantage of the method is that it is able to charge even "half dead" batteries. However, it also has two significant drawbacks: a long charging time and the danger of overcharging. True, the latter is no longer connected with the method, but with the person - often they simply forget to turn off the charger in time. This method has only one unclear point - where did this 0,1C come from? There is no clear answer, and it is hardly possible to get one after so many years, so it remains only to assume that such a regime was chosen simply for compromise reasons. With a lower charging current, the charging time unacceptably increased (at 0.05C - 30 hours), and with a larger one, it was necessary to increase the power of the charger and, accordingly, its dimensions, weight and price. Experiments carried out by the author with AB 7D... showed that charging with a current equal to the capacity of the battery does not damage it. A very interesting and promising method is the method of charging batteries from a source of stable voltage. For definiteness, let's call it stable voltage charging (ZSN). It is possible to completely eliminate overcharging by using the ZSN method equal to the maximum battery voltage. True, it is not entirely clear what exactly this voltage should be: UM3 or UMP, and for insurance it is better to take the smaller of them - UMP. At the beginning of charging, the current is maximum, after a short time, in most cases, it increases a little more (apparently, the internal resistance of the battery decreases). Then, as the battery charges and its voltage increases, the current decreases and at the end of charging asymptotically approaches zero, more precisely, to the battery self-discharge current. When charging a completely discharged battery, the initial current surge may be unacceptably large and should be limited, for example, by including a current-limiting resistor in the charging circuit. The main disadvantage of this method is that it provides a charge of 60 ... 70% of the nominal capacity. Therefore, it is advisable to use it for backup batteries, for example, in electronic watches. A slight decrease in battery capacity for such devices is not significant, it is much more important to ensure its long and reliable operation. This method is also advisable to use when it is required to bring the battery into working condition in 15 ... 20 minutes. The reason why this mode does not fully charge the batteries is quite obvious - it is necessary to increase the supply voltage. In this case, the charging current asymptotically tends not to zero, but to some minimum value. This, in essence, stabilization of the charging current can serve as a criterion for the end of charging. There is another, more reliable and easy-to-implement criterion - reducing the charging current to a value close to the minimum. For the practical implementation of the proposed method, it is necessary to experimentally select the charging mode for a particular battery: determine the charging voltage and current of the end of charging. The diagram of an automatic charger (charger) is shown in fig. 1. It allows you to charge batteries with any degree of discharge, including fully discharged ones. The nominal charging time of AB 7D-0.125, discharged to 1 V per battery, is approximately 1,5 hours. For AB with a lower degree of discharge, it is reduced accordingly. The capacity to which the battery can be charged is approximately equal to 0,85 ... 0,95 of the nominal. It depends on the state of the battery and on the accuracy of setting the current at which the device is turned off.
Working with the charger is extremely simple - after connecting the power supply and the battery being charged, briefly press the SB1 button. This turns on the signal LED HL1 and starts charging. When the battery is charged, the device will automatically turn off, which completely eliminates the danger of overcharging, and the signal LED will turn off. The basis of the memory is the voltage regulator DA1. The exact value of the output voltage is set by the tuning resistor R9. Diode VD1 prevents the battery from discharging after the charger is turned off. To reduce losses, a Schottky diode is used, which has a lower voltage drop compared to conventional silicon diodes. An indicator - LED HL10 - is connected to the output of the memory through a current-limiting resistor R1. Capacitor C2 smooths out the ripple of an unregulated power supply at the input of the stabilizer, and also prevents it from self-excitation. The shutdown unit is a trigger assembled on transistors VT1 and VT2 of different structures. In the initial state, after connecting the power source and charging battery, the trigger is off. To turn it on, just briefly press the SB1 button. This opens the transistor VT1 and its collector current through the resistor R2 opens the transistor VT2 - the memory starts working. The current flowing through the device creates a voltage drop across the resistor R5, which is fed through the resistor R6 and the resistive voltage divider R3R4 to the base of the transistor VT1. The trigger turns on, and the device continues to work even after releasing the SB1 button. "Concurrently" the resistor R5 performs the function of a maximum current limiter at the beginning of charging a fully discharged battery. During charging, the voltage on the battery increases, which leads to a decrease in the charging current, and when it reaches the set minimum value, the voltage drop across the resistor R5 becomes insufficient to keep the trigger on - the charger turns off and charging stops. The exact value of the minimum current is set by the tuning resistor R4. Capacitor C1 smooths out the voltage ripple across resistor R5 that appears when the charger is powered from an unstabilized power source. In the author's version, an unstabilized source of domestic production BPN-12-1 with an open circuit output voltage of 18 V is used to power the memory. Other power supplies, including stabilized ones, with an output voltage of about 15 V (for stabilized power supplies, it can be slightly less) at a current of at least 0,2 A. The device is mounted on a printed circuit board made of one-sided foil fiberglass 1,5 mm thick. The PCB drawing is shown in fig. 2.
The device uses tuning resistors SPZ-19a. Resistor R5 - MLT-0,5 or MT-0,5, R2 - MLT-0,25 or MT-0,25; they are installed perpendicular to the board. The rest of the fixed resistors are leadless for surface mounting, size 1206. They are installed from the side of the printed conductors. Capacitors - K50-35 or similar imported ones. In place of the VD1 diode, you can use any Schottky diode with a permissible current of at least 1 A. LED - any. Button SB1 - any non-latching. The connector for connecting the power supply can also be any - most importantly, it must match the connector of the power supply. To establish, you will need a wire-wound variable resistor with a resistance of 560 ohms and a power of 1 W. It is connected to the output of the charger and the resistance is gradually reduced until the trigger is securely held after the SB1 button is released. With a tuned resistor R9, the output voltage is set (it is measured directly at the output of the stabilizer) equal to 10,9 V. It is somewhat more difficult to set the turn-off current. Since the shunt of the milliammeter introduces a large error in measuring the charging current, the milliammeter should be connected at the input of the device. And although in this case the current consumed by the charger itself is added to the actual charging current, the result is more accurate. To do this, measure the current at the input of the memory at the middle position of the trimmer resistor R4, and then set it to approximately 43 mA. These operations will have to be performed several times until the desired result is obtained, since it is impossible to "catch" the turn-off current at a time. A more precise adjustment can be performed during direct work with the battery, after several control charge-discharge cycles. It is permissible to replace the KR142EN22 stabilizer with KR142EN12A or KR142EN12B. In this case, the supply voltage of the charger should be increased to 16 ... 17 V. Literature
Author: A. Mezhlumyan, Moscow
Air trap for insects
01.05.2024 The threat of space debris to the Earth's magnetic field
01.05.2024 Solidification of bulk substances
30.04.2024
▪ Fujitsu ETERNUS CD10000 56 petabyte storage ▪ Tablets are more attractive than personal computers ▪ Bilingualism improves perception of information and attentiveness
▪ section of the site Intercoms. Article selection ▪ article Mill-mill. Drawing, description ▪ article What part of hereditary information reflects the individuality of a person? Detailed answer ▪ article Head of the dining room. Job description ▪ article Putty for barrels. Simple recipes and tips
Home page | Library | Articles | Website map | Site Reviews
www.diagram.com.ua |
See other articles Section 
Latest news of science and technology, new electronics:
All languages of this page