Chargers for chemical current sources. Problems and solutions..
KOTOMIN Vladimir Ernestovich
CHARGERS FOR CHEMICAL CURRENT SOURCES.
PROBLEMS AND SOLUTIONS
Charging the battery is very simple:
if you have not mixed up the polarity and have provided an acceptable current, then
the main thing is to stop in time.
Today, few people can imagine their everyday life without batteries. In addition to mobile phones, portable computers and radio stations, power tools and many other things, where the use of batteries is self-evident, the latter are beginning to displace primary elements — good old batteries. The appearance of alkaline batteries in the size of household batteries is a clear example of this. Naturally, who would refuse to use a battery several hundred times, the price of which is only 2 — 3 times higher?! In a word, the further, the more batteries will be used in everyday life and technology. But in order to get these very several hundred cycles of operation, which distinguish a battery from a primary element, you need to use them correctly.
Correct use of batteries means observing the conditions of discharge and charge. As for discharge — firstly, you should not exceed the maximum permissible discharge currents and, secondly, overdischarge the battery. The first is achieved by choosing the correct type and capacity of the battery, the second can be ensured by using special service devices that automatically disconnect the battery from the load when completely discharged. Most mobile phones, laptops and video cameras have such devices. In addition, if the user (operator) is sufficiently attentive and turns off, for example, his battery-powered flashlight, the light of which has begun to dim, then nothing will happen to his battery.
Discharged batteries need to be charged correctly. Household alkaline batteries usually have instructions on how to do this. As a rule, manufacturers recommend a standard charge — current 0.1 Cnom (where C is the nominal capacity of the battery), time 14 — 16 hours. In the vast majority of cases, this charging mode is correct from the point of view of ensuring the durability of the battery, but not every user will agree to wait half a day charging the battery of a mobile phone or video camera. Therefore, in most cases, the almost correct so-called «fast» charge is used. Since each type of battery has its own characteristics that determine the area of their application and charging requirements, it makes sense to talk about charging methods for each type of battery separately. Further in this article I will not consider primary elements, therefore, to avoid confusion, we will agree that an accumulator or element is an elementary rechargeable galvanic current source, and a battery is two or more accumulators connected in series. I will refrain from describing the operational properties of accumulators and will consider their different types only from the point of view of the charging process.
Lead-acid batteries
A lead-acid battery is a galvanic cell in which the active substance of the positive electrode is lead dioxide, and the active substance of the negative electrode is spongy lead. Let's consider the chemical reactions during charging of an acid battery:
PbSO4 + PbSO4 + 2H2O = Pb + PbO2 + 2H2SO4 (1)
during overcharging:
2H2O = 2H2+O2 (2)
side reaction:
H2SO4 = SO3 + H2O (3)
From equation (1) it is evident that during charging, spongy metallic lead is reduced on the negative electrode, and lead dioxide on the positive electrode.
Lead-acid batteries are either flooded or sealed. Flooded batteries are cheaper and allow replacement and topping up of the electrolyte. Currently, there is a tendency to reduce their use, since they are suitable only for stationary use and are not applicable in residential and work premises due to the release of various gases during operation (see equations (2) and (3)). Of course, the sulfur dioxide molecule SO3 is heavy and low-mobility. Most likely, it will react with water vapor and return to the electrolyte solution, but during recharging (2), when gas formation is active, it is quite possible that toxic gas will be carried into the surrounding space. Its quantities are small, but in a closed room… In addition, the mixture of gases resulting from reaction (2) is explosive. However, from the point of view of charging, these are the most unpretentious batteries. They can be charged with currents up to 0.25 Cn, and the condition for the end of charging can be considered approximately the achievement of a certain voltage, for example, for a temperature of 20 ° C this voltage will be 2.43 2.53 V. In addition, even if this threshold is exceeded, then overcharging will lead to «boiling electrochemical decomposition of water. Provided that the room is well ventilated, the only problem will be restoring the normal level of electrolyte.
Sealed lead-acidThe batteries differ from flooded batteries mainly by the use of helium electrolyte and the tightness of the container. This type should be discussed in more detail. Apart from its slightly higher cost, a sealed acid battery is devoid of the disadvantages of a flooded battery, which significantly expands its scope of application. What this type is and what features it has is described in detail in [1]. In terms of charging, this is the best battery in terms of simplicity. Firstly, the state of charge is clearly indicated by the voltage on the battery of 2.43–2.53 V for the cyclic charging mode, and secondly, even if the charging voltage is exceeded, no gases are released – recombination in the thickness of the helium layer and on the valve plugs made of catalytic rubber. However, at significant charging currents, the rate of gas release may exceed the recombination rate, and the safety valve will operate. Acid batteries can be charged in a wide range of temperatures from -20 to +50 ° C. Maximum charging currents – up to 0.35 Cnom.
Acid batteries can be charged from any current source, provided that the condition is met — do not exceed the maximum charging current. The condition for the end of charging — reaching the cut-off voltage threshold can be monitored using a voltmeter or the beginning of «boiling», which can be determined by ear. In order to free a person from monitoring the battery charging process, automatic chargers are used.
A simple automatic charger (hereinafter referred to as the CC) is shown in diagram 1.
Diagram 1
The power source charges the battery (hereinafter referred to as the AB) through a controlled key. The control device monitors the voltage on the AB and, when the voltage is reached,
U = (2.35 — 2.43) x N Volts
(where N is the number of elements in the battery), gives a signal to open the key. The dependences of currents and voltages on time are shown in Fig. 1a.
Fig. 1a
Fig. 1b
The efficiency of such a charge is 80 — 90% depending on the current value at the end of the charge. To obtain a full charge, a second stage of discrete or continuous additional charging is required. With discrete additional charging, the control device continues to monitor the voltage on the battery after the key is turned off and, when it drops to the turn-on threshold, closes the key again. As the full charge is reached, the duration of the additional charging pulses decreases and the pauses increase, since the cut-off threshold is reached faster as the charge progresses, and the lower turn-on threshold is reached more slowly (Fig. 1b). This method is used at low charging currents, since high currents at the final stage of charging acid batteries are undesirable. In addition, the internal resistance of used batteries usually increases, which leads to premature reaching of the cut-off threshold.
The best, but more complicated is the so-called “fast” automatic charger (diagram 2). Here, the current and voltage stabilizers are fed from the power source. The current stabilizer produces the maximum permissible charging current, reduces it as the voltage on the battery increases, and when the threshold is reached, transfers the battery to the second stage — charging with a constant voltage U = (2.26 — 2.31) * N from the voltage stabilizer. This charging scheme is recommended by all battery manufacturers, with various variations. For example, PANASONIC (Japan) recommends a constant stable current for the entire charging time, and Sonnensheinn (Germany) recommends a slightly higher threshold voltage for batteries of the A400 and A500 series. Of course, this device is more complex and more expensive than others.
Scheme 2
The optimal one in terms of the sum of parameters, including cost, is an automatic charger (diagram 3). Here, the power source, as in the charger in diagram 1, provides current limitation and feeds the voltage stabilizer, adjusted to the voltage Uз=(2.26 – 2.31) x N Volts. Until the specified voltage is reached, the charging current will be determined by the power source, and when the voltage Uз is reached on the battery, the voltage stabilizer will enter the mode and will limit the current, maintaining the voltage on the battery constant. This method is used to charge batteries operating in buffer mode or in hot reserve. The dependences of currents and voltages on the charging time are shown in Fig. 1a, 2 and 3, respectively, for the chargers in diagrams 1, 2 and 3.
Scheme 3
Fig. 2
Fig. 3
There are many publications on charging acid batteries with asymmetric current — alternating charge and discharge pulses. Allegedly, this charging method increases the service life of batteries, but the authors do not have a unified opinion on the size and shape of these pulses. Considering that acid batteries are the cheapest, the use of expensive devices for a dubious extension of their service life is hardly advisable.
Nickel-cadmium sealed batteries
Nickel-cadmium sealed batteries are alkaline batteries with nickel oxide and cadmium electrodes. The main reaction that occurs during charging on the positive electrode in a nickel-cadmium battery can be written as follows:
Ni(OH)2 + OH- = NiOOH + H2O + e-
During charging on the negative cadmium electrode, the reaction occurs:
Cd(OH)2 + 2e- = Cd + 2OH-
The overall charging reaction is as follows:
2Ni(OH)2 + Cd(OH)2 = 2NiOOH + Cd + 2H2O
When recharging, a side process of oxygen generation occurs on the positive electrode:
2OH- = O + H2O
Oxygen reaches the negative electrode through the separator and oxidizes cadmium:
O + Cd + H2O = Cd(OH)2
The latter reactions form a closed cycle and provide gas balance in the battery. However, the pressure in the battery depends not so much on the intensity of the processes, but on the ratio of the generation rate and the oxygen transport rate. In addition, the oxidation reaction of cadmium is exothermic. During fast charging, significant heat release and heating of the battery case are observed.
Until recently, nickel-cadmium batteries were produced with a lamellar design of electrodes and did not allow fast charging. In batteries with thick lamellar electrodes, this is due to high internal resistance, a significant voltage drop when the charging current exceeds a certain limit, and the impossibility of quickly charging deep layers of the active mass of the electrodes. Domestic nickel-cadmium batteries NKGK and NGK allow a charging current of more than 0.15 Cnom, but they are almost always charged with a current of 0.1 Cnom. For example, the widely known NKGK-11D, used in mine lanterns, create significant problems in maintenance. The need to charge lantern batteries, discharged to an unknown degree, in one shift led to their premature failure. About half of the batteries of this type that I have seen were seriously swollen. You can imagine what internal pressure arose in them in order to turn the prismatic case almost into a cylinder. The thing is that to charge by time, you need to have a discharged battery, and service departments did not have time to discharge and charge it in one shift. At mine charging stations, the voltage charging method is used, which allows you to start charging a nickel-cadmium battery with any degree of residual charge. In this case, a memory effect accumulates, leading to a decrease in capacity, but it is removed by preventive discharges, also called training. The problem of charging nickel-cadmium batteries by voltage is associated with a very small voltage gradient when charging with small currents. The voltage on a 20% charged nickel-cadmium battery is 1.4 V, and on a fully charged one — 1.46 — 1.47 V at 20 ° C. Temperature fluctuations of this value are quite serious, but even at a fairly stable temperature, the values u200bu200bcrawl during aging. By the way, the problem of mine charging stations and lanterns is still quite acute.
Modern technology has made it possible to mass-produce nickel-cadmium batteries with rolled electrodes. Reducing the thickness of the electrodes while increasing their area has made it possible to increase the charging currents. In modern nickel-cadmium batteries, the memory effect is noticeably reduced (but not eliminated), it is only observed at high charging currents, and it is very important that modern nickel-cadmium batteries allow significant overcharging, up to several Csom, when charging with low currents. This value varies for different manufacturers from 20 to 50 hours, without significant deterioration in parameters during a single overcharge, and up to several months without damaging the battery.
To select the type of charger and charging method, you need to know what type of battery you are dealing with, even if it is known that it is nickel-cadmium. If there is no such information, then the universal (correct) charging method is to discharge the batteries or battery to a voltage of 1 V/cell and charge for 10 — 12 hours with a current of 0.1 Cnom. Recommendations to charge a nickel-cadmium battery with a current of 0.1 Cnom for 14 and even 16 hours are obvious overcaution associated with the instability of currents of most chargers and some excess of real capacity over nominal when using fresh batteries. Moscow company Elpi-Pro, which develops and manufactures electronic equipment for chemical current sources, conducted independent studies of charging cylindrical nickel-cadmium batteries from various manufacturers and it can be stated that the charging efficiency at currents of 0.1 — 1 Cnom is 85 — 95%. In other words, with a stable current, charging for 12 hours is quite sufficient.
For modern cylindrical nickel-cadmium batteries, it is permissible to charge with currents up to 0.2 Cnom without preliminary discharge with a time limit of about 6 hours. This is due to the fact that the memory effect is reduced, and some overcharging is permissible at low currents. An example of such a device is shown in diagram 4. The power source, which together with the current stabilizer provides a stable current, charges the battery through the key. The timer counts the charging time and, upon reaching the end of the interval, locks the key.
Diagram 4
An analogue of this method is the “coulomb” or “integral” method, which uses an ampere-hour counter (a digital or analog integrator that takes into account the charging current over time). Scheme 5 shows a charger that uses this method. When current passes through the current sensor, a signal is formed at its output, increasing the value at the integrator output. When the latter reaches the threshold level, the comparator locks the key and can send a signal to the indicator. Here, to ensure the charge transferred to the battery, a timer is not required and a current stabilizer is not needed, but this is a significant drawback of a charger built on this principle. At low currents, the charging time increases, and at high currents, there is a risk of overcharging in the absence of a preliminary charge. When using a current stabilizer, the problem is completely eliminated, but in this case, the use of an integrator compared to a timer is not justified from any point of view. The integrator is more complex, as a result, more expensive and less reliable, in addition, the accuracy of the integrator is much lower than that of the timer. The result in both cases is ampere hours or coulombs transferred from the power source to the battery. There is no more accurate method, but it is suitable only for relatively small charging currents, provided there is no pre-discharge.
Scheme 5
However, the requirements for the charging speed are becoming more stringent and charging with low currents is not always acceptable. Of course, with high charging currents, the service life of a nickel-cadmium battery is significantly reduced, and the best solution to the problem would be to have a sufficient number of batteries or batteries for standard charging, but fast charging is still needed, and in some cases is simply necessary.
The danger of overcharging with high currents is a sharp increase in pressure and temperature at the end of charging. When studying the effect of overcharging on battery capacity, the company Elpi-Procharged a battery of five AA cells from PowerSonic (USA) with a current of 0.5 Cnom for eight hours. As a result, the battery capacity dropped from 850 to 300 milliampere hours, and the capacity of individual cells decreased approximately equally, which excludes an accidental failure of one of the cells. The most likely cause is the loss of a significant portion of the electrolyte ejected through the safety valve. Another experiment on overcharging a domestic 10НКГЦ-1Д battery led to the explosion of one of the cells. Moreover, the explosion occurred some time after the battery was removed from charge due to high temperature. Here, most likely, there was damage to the separator and, as a result, an internal short circuit. As far as I know, НКГЦ batteries explode quite often when used incorrectly, since they do not have a safety valve. Overcharging with high currents can lead to the destruction of the battery shell, the release of electrolyte and an explosion. Thus, the charge level of batteries should be monitored by time only under the condition of preliminary discharge, and by some other parameters except for time for refusing preliminary discharge. From Fig. 4 it is clear that by the end of charging with a high current the temperature and pressure increase and some voltage drop is observed. Pressure sensors are built in only high-capacity batteries, and any developer can equip their battery with a temperature sensor. Some of the first chargers for fast charging used the criterion of exceeding the temperature of 45-50 ° C to decide to stop charging. This simple and cheap method has some disadvantages. The fact is that undercharging or overcharging is possible at too high or low ambient temperatures. Therefore, not the temperature value itself is often used, but the rate of its increase, equal to 0.5 — 1 deg/min, as a condition for the end of charging. An example of such a charger is shown in diagram 6. The power source charges the battery via the key, the control device monitors the temperature on the battery via the temperature sensor and, upon reaching the expected value or rate of its growth, issues a signal to open the key and can turn on an indicator.
Fig. 4
Diagram 6
Another parameter is the voltage drop at the end of charging (see the charging curves in Fig. 4). It is noticeable only at high currents, is practically absent at temperatures above 35 °C, and is weakly expressed in batteries with a large number of elements due to the fact that, as a result of some variation in capacity, when the voltage of one element rises, another may fall, distorting the overall picture. However, this method has become widespread for charging batteries with a small number of elements at normal temperatures. The recommended value for completing charging is a voltage drop of 10 mV/element. The advantage of this method is the ability to control the voltage on the battery or battery via the same wires that are used for charging. In fairness, it should be noted that almost all chargers using this parameter simultaneously monitor the battery temperature and are equipped with a protective shutdown when the charging time is exceeded.
The task of monitoring the negative voltage drop is a complex matter and is mainly performed by specialized fast charge controller microcircuits. In addition to the negative voltage drop, microcontrollers can monitor the temperature or its increase, maximum voltage and charging time. Exceeding one of these parameters of the set value leads to the end of the charging process.
There are other parameters by which one can judge the state of charge of the battery, but they are either poorly studied or difficult to measure, so I will not consider them in this article. It is also worth mentioning the special design of batteries, where each battery is charged separately, which gives slightly better results compared to group charging both in terms of control efficiency and avoiding overcharging of the weakest “battery elements”. This design is not widely used due to the cumbersome internal switching and the high cost of a specialized controller included in the battery.
Thus, the best way to charge nickel-cadmium batteries and accumulators is time-based charging with preliminary discharge. The second in terms of the sum of parameters is temperature-based charging or its growth rate. However, training discharge of accumulators that have been subjected to rapid charging must still be carried out after 5-10 operating cycles.
Nickel-metal hydride sealed accumulators.
Nickel-metal hydride sealed batteries are alkaline batteries where, instead of a cadmium electrode, an electrode made of nickel alloy with rare earth metals capable of absorbing hydrogen is used. The positive electrode, as in a nickel-cadmium battery, is nickel oxide. The reactions occurring on it can be written as follows:
Ni(OH)2 + OH- = NiOOH + H2O + e-
On the negative electrode, the metal reacts with hydrogen in water and forms a metal hydride:
M + H2O + e- = MH + OH-
The overall charging reaction looks like this:
Ni(OH)2 + M = NiOOH + MH
When recharging, as in a nickel-cadmium battery, a side process of oxygen generation occurs on the positive electrode:
2OH- = O + H2O + 2e-
Oxygen reaches the negative electrode through the separator and reacts:
O + H2O + 2e+ = 2OH
The latter reactions form a closed cycle and provide gas balance in the battery. However, the pressure in the battery depends not so much on the intensity of the processes taking place, but on the ratio of the generation rate and the oxygen transport rate. In addition, as described in [2], when oxygen is absorbed, an additional increase in the capacity of the metal hydride electrode is also provided due to the formation of the OH group. However, the heating of the metal hydride battery during recharging still occurs.
The features of a metal hydride battery compared to a nickel-cadmium battery include a higher capacity (up to 1.6 times), a less pronounced voltage drop at the end of the charge, a temperature limitation during charging at 40 ° C, no memory effect, and the dependence of the number of cycles on the depth of discharge — metal hydride batteries «do not like a full discharge.
The last two features make charging a metal hydride battery by time with a preliminary discharge not only unnecessary, but also harmful.
Almost all cylindrical and prismatic nickel-metal hydride batteries can be charged with currents up to 0.2 Csom without preliminary discharge with a time limit of about 6 hours. This is due to the fact that there is no memory effect, and some overcharging at low currents is acceptable. A charger built using this method is similar to a device for nickel-cadmium batteries shown in diagram 4. The power supply parameters are the same nominal voltages of nickel-metal hydride and nickel-cadmium batteries are almost the same.
The less pronounced voltage drop at the end of the charge makes the negative slope charge control difficult and dangerous for the battery. The development of batteries from more than 10 NiMH batteries is not recommended due to the risk of overheating when charging one of the batteries, which increases with the increase in capacity spread as a result of long-term use.
In light of the above, the best charging methods for NiMH batteries are: standard charge by time and fast charge by temperature to a value of 40 — 60 ° C or its gradient of 1 — 2 ° C/min.
Different manufacturers give different recommendations for fast charging of their batteries. For example:
— Panasonic (Japan): charging currents 0.5 – 1 Cnom. Maximum temperature – 55°C for sizes A and AA and 60 for L-A, fast charge timer – 90 min for charging current 1Cnom (pretty bold, but they know better), end of charge voltage 1.8 V/cell, negative voltage drop 5 – 10 millivolts/cell.
— Gold Peak Group(China) recommends charging your batteries using different methods, depending on the ambient temperature:
by temperature — at 25 — 45 ° C;
by temperature gradient — at 20 — 30 ° C;
by negative voltage drop — at 0 — 30 ° C.
The maximum battery temperature at a charging current of 0.5 — 1 Cnom is 55 ° C, and at a charging current of 0.2 — 0.4 Cnom 50 ° C, negative voltage drop 10 — 15 millivolts/cell, fast charge timer — 120% capacity.
Batteries from sealed alkaline batteries
The nominal voltage of alkaline batteries is 1.2 V. This is usually not enough to power the consumer. To increase the voltage, batteries are connected into batteries, which have their own operating characteristics. I will talk about the characteristics of batteries during discharge and storage another time — our task is to describe the design features of batteries from the point of view of charging.
The first and most important thing is the choice of battery type and manufacturer. Almost all serious battery manufacturers present series with different sizes, and, consequently, battery capacity with the same properties. Series can be standard, high-capacity, high-temperature, for fast charging, etc.
Secondly, the elements in the battery must be correctly assembled and connected. The use of soldering for sealed batteries is unacceptable.
As rightly noted in [3]: “Never solder wires or any other contacts directly to the battery, as this will damage the internal safety valve, separator, and other parts made of organic materials.” Use spot welding to connect batteries.
Third — when connecting batteries into a battery, use alkali-resistant materials: nickel, stainless steel, nickel-plated steel. Avoid materials: tin, aluminum, copper, zinc, brass, as leakage of electrolyte through the valve during overcharging will cause corrosion problems.
Fourth, try to design batteries with as few elements as possible. With proper operation, the durability and reliability of small batteries is much higher.
Fifth, do not forget to consider installing protective elements and a temperature sensor in the battery. You can save a lot on the charger.
I will not stop you from trying to make a battery from both old and new elements, elements of different capacity or chemical system, to give the batteries the shape you need with a hammer, to allow for the constructive possibility of short-circuiting the battery or its elements, to short-circuit a discharged battery overnight to better eliminate the memory effect, to store the battery in a refrigerator in a jar of brine or to charge an alkaline battery outside in twenty-degree frost. Although all this was done by people without obvious mental deviations and with mental abilities not below average, I recommend only to consult with specialists. And not at the stage when the device for which the battery is designed is already ready, and it is necessary to shove a large battery into a small compartment, but at the stage when it is still possible to make the necessary adjustments.
Literature
1. Technical manual for the use of SLA batteries. PANASONIC’2000
2. Handbook of sealed current sources. KHIMIZDAT, St. Petersburg, 2000
3. Technical guide to the use of NiCd and NiMH batteries. PANASONIC’2000