Features of charging Ni-MH batteries, requirements for the charger and basic parameters. How to charge NiMH batteries correctly How to properly discharge nimh batteries

Nickel metal hydride batteries (ni mh) belong to the alkaline group. Such chemical-type devices produce current, where nickel oxide acts as the cathode and a metal hydride hydrogen electrode acts as the anode. These devices are similar in structure to nickel-hydrogen devices, but they exceed metal hydride devices several times in capacity.

History of creation and development

Nickel-metal hydride batteries began to be manufactured in the 60s of the 20th century. And production began due to significant shortcomings of their predecessors - nickel-cadmium devices. Metal hydride batteries could use different sets of metals. Special alloys have been developed for mass production, which could operate at room temperature.

Serious mass production began in the 1980s. Although such devices are still being improved today. Modern Nickel-metal hydride batteries can provide up to 500 charge and discharge cycles thanks to the use of alloys of nickel and other rare earth metals.

In such devices such as Krona, the voltage is generally initially 8.2 V. Over time, it gradually decreases to 7.4 V. After prolonged use, the subsequent decrease occurs much faster. Metal hydride batteries have a higher capacity (approximately 20% higher) than cadmium devices, but have a shorter service life (200−500 charge/discharge cycles). They also have a higher self-discharge rate, approximately 1.5-2 times.

If we talk about such a factor as the “memory effect”, then it is almost invisible here. If The battery is constantly in use, you can charge it even when it is already half charged, but when it has not been used for some time, then it needs to be prevented by completely discharging it and then charging it.

Such power supplies are often used for various equipment that require autonomous operation. As a rule, such technologies are used in AAA or AA batteries, but there are other options, for example, barathea for industry. The areas of use of such power sources are much larger than their predecessors. Ni MH batteries have no toxic components Due to this, they are used for many tasks.

Today there are 2 types of such devices:

1. 1500−3000 milliamps per hour. This group applies to devices that have increased consumption energy in a short period of time. Video cameras and cameras, equipment for remote control and other devices that require a lot of energy.
2. 300−1000 milliamps per hour. Such batteries are used for devices that consume electricity after a certain period of time, for example, walkie-talkie lights or toys. They consume energy very slowly.

You can charge them using the drip method and fast. But in the instructions, as a rule, the manufacturer indicates that charging using the first method is not recommended, since difficulties may subsequently arise in determining when the current supply to the device stops.

If you charge them in this way, severe overcharging may occur, and this will lead to partial breakdown of the device or a decrease in its capacity. You need to charge the ni mh battery using a quick method. The efficiency in this case will be slightly higher than with the drip option.

The battery charging process can be divided into several points:

• installing the battery into the charger;
• Battery Type;
• initial charging;
• fast charging;
• recharging;
• maintenance charging.

If fast charging is going on, it is desirable that the battery has good support. Nickel-cadmium batteries have enough delta control. Ni mh batteries must have temperature and delta control at a minimum.

For long-term operation of ni mh batteries, you need to know and follow several tips, the regular use of which can guarantee long-term use. To do this you need to know just a few things.

Initially, you need to prepare for the fact that the batteries should not overheat, be heavily discharged, or be recharged. Under these conditions, the operating time can be increased several times.

For long-term operation, the following methods are used:

In order to correctly calculate the formula for charging a ni mh battery, you need to apply the following formula: charging time is equal to the capacity divided by the current of the charger. For example, there is a battery with a capacity of 4000 milliamps per hour. The charger has a current of 1000 milliamps per hour: 4000 / 1000 = 4.

Necessary rules that must be followed during battery operation:

1. Such devices are very sensitive to overheating, and it will have a very bad effect on their operation. They lose current output and the ability to release the available charge.
2. Before actively using a battery cell to better work You can perform several cycles of discharging and charging the device. This will maximize the capacity that was lost during transportation and storage after production.
3. During long-term storage without use, the battery should be left charged to no more than 30-40% of its maximum capacity.
4. After charging or discharging the battery, you need to give it time to cool down.
5. It is recommended from time to time (every 8-10 charging cycles) to discharge the battery to 0.98 and fully charge it. This will extend its operating time.
6. Such batteries need to be discharged to a maximum of 0.98. If this figure is lower, then the device may simply stop charging.

Due to a phenomenon called the “memory effect,” batteries from time to time lose some starting performance and characteristics. This effect occurs as a consequence of repeated cycles of incomplete charging and discharging.

The battery remembers the smaller (upper and lower) limits and significantly reduces its capacity.

But if a problem has already arisen, you need to properly train and restore the battery to solve it. The following steps are performed:

• Using a charger or a light bulb, you need to discharge the battery to 0.801 V;
• fully charged.

If a certain battery has not undergone such prophylaxis for a long time, then several procedures need to be done. It is advisable to carry out charging and discharging training once every 3-4 weeks.

Manufacturers of NiMh batteries claim that such an effect cannot take away more than 5% of the capacity. When training, it is important to use chargers with the ability to discharge with a set minimum threshold. This is necessary to ensure that the battery does not completely discharge, since it may subsequently not be charged at all. Such a charger is very useful when the state of charge of the battery is unknown and it is impossible to estimate it approximately.

If the charge level is unknown, then the discharge must be carried out under careful control of the charger, as this can lead to a deep discharge. When carrying out maintenance on a whole battery, you must first fully charge it in order to equalize the capacity.

In the case when the battery has already worked for a long time (2-3 years), restoring it in this way may be useless. Such actions can only help in the process of battery operation. When using the battery, in addition to the memory effect, the amount of electrolyte filled in also changes downward. It is important to note that it is better to carry out maintenance of each element separately than the entire battery at once. This will enhance the effect. Such batteries can operate for 1-5 years. This depends on the specific manufacturer and model.

Pros and cons of metal hydride devices

If we compare nickel-metal hydride batteries with cadmium batteries, then the significant advantage in the energy reserve of the former is not their only advantage. Battery manufacturers, having abandoned the use of cadmium, have taken a big step towards the use of environmentally friendly materials.

This makes it much easier to resolve the issue of disposal of used products.

Thanks to such advantages as durability, environmental friendliness, high performance, and the use of materials such as nickel, Ni MH batteries are gaining popularity every day. They are also good because with frequent charging and discharging, preventative maintenance to restore capacity must be carried out once every 3-4 weeks.

They also have their disadvantages:

1. Manufacturers of such batteries have limited one set to 10 cells due to the fact that the possibility of device polarity reversal increases over time.
2. Such batteries operate under narrower temperature conditions. Already at -10 °C or +40 °C they lose their performance.
3. When charging such batteries, they generate a lot of heat, so they need special fuses to prevent overheating.
4. They often self-discharge unnecessarily. This happens due to the reaction of the nickel electrode with the hydrogen of the electrolyte.

During the charge/discharge cycle, the amount of crystal lattice decreases over time. This contributes to the appearance of rust and cracks during interaction with the electrolyte.

Advantages of large and small containers

When buying such batteries, you don’t always need to look at their capacity. As the battery capacity increases, its self-discharge also increases. An example is a battery with a capacity of 2400 mAh and 1500 mAh. After several months of use, the stronger battery will lose more capacity than weak. A 2400 mAh battery in a few months will be comparable in capacity to a 1500 mAh device and after a while will even have a charge strength lower than a weaker battery.

If we consider the practice of using such devices, then it is used in devices that require high power consumption in a short time. For example, these could be players, radio controlled models or VCRs.

This article about Nickel-metal hydride (Ni-MH) batteries has long been a classic on the Russian Internet. I recommend checking out...

Nickel-metal hydride (Ni-MH) batteries are similar in design to nickel-cadmium (Ni-Cd) batteries, and in electrochemical processes - nickel-hydrogen batteries. The specific energy of a Ni-MH battery is significantly higher than the specific energy of Ni-Cd and hydrogen batteries (Ni-H2)

VIDEO: Nickel-metal hydride (NiMH) batteries

Comparative characteristics of batteries

***The wide spread of some parameters in the table is caused by different purposes (designs) of batteries. In addition, the table does not take into account data on modern batteries with low self-discharge

History of Ni-MH battery

The development of nickel-metal hydride (Ni-MH) batteries began in the 50-70s of the last century. The result was a new way to store hydrogen in nickel-hydrogen batteries used in spacecraft. In the new element, hydrogen accumulated in alloys of certain metals. Alloys that absorb hydrogen up to 1,000 times their own volume were discovered in the 1960s. These alloys consist of two or more metals, one of which absorbs hydrogen, and the other is a catalyst that promotes the diffusion of hydrogen atoms into the metal lattice. The number of possible combinations of metals used is practically unlimited, which makes it possible to optimize the properties of the alloy. To create Ni-MH batteries, it was necessary to create alloys that operate at low hydrogen pressure and room temperature. Currently, work on the creation of new alloys and their processing technologies continues throughout the world. Nickel alloys with rare-earth metals can provide up to 2000 battery charge-discharge cycles while reducing the capacity of the negative electrode by no more than 30%. The first Ni-MH battery, which used LaNi5 alloy as the main active material of the metal hydride electrode, was patented by Bill in 1975. In early experiments with metal hydride alloys, Ni-MH batteries were unstable and the required battery capacity could not be achieved. Therefore, the industrial use of Ni-MH batteries began only in the mid-80s after the creation of the La-Ni-Co alloy, which allows electrochemically reversible absorption of hydrogen for more than 100 cycles. Since then, the design of Ni-MH rechargeable batteries has been continuously improved towards increasing their energy density. Replacing the negative electrode made it possible to increase the active mass content of the positive electrode, which determines the battery capacity, by 1.3-2 times. Therefore, Ni-MH batteries have, compared to Ni-Cd battery with significantly higher specific energy characteristics. The success of the spread of nickel-metal hydride batteries was ensured by the high energy density and non-toxicity of the materials used in their production.

Basic processes of Ni-MH batteries

Ni-MH batteries use a nickel oxide electrode as the positive electrode, just like a nickel-cadmium battery, and use a nickel-rare earth hydrogen-absorbing electrode instead of a negative cadmium electrode. The following reaction occurs on the positive nickel oxide electrode of a Ni-MH battery:

Ni(OH) 2 + OH- → NiOOH + H 2 O + e - (charge) NiOOH + H 2 O + e - → Ni(OH) 2 + OH - (discharge)

At the negative electrode, the metal with absorbed hydrogen is converted into a metal hydride:

M + H 2 O + e - → MH + OH- (charge) MH + OH - → M + H 2 O + e - (discharge)

The overall reaction in a Ni-MH battery is written as follows:

Ni(OH) 2 + M → NiOOH + MH (charge) NiOOH + MH → Ni(OH) 2 + M (discharge)

The electrolyte does not participate in the main current-forming reaction. After reaching 70-80% of the capacity and upon recharging, oxygen begins to be released on the nickel oxide electrode,

2OH- → 1/2O 2 + H2O + 2e - (recharge)

which is restored at the negative electrode:

1/2O 2 + H 2 O + 2e - → 2OH - (recharge)

The last two reactions provide a closed oxygen cycle. When oxygen is reduced, an additional increase in the capacity of the metal hydride electrode is provided due to the formation of the OH - group.

Design of electrodes of Ni-MH batteries

Metal hydrogen electrode

The main material that defines the characteristics of a Ni-MH battery is a hydrogen-absorbing alloy, which can absorb 1000 times its own volume of hydrogen. The most widespread are alloys of the LaNi5 type, in which part of the nickel is replaced by manganese, cobalt and aluminum to increase the stability and activity of the alloy. To reduce the cost, some manufacturing companies use misch metal instead of lanthanum (Mm, which is a mixture of rare earth elements, their ratio in the mixture is close to the ratio in natural ores), which in addition to lanthanum also includes cerium, praseodymium and neodymium. During charge-discharge cycling, expansion and contraction of the crystal lattice of hydrogen-absorbing alloys occurs by 15-25% due to the absorption and desorption of hydrogen. Such changes lead to the formation of cracks in the alloy due to an increase in internal stress. The formation of cracks causes an increase in the surface area, which is subject to corrosion when interacting with an alkaline electrolyte. For these reasons, the discharge capacity of the negative electrode gradually decreases. In a battery with a limited amount of electrolyte, this creates problems associated with electrolyte redistribution. Corrosion of the alloy leads to chemical passivity of the surface due to the formation of corrosion-resistant oxides and hydroxides, which increase the overvoltage of the main current-generating reaction of the metal hydride electrode. The formation of corrosion products occurs with the consumption of oxygen and hydrogen from the electrolyte solution, which, in turn, causes a decrease in the amount of electrolyte in the battery and an increase in its internal resistance. To slow down the undesirable processes of dispersion and corrosion of alloys, which determine the service life of Ni-MH batteries, two main methods are used (in addition to optimizing the composition and production mode of the alloy). The first method is to microencapsulate alloy particles, i.e. in covering their surface with a thin porous layer (5-10%) - by weight of nickel or copper. The second method, which is most widely used at present, involves treating the surface of alloy particles in alkaline solutions to form protective films permeable to hydrogen.

Nickel oxide electrode

Nickel oxide electrodes in mass production are manufactured in the following design modifications: lamella, lamella-free sintered (cermet) and pressed, including tablet electrodes. IN recent years lamella-free felt and foam-polymer electrodes are beginning to be used.

Lamellar electrodes

Lamellar electrodes are a set of interconnected perforated boxes (lamellas) made from thin (0.1 mm thick) nickel-plated steel strip.

Sintered (cermet) electrodes

electrodes of this type consist of a porous (with a porosity of at least 70%) metal-ceramic base, in the pores of which the active mass is located. The base is made from carbonyl nickel fine powder, which, mixed with ammonium carbonate or urea (60-65% nickel, the rest is filler), is pressed, rolled or sprayed onto a steel or nickel mesh. Then the mesh with the powder is subjected to heat treatment in a reducing atmosphere (usually in a hydrogen atmosphere) at a temperature of 800-960 ° C, while ammonium carbonate or urea decomposes and volatilizes, and the nickel is sintered. The bases obtained in this way have a thickness of 1-2.3 mm, a porosity of 80-85% and a pore radius of 5-20 microns. The base is alternately impregnated with a concentrated solution of nickel nitrate or nickel sulfate and an alkali solution heated to 60-90 ° C, which encourages the precipitation of nickel oxides and hydroxides. Currently, the electrochemical impregnation method is also used, in which the electrode is subjected to cathodic treatment in a solution of nickel nitrate. Due to the formation of hydrogen, the solution in the pores of the plate becomes alkalized, which leads to the precipitation of nickel oxides and hydroxides in the pores of the plate. Foil electrodes are among the types of sintered electrodes. Electrodes are produced by applying an alcohol emulsion of nickel carbonyl powder containing binders to a thin (0.05 mm) perforated nickel tape on both sides, by spraying, sintering and further chemical or electrochemical impregnation with reagents. The thickness of the electrode is 0.4-0.6 mm.

Pressed electrodes

Pressed electrodes are made by pressing the active mass under a pressure of 35-60 MPa onto a mesh or perforated steel tape. The active mass consists of nickel hydroxide, cobalt hydroxide, graphite and a binder.

Metal felt electrodes

Metal felt electrodes have a highly porous base made of nickel or carbon fibers. The porosity of these bases is 95% or more. The felt electrode is made on the basis of nickel-plated polymer or carbon-graphite felt. The thickness of the electrode, depending on its purpose, is in the range of 0.8-10 mm. The active mass is introduced into the felt using different methods depending on its density. Can be used instead of felt nickel foam, obtained by nickel plating of polyurethane foam followed by annealing in a reducing environment. A paste containing nickel hydroxide and a binder are usually added to a highly porous medium by spreading. After this, the base with the paste is dried and rolled. Felt and foam polymer electrodes are characterized by high specific capacity and long service life.

Ni-MH battery design

Cylindrical Ni-MH batteries

The positive and negative electrodes, separated by a separator, are rolled into a roll, which is inserted into the housing and closed with a sealing lid with a gasket (Figure 1). The cover has a safety valve that is triggered at a pressure of 2-4 MPa in the event of a failure during battery operation.

Fig.1. Nickel-metal hydride (Ni-MH) battery design: 1-body, 2-cap, 3-valve cap, 4-valve, 5-positive electrode collector, 6-insulating ring, 7-negative electrode, 8-separator, 9- positive electrode, 10-insulator.

Prismatic Ni-MH batteries

In prismatic Ni-MH batteries, positive and negative electrodes are placed alternately, and a separator is placed between them. The electrode block is inserted into a metal or plastic case and closed with a sealing cap. A valve or pressure sensor is usually installed on the lid (Figure 2).

Fig.2. Ni-MH battery design: 1-body, 2-cover, 3-valve cap, 4-valve, 5-insulating gasket, 6-insulator, 7-negative electrode, 8-separator, 9-positive electrode.

Ni-MH batteries use an alkaline electrolyte consisting of KOH with the addition of LiOH. Non-woven polypropylene and polyamide with a thickness of 0.12-0.25 mm, treated with a wetting agent, are used as a separator in Ni-MH batteries.

Positive electrode

Ni-MH batteries use positive nickel oxide electrodes similar to those used in Ni-Cd batteries. Ni-MH batteries mainly use metal-ceramic, and in recent years, felt and polymer foam electrodes (see above).

Negative electrode

Five designs of negative metal hydride electrode (see above) have found practical application in Ni-MH batteries: - lamellar, when the powder of a hydrogen-absorbing alloy with or without a binder is pressed into a nickel mesh; — nickel foam, when a paste with an alloy and a binder is introduced into the pores of a nickel foam base, and then dried and pressed (rolled); — foil, when a paste with an alloy and a binder is applied to perforated nickel or nickel-plated steel foil, and then dried and pressed; - rolled, when the powder of the active mass, consisting of an alloy and a binder, is applied by rolling (rolling) onto a tensile nickel grid or copper mesh; - sintered, when alloy powder is pressed onto a nickel mesh and then sintered in a hydrogen atmosphere. The specific capacitances of metal hydride electrodes of different designs are close in value and are determined mainly by the capacitance of the alloy used.

Characteristics of Ni-MH batteries. Electrical characteristics

Open circuit voltage

Open circuit voltage value Uр.к. Ni-MH systems are difficult to accurately determine due to the dependence of the equilibrium potential of the nickel oxide electrode on the degree of oxidation of nickel, as well as the dependence of the equilibrium potential of the metal hydride electrode on the degree of its saturation with hydrogen. 24 hours after charging the battery, the open circuit voltage of a charged Ni-MH battery is in the range of 1.30-1.35V.

Rated discharge voltage

Uр at a normalized discharge current Iр = 0.1-0.2C (C is the nominal capacity of the battery) at 25°C is 1.2-1.25V, the usual final voltage is 1V. Voltage decreases with increasing load (see Figure 3)

Fig.3. Discharge characteristics of a Ni-MH battery at a temperature of 20°C and different normalized load currents: 1-0.2C; 2-1C; 3-2C; 4-3С

Battery capacity

With increasing load (decreasing discharge time) and decreasing temperature, the capacity of the Ni-MH battery decreases (Figure 4). The effect of temperature reduction on capacity is especially noticeable at high discharge rates and at temperatures below 0°C.

Fig.4. Dependence of the discharge capacity of a Ni-MH battery on temperature at different currents discharge: 1-0.2C; 2-1C; 3-3C

Safety and service life of Ni-MH batteries

During storage, the Ni-MH battery self-discharges. After a month at room temperature, the loss of capacity is 20-30%, and with further storage the losses decrease to 3-7% per month. The self-discharge rate increases with increasing temperature (see Figure 5).

Fig.5. Dependence of the discharge capacity of a Ni-MH battery on storage time at different temperatures: 1-0°C; 2-20°C; 3-40°С

Charging Ni-MH battery

The operating time (number of discharge-charge cycles) and service life of a Ni-MH battery are largely determined by operating conditions. The operating time decreases with increasing discharge depth and speed. The operating time depends on the charging speed and the method of monitoring its completion. Depending on the type of Ni-MH batteries, operating mode and operating conditions, the batteries provide from 500 to 1800 discharge-charge cycles at a discharge depth of 80% and have a service life (on average) of 3 to 5 years.

To ensure reliable operation of the Ni-MH battery during the guaranteed period, you must follow the manufacturer's recommendations and instructions. The greatest attention should be paid temperature conditions. It is advisable to avoid overdischarges (below 1V) and short circuits. It is recommended to use Ni-MH batteries for their intended purpose, avoid combining used and unused batteries, and do not solder wires or other parts directly to the battery. Ni-MH batteries are more sensitive to overcharging than Ni-Cd batteries. Overcharging can lead to thermal runaway. Charging is usually carried out with current Iз=0.1С for 15 hours. Compensatory recharging is carried out with current Iз=0.01-0.03С for 30 hours or more. Accelerated (4 - 5 hours) and fast (1 hour) charges are possible for Ni-MH batteries with highly active electrodes. With such charges, the process is controlled by changes in temperature ΔT and voltage ΔU and other parameters. Fast charging is used, for example, for Ni-MH batteries that power laptops, cell phones, and electrical tools, although in laptops and cell phones Lithium-ion and lithium polymer batteries. A three-stage charging method is also recommended: the first stage of fast charging (1C and above), a charge at a speed of 0.1C for 0.5-1 hour for the final recharge, and a charge at a speed of 0.05-0.02C as a compensatory recharge. Information on how to charge Ni-MH batteries is usually contained in the manufacturer's instructions, and the recommended charging current is indicated on the battery case. Charging voltage Uз at Iз=0.3-1С lies in the range of 1.4-1.5V. Due to the release of oxygen on the positive electrode, the amount of electricity transferred during charging (Q3) is greater than the discharge capacity (Cp). At the same time, the return on capacity (100 Sr/Qz) is 75-80% and 85-90%, respectively, for disk and cylindrical Ni-MH batteries.

Charge and discharge control

To prevent overcharging of Ni-MH batteries, the following charge control methods can be used with appropriate sensors installed in the batteries or chargers:

• charging termination method based on absolute temperature Tmax. The battery temperature is constantly monitored during the charging process, and when the maximum value is reached, the fast charge is interrupted;
• charging termination method based on the rate of temperature change ΔT/Δt. When using this method, the slope of the temperature curve battery is constantly monitored during the charging process, and when this parameter becomes higher than a specifically set value, the charging is interrupted;
• method of stopping the charge using a negative voltage delta -ΔU. At the end of the battery charge, during the oxygen cycle, its temperature begins to increase, leading to a decrease in voltage;
• charging termination method based on maximum charging time t;
• charging termination method maximum pressure Pmax. Typically used in prismatic batteries of large size and capacity. The level of permissible pressure in a prismatic accumulator depends on its design and lies in the range of 0.05-0.8 MPa;
• charging termination method based on maximum voltage Umax. It is used to cut off the charge of batteries with high internal resistance, which appears at the end of their service life due to a lack of electrolyte or at low temperatures.

When using the Tmax method, the battery may be overcharged if the temperature environment decreases, or the battery may not receive enough charge if the ambient temperature rises significantly. The ΔT/Δt method can be used very effectively to stop charging when low temperatures environment. But if at higher temperatures this method alone is used, the batteries inside the batteries will be subject to undesirably high temperatures before the ΔT/Δt value for shutdown can be reached. For a given value of ΔT/Δt, a larger input capacitance can be obtained at a lower ambient temperature than at a higher ambient temperature. high temperature. At the beginning of a battery charge (as well as at the end of a charge), the temperature rises rapidly, which can lead to premature charge shutdown when using the ΔT/Δt method. To eliminate this, charger developers use timers for the initial delay of sensor response using the ΔT/Δt method. The -ΔU method is effective in stopping charging at low ambient temperatures rather than at elevated temperatures. In this sense, the method is similar to the ΔT/Δt method. To ensure charging termination in cases where unforeseen circumstances prevent normal charging interruption, it is also recommended to use a timer control that regulates the duration of the charging operation (t method). Thus, to quickly charge batteries with normalized currents of 0.5-1C at temperatures of 0-50 °C, it is advisable to simultaneously use the Tmax methods (with a shutdown temperature of 50-60 °C depending on the design of the batteries and batteries), -ΔU (5- 15 mV per battery), t (usually to obtain 120% of the rated capacity) and Umax (1.6-1.8 V per battery). Instead of the -ΔU method, the ΔT/Δt method (1-2 °C/min) with an initial delay timer (5-10 min) can be used. For charge control, also see the corresponding article. After fast charging the battery, the chargers provide for switching them to recharging with a normalized current of 0.1 C - 0.2 C for a certain time. For Ni-MH batteries it is not recommended to charge at constant voltage, as “thermal runaway” of the batteries may occur. This is due to the fact that at the end of the charge there is an increase in current, which is proportional to the difference between the power supply voltage and the battery voltage, and the battery voltage at the end of the charge decreases due to the increase in temperature. At low temperatures, the charging rate must be reduced. Otherwise, the oxygen will not have time to recombine, which will lead to an increase in pressure in the battery. For operation in such conditions, Ni-MH batteries with highly porous electrodes are recommended.

Advantages and disadvantages of Ni-MH batteries

A significant increase in specific energy parameters is not the only advantage of Ni-MH batteries over Ni-Cd batteries. Refusal from cadmium also means a transition to more environmentally friendly production. The problem of recycling worn-out batteries is also easier to solve. These advantages of Ni-MH batteries have determined the faster growth of their production volumes among all the world's leading battery companies compared to Ni-Cd batteries.

Ni-MH batteries do not have the “memory effect” inherent in Ni-Cd batteries due to the formation of nickelate in the negative cadmium electrode. However, the effects associated with recharging the nickel oxide electrode remain. The decrease in discharge voltage observed with frequent and long recharges, just like with Ni-Cd batteries, can be eliminated by periodically performing several discharges up to 1V - 0.9V. It is enough to carry out such discharges once a month. However, nickel-metal hydride batteries are inferior to nickel-cadmium batteries, which they are intended to replace, in some performance characteristics:

• Ni-MH batteries operate effectively in a narrower range of operating currents, which is associated with limited desorption of hydrogen from the metal hydride electrode at very high discharge rates;
• Ni-MH batteries have a narrower temperature range of operation: most of them are inoperable at temperatures below -10 °C and above +40 °C, although in some series of batteries, adjustments to the recipes have expanded the temperature limits;
• During the charging of Ni-MH batteries, more heat is generated than when charging Ni-Cd batteries, therefore, in order to prevent overheating of batteries from Ni-MH batteries during fast charging and/or significant overcharging, thermal fuses or thermal relays are installed in them, which are located on the wall of one of the batteries in the central part of the battery (this applies to industrial battery assemblies);
• Ni-MH batteries have increased self-discharge, which is determined by the inevitable reaction of hydrogen dissolved in the electrolyte with the positive nickel oxide electrode (but, thanks to the use of special alloys of the negative electrode, it was possible to reduce the self-discharge rate to values ​​close to those for Ni-Cd batteries );
• the danger of overheating when charging one of the Ni-MH batteries of the battery, as well as reversal of the battery with a lower capacity when the battery is discharged, increases with the mismatch of battery parameters as a result of prolonged cycling, therefore the creation of batteries from more than 10 batteries is not recommended by all manufacturers;
• the loss of capacity of the negative electrode that occurs in a Ni-MH battery when discharged below 0 V is irreversible, which puts forward more stringent requirements for the selection of batteries in the battery and control of the discharge process than in the case of using Ni-Cd batteries; as a rule, it is recommended to discharge to 1 V/ac in low-voltage batteries and up to 1.1 V/ac in a battery of 7-10 batteries.

As noted earlier, the degradation of Ni-MH batteries is determined primarily by a decrease in the sorption capacity of the negative electrode during cycling. During the charge-discharge cycle, the volume of the alloy crystal lattice changes, which leads to the formation of cracks and subsequent corrosion during reaction with the electrolyte. The formation of corrosion products occurs with the absorption of oxygen and hydrogen, as a result of which the total amount of electrolyte decreases and the internal resistance of the battery increases. It should be noted that the characteristics of Ni-MH batteries significantly depend on the alloy of the negative electrode and the processing technology of the alloy to increase the stability of its composition and structure. This forces battery manufacturers to carefully select alloy suppliers, and battery consumers to carefully select a manufacturing company.

Based on materials from the sites powerinfo.ru, “Chip and Dip”

The main difference between Ni-Cd batteries and Ni-Mh batteries is the composition. The base of the battery is the same - it is nickel, it is the cathode, but the anodes are different. For a Ni-Cd battery, the anode is cadmium metal; for a Ni-Mh battery, the anode is a hydrogen metal hydride electrode.

Each type of battery has its pros and cons, knowing them you can more accurately select the battery you need.

Will the old charger fit the new battery if I change the Ni-Cd to a Ni-Mh battery or vice versa?

The charging principle for both batteries is absolutely the same, so the charger can be used from the previous battery. The basic rule for charging these batteries is that they can only be charged after they are completely discharged. This requirement is a consequence of the fact that both types of batteries are subject to the “memory effect”, although with Ni-Mh batteries this problem is minimized.

How to properly store Ni-Cd and Ni-Mh batteries?

The best place to store a battery is in a cool, dry room, since the higher the storage temperature, the faster the battery self-discharges. The battery can be stored in any condition other than completely discharged or fully charged. The optimal charge is 40-60%%. Once every 2-3 months you should recharge (due to the presence of self-discharge), discharge and charge again to 40-60% of the capacity. Storage for up to five years is acceptable. After storage, the battery should be discharged, charged and then used normally.

Can I use batteries with a larger or smaller capacity than the battery from the original kit?

Battery capacity is the operating time of your power tool on battery power. Accordingly, there is absolutely no difference in battery capacity for a power tool. The actual difference will only be in the charging time of the battery and the operating time of the power tool from the battery. When choosing a battery capacity, you should proceed from your requirements; if you need to work longer using one battery, choose more capacious batteries; if the supplied batteries are completely satisfactory, then you should choose batteries of equal or similar capacity.

NiMH stands for nickel metal hydride. Proper charging is key to maintaining performance and longevity. You need to know this technology in order to charge NiMH. Recycling NiMH cells is a rather complex process because the voltage peak and subsequent drop are smaller, and therefore the indicators are more difficult to determine. Overcharging causes the cell to overheat and become damaged, leading to loss of capacity and subsequent loss of functionality.

A battery is an electrochemical device in which electrical energy is converted and stored in chemical form. Chemical energy Easily converts to electric. NiMH works on a principle based on the absorption, release and transfer of hydrogen within two electrodes.

NiMH batteries consist of two metal strips that act as positive and negative electrodes, and an insulating foil separator between them. This energy “sandwich” is wound and placed in the battery along with liquid electrolyte. The positive electrode is usually made of nickel, the negative electrode is made of metal hydride. Hence the name NiMH, or nickel metal hydride.

Advantages:

1. Contains fewer toxins and is environmentally friendly and recyclable.
2. The memory effect is higher than Ni-Cad.
3. Much safer than lithium batteries.

Flaws:

1. Deep discharging reduces lifespan and generates heat during fast charging and high load.
2. Self-discharge is higher compared to other batteries and must be taken into account before charging NiMH.
3. High level required maintenance. The battery must be completely discharged to prevent crystal formation during charging.
4. More expensive than Ni-Cad battery.

The NiMH cell has many characteristics similar to NiCd, such as the discharge curve (subject to additional charging) that the battery can accept. It is intolerant of overcharging, which causes capacity degradation, which poses a serious problem for charger designers.

The current characteristics that are necessary in order to properly charge a NiMH battery are:

1. Rated voltage - 1.2V.
2. Specific energy - 60-120 W-hour/kg.
3. Energy density - 140-300 W-hour/kg.
4. Specific power - 250-1000 W/kg.
5. Charge/discharge efficiency - 90%.

The charging efficiency of nickel batteries ranges from 100% to 70% of full capacity. Initially there is a slight increase in temperature, but later as the charge level rises, the efficiency drops, generating heat, which must be taken into account before charging NiMH.

When a NiCD battery is discharged to a certain minimum voltage and then charged, steps must be taken to reduce the conditioning effect (approximately every 10 charge/discharge cycles) otherwise it will begin to lose capacity. NiMH does not require such a requirement because the effect is negligible.

However, this recovery process is also convenient for nickel-metal hydride devices and is recommended to be taken into account before charging NiMH batteries. The process is repeated three to five times before they reach full capacity. The conditioning process of rechargeable batteries ensures that they will last for many years.

There are several charging methods that can be used with NiMH batteries. They, like NiCds, require a source DC. The speed is usually indicated on the cell body. It should not exceed technological standards. Charging limits are clearly regulated by manufacturers. Before using batteries, you need to clearly know what current to charge NiMH batteries with. There are several methods that are used to prevent failure:

Parallel charging of batteries makes it difficult qualitative definition end of the process. This is because you cannot be sure that each cell or stack has the same resistance, and so some will draw more current than others. This means that you need to use a separate charging circuit for each line in parallel block. It is necessary to establish what current to charge the NiMH by determining the balancing, for example, using resistors of such resistance that they will dominate the control of the parameters.

Modern algorithms have been developed to provide accurate charging without the use of a thermistor. These devices are similar to Delta V, but have special measurement methods to detect full charge, usually involving some kind of cycling where the voltage is measured over time and between pulses. For multi-element packets, if they are not in the same state and are not balanced in capacity, they may be filled one at a time, signaling the end of the stage.

It will take several cycles to balance them. When the battery reaches the end of its charge, oxygen begins to form at the electrodes and recombine at the catalyst. New chemical reaction creates heat that is easily measured by a thermistor. This is the safest way to determine the end of a process during a fast recovery.

Overnight charging is the cheapest way to charge a NiMH battery at C/10, which is below 10% of rated capacity per hour. This must be taken into account in order to charge NiMH correctly. So a 100mAh battery will charge at 10mA for 15 hours. This method does not require an end-of-process sensor and ensures a full charge. Modern cells have an oxygen recirculation catalyst, which prevents damage to the battery when exposed to electric current.

This method cannot be used if the charging rate exceeds C/10. The minimum voltage required for full reaction depends on temperature (at least 1.41 V per cell at 20 degrees), which must be taken into account in order to properly charge NiMH. Prolonged recovery does not cause ventilation. It warms up the battery slightly. To maintain service life, it is recommended to use a timer with a range of 13 to 15 hours. The Ni-6-200 charger has a microprocessor that reports the state of charge via an LED and also performs a timing function.

Fast charging process

Using the timer, you can charge the C/3.33 for 5 hours. This is a bit risky as the battery must be completely discharged first. One way to ensure this doesn't happen is to automatically discharge the battery by the charger, which then starts the recovery process for 5 hours. The advantage of this method is to eliminate any possibility of creating negative battery memory.

Currently, not all manufacturers produce such chargers, but the microprocessor board is used, for example, in the C/10 /NiMH-NiCad-solar-charge-controller charger and can be easily modified to perform the discharge. A power dissipator will be required to dissipate the energy of a partially charged battery for a reasonable amount of time.

If a temperature monitor is used, NiMH batteries can be charged at rates up to 1C, in other words, 100% amp-hour capacity in 1.5 hours. The PowerStream battery charge controller does this in conjunction with a control board that can measure voltage and current for more complex algorithms. When the temperature rises, the process should be stopped, and the dT/dt value should be set to 1-2 degrees per minute.

There are new algorithms that use microprocessor control using the -dV signal to determine the end of charge. In practice they work very well, which is why modern devices use this technology, which involves switching on and off processes to measure voltage.

Adapter Specifications

An important issue is the battery life or the total lifetime cost of the system. In this case, manufacturers offer devices with microprocessor control.

Algorithm for an ideal charger:

1. Soft start. If the temperature is above 40 degrees or below zero, start with charging C/10.
2. Option. If the voltage of the discharged battery is higher than 1.0 V/cell, discharge the battery to 1.0 V/cell and then proceed to fast charging.
3. Fast charging. At 1 degree until the temperature reaches 45 degrees or dT indicates full charge.
4. After fast charging is completed, charge at C/10 for 4 hours to ensure a full charge.
5. If the voltage of a charged NiMH battery rises to 1.78 V/cell, stop working.
6. If the fast charging time exceeds 1.5 hours without interruption, it is stopped.

In theory, trickle charging is a charge rate that is high enough to keep the battery fully charged, but low enough to avoid overcharging. Determining the optimal charging rate for a particular battery is a little difficult to describe, but it is generally accepted that it is around ten percent of the battery's capacity, for example, for a Sanyo 2500mAh AA NiMH, the optimal charging rate is 250mA or lower. This must be taken into account in order to properly charge NiMH batteries.

Most common cause premature failure of the battery is overcharging. The types of chargers that most often cause it are the so-called "fast chargers" of 5 or 8 hours. The problem with these instruments is that they don't really have a process control mechanism.

Most of them have simple functionality. They charge at full speed for a fixed period of time (usually five or eight hours) and then switch off or switch to a lower "manual" speed. If they are used properly then everything is fine. If they are applied incorrectly, battery life will be reduced in several ways:

1. If fully charged or partially charged batteries are inserted into the device, it cannot sense this, so it fully charges the batteries it is designed for. So, the battery capacity decreases.
2. Another common situation is when the charging cycle is interrupted while it is in progress. However, after this, a reconnection follows. Unfortunately, this leads to relaunch full cycle charging even if the previous cycle is almost complete.

The easiest way to avoid these scenarios is to use a microprocessor-controlled smart charger. It can detect when the battery is fully charged and then - depending on its design - either shut down completely or switch to recharging mode.

In order to charge the NiMH iMax you will need a special charger as using the wrong method can render the battery useless. Many users consider iMax B6 best choice For NiMH charging. It supports the process of up to 15 cell batteries, as well as many settings and configurations for different types batteries. The recommended charging time should not exceed 20 hours.

Typically, the manufacturer guarantees 2000 charge/discharge cycles from a standard NiMH battery, although this number may vary depending on operating conditions.

Work algorithm:

1. Charging NiMH iMax B6. You must connect the power cord to the outlet on the left side of the device, taking note of the shape at the end of the cord to ensure that the connection is correct. We insert it all the way and stop pressing when it appears beep and a welcome message on the display screen.
2. Use the silver button on the far left to view the first menu and select the type of battery to charge. Pressing the leftmost button will confirm the selection. The button on the right will scroll through the options: charge, discharge, balance, fast charge, storage and others.
3. Two central control buttons will help you select the desired number. By pressing the rightmost button to enter, you can move to the voltage setting by scrolling again using the two center buttons and pressing enter.
4. Use several cables to connect the battery. The first set looks like lab wiring equipment. It often comes complete with alligator clips. Connection sockets are located on the right side of the device near the bottom. They are fairly easy to spot. This is how you can charge NiMH with the iMax B6.
5. Then you need to connect the free battery cable to the end of the red and black clamps, creating a closed loop. This can be a bit risky, especially if the user makes the wrong settings the first time. Press and hold the enter button for three seconds. The screen should then inform you that it is checking the battery, after which the user will be asked to confirm the mode setting.
6. While the battery is charging, you can scroll through the different display screens using the two central buttons, which provide information about the charging process in different modes.

The most standard advice is to completely discharge the batteries and then charge them. Although this is a treatment for the "memory effect", care must be taken in NiCad batteries as they are easily damaged by over-discharging, resulting in "pole reversal" and irreversible processes. In some cases, the battery electronics are designed in such a way that they prevent negative processes by turning off before they happen, but more simple devices, for example, for flashlights, this is not done.

Necessary:

1. Be prepared to replace them. Nickel-metal hydride batteries do not last forever. After the resource expires, they will stop working.
2. Buy a “smart” charger that electronically controls the process and prevents overcharging. This is not only better for batteries, but also uses less energy.
3. Remove the battery when recharging is complete. Unnecessary time on the device means more "trickle" energy is used to charge it, therefore increasing wear and tear and wasting more energy.
4. Do not completely discharge batteries to extend their life. Despite all the advice to the contrary, completely discharging them actually shortens their lifespan.
5. Store NiMH batteries at room temperature in a dry place.
6. Excessive heat can damage batteries and cause them to drain quickly.
7. Consider using a model with low level charge.

Thus, we can draw a line. Indeed, nickel-metal hydride batteries are better prepared by the manufacturer for operation in modern conditions, and proper charging of batteries using a smart device will ensure their performance and durability.

Ni-MH batteries (nickel metal hydride) are included in the alkaline group. They are current sources of a chemical type, where nickel oxide acts as the cathode, and a hydrogen metal hydride electrode acts as the anode. Alkali is an electrolyte. They are similar to nickel-hydrogen batteries, but are superior in energy capacity.

The production of Ni-MH batteries began in the mid-twentieth century. They were developed taking into account the shortcomings of outdated nickel-cadmium batteries. NiNH can use different combinations of metals. For their production, special alloys and metals have been developed that operate at room temperature and low hydrogen pressure.

Industrial production began in the eighties. Alloys and metals for Ni-MH are still being manufactured and improved today. Modern devices This type can provide up to 2 thousand charge-discharge cycles. A similar result is achievable due to the use of nickel alloys with rare earth metals.

How are these devices used?

Nickel metal hydride devices are widely used for power supply different types electronics that operate autonomously. They are usually made in the form of AAA or AA batteries. Other versions are also available. For example, industrial batteries. The scope of use of Ni-MH batteries is slightly wider than that of nickel-cadmium batteries, because they do not contain toxic materials.

IN at the moment Nickel-metal hydride batteries sold on the domestic market are divided into 2 groups by capacity - 1500-3000 mAh and 300-1000 mAh:

1. First used in devices that have increased energy consumption in a short time. These are all kinds of players, radio-controlled models, cameras, video cameras. In general, devices that quickly consume energy.
2. Second used when energy consumption begins after a certain time interval. These are toys, flashlights, walkie-talkies. Battery-powered devices operate that consume moderate amounts of electricity and remain offline for a long time.

Charging Ni-MH devices

Charging can be drip and fast. Manufacturers do not recommend the first because it makes it difficult to accurately determine when the current supply to the device has stopped. For this reason, a powerful overcharge may occur, which will lead to battery degradation. using the quick option. The efficiency here is slightly higher than that of the drip type of charging. The current is set to 0.5-1 C.

How to charge a hydride battery:

• the presence of a battery is determined;
• device qualification;
• pre-charge;
• fast charging;
• recharging;
• maintenance charging.

When fast charging you need to have a good charger. It must control the end of the process according to different criteria independent of each other. For example, Ni-Cd devices have enough voltage delta control. And with NiMH, the battery needs to monitor temperature and delta at a minimum.

For proper operation Ni-MH should remember the “Rule of the Three Ps”: “ Do not overheat”, “Do not overcharge”, “Do not overdischarge”.

To prevent battery overcharging, the following control methods are used:

1. Termination of charge based on temperature change rate . Using this technique, the battery temperature is constantly monitored during charging. When the readings rise faster than necessary, charging stops.
2. Method of stopping charging based on its maximum time .
3. Termination of charge based on absolute temperature . Here the temperature of the battery is monitored during the charging process. When the maximum value is reached, fast charging stops.
4. Negative delta voltage termination method . Before the battery completes charging, the oxygen cycle raises the temperature of the NiMH device, causing the voltage to drop.
5. Maximum voltage . The method is used to turn off the charge of devices with increased internal resistance. The latter appears at the end of the battery life due to lack of electrolyte.
6. Maximum pressure . The method is used for high-capacity prismatic batteries. The level of permitted pressure in such a device depends on its size and design and is in the range of 0.05-0.8 MPa.

To clarify the charging time of a Ni-MH battery, taking into account all the characteristics, you can use the formula: charging time (h) = capacity (mAh) / charger current (mA). For example, there is a battery with a capacity of 2000 milliamp-hours. The charge current in the charger is 500 mA. The capacity is divided by the current and the result is 4. That is, the battery will charge in 4 hours.

Mandatory rules that must be followed for the proper functioning of the nickel-metal hydride device:

1. These batteries are much more sensitive to heat than nickel-cadmium batteries; they cannot be overloaded . Overload will negatively affect current output (the ability to hold and release accumulated charge).
2. Metal hydride batteries can be “trained” after purchase . Perform 3-5 charge/discharge cycles, which will allow you to reach the limit of capacity lost during transportation and storage of the device after leaving the conveyor.
3. Batteries should be stored with a small amount of charge. , approximately 20-40% of the nominal capacity.
4. After discharging or charging, allow the device to cool down. .
5. If in electronic device the same battery assembly is used in recharging mode , then from time to time you need to discharge each of them to a voltage of 0.98, and then fully charge them. It is recommended to perform this cycling procedure once every 7-8 battery recharging cycles.
6. If you need to discharge NiMH, you should stick to the minimum value of 0.98 . If the voltage drops below 0.98, it may stop charging.

Reconditioning of Ni-MH batteries

Due to the “memory effect”, these devices sometimes lose some characteristics and most of containers. This occurs during repeated cycles of incomplete discharge and subsequent charging. As a result of this operation, the device “remembers” a lower discharge limit, for this reason its capacity decreases.

To get rid of this problem, you need to constantly perform training and recovery. The light bulb or charger discharges to 0.801 volts, then the battery is fully charged. If the battery has not undergone the recovery process for a long time, then it is advisable to perform 2-3 similar cycles. It is advisable to train it once every 20-30 days.

Manufacturers of Ni-MH batteries claim that the “memory effect” takes up approximately 5% of the capacity. You can restore it with the help of training. An important point when Ni-MH reduction is that the charger has a discharge function with minimum voltage control. What is needed to prevent the device from being severely discharged during restoration. This is indispensable when the initial state of charge is unknown and it is impossible to guess the approximate discharge time.

If the state of charge of the battery is unknown, it should be discharged under full voltage control, otherwise such restoration will lead to deep discharge. When reconditioning a whole battery, it is recommended to first fully charge it to equalize the charge level.

If the battery has been used for several years, then restoration by charging and discharging may be useless. It is useful for prevention during operation of the device. When using NiMH, along with the appearance of the “memory effect,” changes in the volume and composition of the electrolyte occur. It is worth remembering that it is wiser to restore battery cells individually than to restore the entire battery. The battery life is from one to five years (depending on the specific model).

Advantages and disadvantages

A significant increase in the energy parameters of nickel-metal hydride batteries is not their only advantage over cadmium batteries. Having abandoned the use of cadmium, manufacturers began to use a more environmentally friendly metal. It is much easier to resolve issues with .

Due to these advantages and the fact that the metal used in manufacturing is nickel, the production of Ni-MH devices has increased sharply when compared with nickel-cadmium batteries. They are also convenient because in order to reduce the discharge voltage during long-term recharges, a full discharge (up to 1 volt) must be carried out once every 20-30 days.

A little about the disadvantages:

1. Manufacturers limited Ni-MH batteries to ten cells , because with increasing charge-discharge cycles and service life, there is a danger of overheating and polarity reversal.
2. These batteries operate in a narrower temperature range than nickel-cadmium batteries. . Already at -10 and +40°C they lose their performance.
3. Ni-MH batteries generate a lot of heat when charging , therefore they need fuses or temperature relays.
4. Increased self-charging , the presence of which is due to the reaction of the nickel oxide electrode with hydrogen from the electrolyte.

Degradation of Ni-MH batteries is determined by a decrease in the sorption capacity of the negative electrode during cycling. During the discharge-charge cycle, the volume of the crystal lattice changes, which contributes to the formation of rust and cracks during the reaction with the electrolyte. Corrosion occurs when the battery absorbs hydrogen and oxygen. This leads to a decrease in the amount of electrolyte and an increase in internal resistance.

It must be taken into account that the characteristics of batteries depend on the processing technology of the negative electrode alloy, its structure and composition. The metal for alloys also matters. All this forces manufacturers to very carefully choose alloy suppliers, and consumers - the manufacturer.