Nickel-cadmium battery sph fr 130. Ni-Cd, Ni-MH and Li-Ion batteries

For a full fifty years, portable devices could rely solely on nickel-cadmium power supplies for battery life. But cadmium is a very toxic material, and in the 1990s nickel-cadmium technology was replaced by a more environmentally friendly nickel-metal hydride technology. In fact, these technologies are very similar, and most of the characteristics of nickel-cadmium batteries were inherited by nickel-metal hydride. But nevertheless, for some applications, nickel-cadmium batteries remain indispensable and are used to this day.

1. Nickel-cadmium batteries (NiCd)

Invented by Waldmar Jungner in 1899, the nickel-cadmium battery had several advantages over the only lead-acid battery available at the time, but was more expensive due to the cost of materials. The development of this technology was rather slow, but in 1932 a significant breakthrough was made - a porous material with an active substance inside was used as an electrode. A further improvement was made in 1947 and solved the problem of gas absorption, which made it possible to create a modern sealed maintenance-free nickel-cadmium battery.

For many years, NiCd batteries have served as power sources for two-way radios, emergency medical equipment, professional video cameras and power tools. In the late 1980s, ultra high-capacity NiCd batteries were developed, which shocked the world with their capacity, 60% higher than that of a standard battery. This was achieved by placing a larger amount of active substance in the battery, but there were also disadvantages - internal resistance increased and the number of charge / discharge cycles decreased.

The NiCd standard remains one of the most reliable and unassuming of batteries, and the aviation industry remains true to this system. However, the longevity of these batteries depends on proper maintenance. NiCd, and to some extent NiMH batteries, are subject to the “memory” effect, which leads to a loss of capacity if the battery is not cycled through periodically. If the recommended charging mode is violated, the battery seems to remember that in the previous cycles of operation its capacity was not fully used, and when discharged, it gives off electricity only to a certain level. ( See: How to repair a nickel battery). Table 1 lists the advantages and disadvantages of a standard nickel-cadmium battery.

Advantages

Reliable; high number of cycles with proper maintenance The only battery capable of ultra-fast charging with minimal stress Good load characteristics, forgive their exaggeration Long shelf life; possibility of storage in a discharged state No special requirements for storage and transportation Good performance at low temperatures Lowest cost per cycle of any battery Available in a wide range of sizes and designs

Flaws

Relatively low energy density compared to newer systems "Memory" effect; the need for periodic maintenance to avoid it Cadmium is a toxic material, special disposal is required High self-discharge; needs recharging after storage Low cell voltage of 1.2 volts, requires building multi-cell systems to provide high voltage

Table 1: Advantages and disadvantages of nickel-cadmium batteries.

2. Nickel-metal hydride batteries (NiMH)

Research into nickel-metal hydride technology began as early as 1967. However, the instability of the metal hydride hampered development, which in turn led to the development of the nickel-hydrogen (NiH) system. New hydride alloys, discovered in the 1980s, solved the safety concerns and made it possible to create a battery with a specific energy content 40% higher than that of standard nickel-cadmium.

Nickel-metal hydride batteries are not without drawbacks. For example, their charging process is more complicated than that of NiCd. With a self-discharge of 20% for the first day and then a monthly rate of 10%, NiMH is one of the leaders in its class. By modifying the hydride alloy, it is possible to achieve a reduction in self-discharge and corrosion, but this will add the disadvantage of reducing the specific energy consumption. But in the case of use in electric vehicles, these modifications are very useful, as they increase reliability and increase battery life.

3. Use in the consumer segment

NiMH batteries are currently among the most readily available. Industry giants such as Panasonic, Energizer, Duracell and Rayovac have recognized the need for an inexpensive and durable battery in the market, and offer nickel-metal hydride power supplies in different sizes, in particular AA and AAA. Manufacturers are working hard to win back some of the market from alkaline batteries.

In this market segment, nickel-metal hydride batteries are an alternative to rechargeable alkaline batteries, which appeared back in 1990, but due to the limited life cycle and weak load characteristics, did not gain success.

Table 2 compares the specific energy intensity, voltage, self-discharge and operating time of batteries and accumulators in the consumer segment. Available in AA, AAA and other sizes, these power supplies can be used in portable devices. Even if they may have slightly different nominal voltages, the state of discharge usually occurs at the same actual voltage value of 1 V for everyone. This voltage range is acceptable, since portable devices have some flexibility in terms of voltage range. The main thing is that it is necessary to use only the same type of electrical elements together. Safety concerns and voltage incompatibilities have hindered the development of AA and AAA Li-Ion batteries.

Table 2: Comparison of different AA batteries.

* Eneloop is a trademark of Sanyo Corporation based on the NiMH system.

The high self-discharge rate of NiMH is a continuing consumer concern. A flashlight or handheld device with a NiMH battery will run out of power if left unused for several weeks. The proposal to charge the device before each use is unlikely to find understanding, especially in the case of flashlights, which are positioned as backup lighting sources. The advantage of an alkaline battery with a shelf life of 10 years seems undeniable here.

The nickel-metal hydride battery from Panasonic and Sanyo under the brand name Eneloop has been able to significantly reduce self-discharge. Eneloop can be stored without recharging six times longer than conventional NiMH. But the disadvantage of such an improved battery is a slightly lower energy density.

Table 3 lists the advantages and disadvantages of the nickel-metal hydride electrochemical system. The table does not take into account the characteristics of Eneloop and other consumer brands.

Advantages	30-40 percent higher capacity than NiCd Less prone to "memory" effect, can be recovered Simple requirements for storage and transportation; lack of regulation of these processes Environmentally friendly; contain only moderately toxic materials Nickel content makes recycling self-sustaining Wide operating temperature range
Flaws	Limited service life; deep discharges contribute to its reduction Sophisticated charging algorithm; sensitive to overcharging Special requirements for recharge mode Generate heat during fast charging and discharging with powerful loads High self-discharge Coulomb efficiency at the level of 65% (for comparison, for lithium-ion - 99%)

Table 3: Advantages and disadvantages of NiMH batteries.

4. Iron-nickel batteries (NiFe)

After the invention of the nickel-cadmium battery in 1899, the Swedish engineer Waldmar Jungner continued his research and tried to replace expensive cadmium with cheaper iron. But the low charge efficiency and excessive hydrogen gassing forced him to abandon further development of the NiFe battery. He didn't even patent the technology.

An iron-nickel battery (NiFe) uses nickel oxide hydrate as the cathode, iron as the anode, and an aqueous solution of potassium hydroxide as the electrolyte. The cell of such a battery generates a voltage of 1.2 V. NiFe is resistant to overcharging and deep discharge; can be used as a backup power source for more than 20 years. Vibration and high temperature resistance have made this battery the most used in the mining industry in Europe; it has also found its use in providing power to railway signaling, and is also used as a traction battery for loaders. It can be noted that during the Second World War, it was iron-nickel batteries that were used in the German V-2 rocket.

NiFe has a low specific power of about 50 W/kg. Also, the disadvantages include poor performance at low temperatures and a high self-discharge rate (20-40 percent per month). It is this, coupled with the high cost of production, that encourages manufacturers to stay true to lead-acid batteries.

But the iron-nickel electrochemical system is actively developing and in the near future can become an alternative to lead-acid in some industries. The experimental model of the lamella design looks promising, it managed to reduce the self-discharge of the battery, it became practically immune to the harmful effects of over- and undercharging, and its service life is expected to be 50 years, which is comparable to the 12-year service life of a lead-acid battery in the mode work with deep cyclic discharges. The expected price of such a NiFe battery would be comparable to that of a lithium-ion battery, and only four times the price of a lead-acid battery.

NiFe batteries, as well as NiCd And NiMH, require special charging rules - the voltage curve has a sinusoidal shape. Accordingly, use the charger for lead acid or lithium ion the battery will not come out, it can even harm. Like all nickel-based batteries, NiFe is afraid of overcharging - it causes the decomposition of water in the electrolyte and leads to its loss.

The capacity of such a battery, reduced as a result of improper use, can be restored by applying high discharge currents (commensurate with the value of the battery capacity). This procedure must be carried out up to three times with a discharge period of 30 minutes. You should also monitor the temperature of the electrolyte - it should not exceed 46 ° C.

5. Nickel-zinc batteries (NiZn)

A nickel-zinc battery is similar to a nickel-cadmium battery in that it uses an alkaline electrolyte and a nickel electrode, but differs in voltage - NiZn provides 1.65 volts per cell, while NiCd and NiMH have 1.20 volts per cell. It is necessary to charge a NiZn battery with a constant current with a voltage value of 1.9 V per cell, it is also worth remembering that this type of battery is not designed to work in recharge mode. The specific energy consumption is 100W/kg, and the number of possible cycles is 200-300 times. NiZn does not contain toxic materials and can be easily recycled. Available in various sizes, including AA.

In 1901, Thomas Edison received a US patent for a rechargeable nickel-zinc battery. Later, his designs were perfected by the Irish chemist James Drumm, who installed these batteries on railcars that ran along the Dublin Brae route from 1932 to 1948. NiZn was not well developed due to its strong self-discharge and short life cycle caused by dendritic formation, which also often led to short circuits. But improving the composition of the electrolyte reduced this problem, which gave rise to NiZn being considered again for commercial use. Low cost, high power output and wide operating temperature range make this electrochemical system extremely attractive.

6. Nickel-hydrogen batteries (NiH)

When the development of nickel-metal hydride batteries began in 1967, researchers were faced with the instability of metal hydrites, which caused a shift towards the development of a nickel-hydrogen (NiH) battery. The cell of such a battery includes an electrolyte encapsulated in a vessel, nickel and hydrogen (hydrogen is enclosed in a steel cylinder under a pressure of 8207 bar) electrodes.

From operating experience

NiMH cells are widely advertised as high energy, cold and memory free. When I bought a Canon PowerShot A 610 digital camera, I naturally equipped it with a capacious memory for 500 high-quality shots, and to increase the duration of shooting, I bought 4 NiMH cells with a capacity of 2500 mA * hour from Duracell.

Let's compare the characteristics of the elements produced by the industry:

Options		Lithium ion Li-ion	Nickel Cadmium NiCd	Nickel- metal hydride NiMH	Lead acid Pb
service duration, charge/discharge cycles		1-1.5 years	500-1000		3 00-5000
Energy capacity, W*h/kg
Discharge current, mA * battery capacity
Voltage of one element, V
Self-discharge rate		2-5% per month	10% for the first day, 10% for each subsequent month	2 times higher NiCd	40% in year
Permissible temperature range, degrees Celsius	charging
	detente		-20... +65
Permissible voltage range, V		2,5-4,3 (coke), 3,0-4,3 (graphite)			5,25-6,85 (for batteries 6 V), 10,5-13,7 (for batteries 12V)

Table 1.

From the table we see NiMH elements have a high energy capacity, which makes them preferable when choosing.

To charge them, an intelligent DESAY Full-Power Harger charger was purchased, which provides charging of NiMH cells with their training. The elements of it were charged with high quality, but ... However, on the sixth charge, it ordered a long life. Burnt out electronics.

After replacing the charger and several charge-discharge cycles, the batteries began to run out in the second or third ten shots.

It turned out that despite the assurances, NiMH elements also have memory.

And most modern portable devices using them have built-in protection that turns off the power when a certain minimum voltage is reached. This prevents the battery from being fully discharged. Here the memory of elements begins to play its role. Cells that are not fully discharged are not fully charged and their capacity drops with each recharge.

High-quality chargers allow you to charge without losing capacity. But I could not find something like this for sale for elements with a capacity of 2500mah. It remains to periodically conduct their training.

Training NiMH elements

Everything written below does not apply to battery cells with a strong self-discharge . They can only be thrown away, experience shows that they cannot be trained.

Training of NiMH elements consists of several (1-3) discharge-charge cycles.

Discharging is performed until the voltage on the battery cell drops to 1V. It is advisable to discharge the elements individually. The reason is that the ability to receive a charge can be different. And it intensifies when charging without training. Therefore, there is a premature operation of the voltage protection of your device (player, camera, ...) and subsequent charging of an undischarged element. The result of this is a progressive loss of capacity.

Discharging must be carried out in a special device (Fig. 3), which allows it to be performed individually for each element. If there is no voltage control, then the discharge was carried out until a noticeable decrease in the brightness of the light bulb.

And if you detect the burning time of the light bulb, you can determine the battery capacity, it is calculated by the formula:

Capacity = Discharge current x Discharge time = I x t (A * hour)

A battery with a capacity of 2500 mAh is capable of delivering a current of 0.75 A to the load for 3.3 hours, if the time obtained as a result of discharging is less, and accordingly the residual capacity is less. And with a decrease in capacity, you need to continue training the battery.

Now, to discharge the battery cells, I use a device made according to the scheme shown in Fig. 3.

It is made from an old charger and looks like this:

Only now there are 4 bulbs, as in Fig. 3. Light bulbs should be mentioned separately. If the light bulb has a discharge current equal to the nominal for a given battery or slightly less, it can be used as a load and an indicator, otherwise the light bulb is only an indicator. Then the resistor must have such a value that the total resistance of El 1-4 and the resistor R 1-4 parallel to it is of the order of 1.6 ohms. Replacing a light bulb with an LED is unacceptable.

An example of a light bulb that can be used as a load is a 2.4 V krypton flashlight bulb.

A special case.

Attention! Manufacturers do not guarantee the normal operation of batteries at charging currents exceeding the accelerated charging current. I charge should be less than the battery capacity. So for batteries with a capacity of 2500 ma * h, it should be below 2.5A.

It happens that NiMH cells after discharging have a voltage of less than 1.1 V. In this case, it is necessary to apply the technique described in the above article in the PC MIR magazine. An element or a series of elements is connected to a power source through a 21 W car light bulb.

Once again, I draw your attention! Such elements must be checked for self-discharge! In most cases, it is elements with low voltage that have an increased self-discharge. These elements are easier to throw out.

Charging is preferably individual for each element.

For two cells with a voltage of 1.2V, the charging voltage should not exceed 5-6V. With forced charging, the light is also an indicator. By reducing the brightness of the light bulb, you can check the voltage on the NiMH element. It will be greater than 1.1 V. Typically, this initial boost charge takes 1 to 10 minutes.

If the NiMH element, during forced charging, does not increase the voltage for several minutes, heats up, this is a reason to remove it from charging and reject it.

I recommend using chargers only with the ability to train (regenerate) elements when recharging. If there are none, then after 5-6 operating cycles in the equipment, without waiting for a complete loss of capacity, train them and reject elements with a strong self-discharge.

And they won't let you down.

In one of the forums commented on this article "badly written but nothing else". So, this is not "stupid", but simple and accessible for everyone who needs help in the kitchen. That is, as simple as possible. Advanced can put a controller, connect a computer, ......, but this is already another story.

To not seem stupid

There are "smart" chargers for NiMH cells.

This charger works with each battery separately.

He can:

1. work individually with each battery in different modes,
2. charge batteries in fast and slow mode,
3. individual LCD display for each battery compartment,
4. charge each battery independently,
5. charge from one to four batteries of different capacities and sizes (AA or AAA),
6. protect the battery from overheating,
7. protect each battery from overcharging,
8. determination of the end of charging by voltage drop,
9. identify faulty batteries
10. pre-discharge the battery to the residual voltage,
11. restore old batteries (charge-discharge training),
12. check battery capacity
13. display on the LCD: - charge current, voltage, reflect the current capacity.

Most importantly, I emphasize that this type of device allows you to work individually with each battery.

According to user reviews, such a charger allows you to restore most of the running batteries, and serviceable ones can be used for the entire guaranteed service life.

Unfortunately, I did not use such a charger, since it is simply impossible to buy it in the provinces, but you can find a lot of reviews in the forums.

The main thing is not to charge at high currents, despite the declared mode with currents of 0.7 - 1A, this is still a small-sized device and can dissipate 2-5 watts of power.

Conclusion

Any recovery of NiMh batteries is strictly individual (with each individual element) work. With constant monitoring and rejection of elements that do not accept charging.

And the best way to deal with their recovery is with smart chargers that allow you to individually reject and charge-discharge cycle with each cell. And since there are no such devices automatically working with batteries of any capacity, they are designed for elements of a strictly defined capacity or must have controlled charging and discharging currents!

Invention history

Research in the field of manufacturing technology for NiMH batteries began in the 70s of the XX century and was undertaken as an attempt to overcome shortcomings. However, the metal hydride compounds used at that time were unstable and the required performance was not achieved. As a result, the NiMH battery development process stalled. New metal hydride compounds stable enough for battery applications were developed in the 1980s. Since the late 1980s, NiMH batteries have been constantly improved, mainly in terms of energy storage density. Their developers noted that NiMH technology has the potential to achieve even higher energy densities.

Options

• Theoretical energy intensity (Wh / kg): 300 Wh / kg.
• Specific energy consumption: about - 60-72 W h / kg.
• Specific energy density (Wh/dm³): approx. - 150 Wh/dm³.
• EMF: 1.25.
• Operating temperature: -60…+55 °C .(-40… +55)
• Service life: about 300-500 charge/discharge cycles.

Description

Nickel-metal hydride batteries of the Krona form factor, as a rule, with an initial voltage of 8.4 volts, gradually reduce the voltage to 7.2 volts, and then, when the energy of the battery is exhausted, the voltage decreases rapidly. This type of battery is designed to replace nickel-cadmium batteries. Nickel-metal hydride batteries have about 20% more capacity with the same dimensions, but a shorter service life - from 200 to 300 charge / discharge cycles. Self-discharge is about 1.5-2 times higher than that of nickel-cadmium batteries.

NiMH batteries are practically free from the "memory effect". This means that you can charge a battery that is not completely discharged if it has not been stored for more than a few days in this state. If the battery was partially discharged and then not used for a long time (more than 30 days), then it must be discharged before charging.

Environmentally friendly.

The most favorable mode of operation: charge with a small current, 0.1 of the rated capacity, charge time - 15-16 hours (typical manufacturer's recommendation).

Storage

Batteries should be stored fully charged in the refrigerator, but not below 0 degrees. During storage, it is advisable to check the voltage regularly (every 1-2 months). It should not fall below 1.37. If the voltage drops, you need to charge the batteries again. The only kind of batteries that can be stored discharged are Ni-Cd batteries.

NiMH batteries with low self-discharge (LSD NiMH)

The low self-discharge nickel-metal hydride battery, LSD NiMH, was first introduced in November 2005 by Sanyo under the brand name Eneloop. Later, many world manufacturers introduced their LSD NiMH batteries.

This type of battery has a reduced self-discharge, which means it has a longer shelf life than conventional NiMH. Batteries are marketed as "ready to use" or "pre-charged" and marketed as a replacement for alkaline batteries.

Compared to conventional NiMH batteries, LSD NiMHs are most useful when more than three weeks can elapse between charging and using the battery. Conventional NiMH batteries lose up to 10% of capacity during the first 24 hours after being charged, then the self-discharge current stabilizes at up to 0.5% of capacity per day. For LSD NiMH, this setting typically ranges from 0.04% to 0.1% capacity per day. Manufacturers claim that by improving the electrolyte and electrode, it was possible to achieve the following advantages of LSD NiMH compared to classical technology:

Of the shortcomings, a relatively slightly smaller capacity should be noted. At present (2012) the maximum achieved LSD capacity is 2700 mAh.

However, when testing Sanyo Eneloop XX batteries with a nameplate capacity of 2500mAh (min 2400mAh), it turned out that all of the batteries in a batch of 16 pieces (made in Japan, sold in South Korea) have an even larger capacity - from 2550 mAh to 2680 mAh . Tested by charging LaCrosse BC-9009.

An incomplete list of long-term storage batteries (with low self-discharge):

• Prolife by Fujicell
• Ready2Use Accu by Varta
• AccuEvolution by AccuPower
• Hybrid, Platinum, and OPP Pre-Charged by Rayovac
• Eneloop by Sanyo
• eniTime by Yuasa
• Infinium by Panasonic
• ReCyko by Gold Peak
• Instant by Vapex
• Hybrio by Uniross
• Cycle Energy by Sony
• MaxE and MaxE Plus by Ansmann
• EnergyOn by NexCell
• ActiveCharge/StayCharged/Pre-Charged/Accu by Duracell
• Pre-Charged by Kodak
• nx-ready by ENIX energies
• Imedion from
• Pleomax E-Lock by Samsung
• Centura by Tenergy
• Ecomax by CDR King
• R2G by Lenmar
• LSD ready to use by Turnigy

Other Benefits of Low Self Discharge NiMH (LSD NiMH) Batteries

Low self-discharge NiMH batteries typically have significantly lower internal resistance than conventional NiMH batteries. This has a very positive effect in applications with high current consumption:

• More stable voltage
• Reduced heat dissipation especially in fast charge/discharge modes
• Higher Efficiency
• High impulse current capability (Example: camera flash charging is faster)
• Possibility of continuous operation in devices with low power consumption (Example: remote controls, watches.)

Charge Methods

Charging is carried out by electric current at a voltage on the cell up to 1.4 - 1.6 V. The voltage on a fully charged cell without load is 1.4 V. The voltage at load varies from 1.4 to 0.9 V. The voltage without load at full discharged battery is 1.0 - 1.1 V (further discharging may damage the cell). To charge the battery, direct or pulsed current with short-term negative pulses is used (to restore the "memory" effect, the "FLEX Negative Pulse Charging" or "Reflex Charging" method).

End-of-charge control by voltage change

One of the methods for determining the end of the charge is the -ΔV method. The image shows a graph of the voltage on the cell when charging. The charger charges the battery with direct current. After the battery is fully charged, the voltage on it begins to drop. The effect is observed only at sufficiently high charging currents (0.5C..1C). The charger should detect this drop and turn off charging.

There is also the so-called "inflexion" - a method for determining the end of fast charging. The essence of the method is that it is not the maximum voltage on the battery that is analyzed, but the maximum derivative of the voltage with respect to time. That is, fast charging will stop at the moment when the voltage growth rate is maximum. This allows you to complete the fast charging phase earlier, when the temperature of the battery has not yet risen significantly. However, the method requires voltage measurement with greater accuracy and some mathematical calculations (calculation of the derivative and digital filtering of the obtained value).

Control of the end of the charge by temperature change

When charging a cell with direct current, most of the electrical energy is converted into chemical energy. When the battery is fully charged, the input electrical energy will be converted into heat. With a sufficiently large charging current, you can determine the end of the charge by a sharp increase in the temperature of the cell by installing a battery temperature sensor. The maximum allowable battery temperature is 60°C.

Areas of use

Replacement of a standard galvanic cell, electric vehicles, defibrillators, rocket and space technology, autonomous power supply systems, radio equipment, lighting equipment.

Selection of battery capacity

When using NiMH batteries, it is far from always necessary to chase after a large capacity. The more capacious the battery, the higher (ceteris paribus) its self-discharge current. For example, consider batteries with a capacity of 2500 mAh and 1900 mAh. Batteries fully charged and not used for, for example, a month, will lose part of their electrical capacity due to self-discharge. A larger battery will lose charge much faster than a smaller one. Thus, after a month, for example, the batteries will have approximately the same charge, and after even more time, the initially more capacious battery will contain a smaller charge.

From a practical point of view, high-capacity batteries (1500-3000 mAh for AA batteries) make sense to use in devices with high power consumption for a short time and without prior storage. For example:

• In radio-controlled models;
• In the camera - to increase the number of pictures taken in a relatively short period of time;
• In other devices in which the charge will be generated in a relatively short period of time.

Batteries of low capacity (300-1000 mAh for AA batteries) are more suitable for the following cases:

• When the use of the charge does not begin immediately after charging, but after a considerable time has passed;
• For occasional use in devices (hand lamps, GPS navigators, toys, walkie-talkies);
• For long-term use in a device with moderate power consumption.

Manufacturers

Nickel-metal hydride batteries are manufactured by various companies, including:

• camelion
• Lenmar
• Our strength
• NIAI SOURCE
• Space

see also

Literature

• Khrustalev D. A. Accumulators. M: Emerald, 2003.

Notes

Links

• GOST 15596-82 Chemical current sources. Terms and Definitions
• GOST R IEC 61436-2004 Sealed nickel-metal hydride batteries
• GOST R IEC 62133-2004 Accumulators and rechargeable batteries containing alkaline and other non-acid electrolytes. Safety requirements for portable sealed batteries and batteries made from them for portable use

Galvanic cell	Galvanic Daniel cell | Alkaline element | | Dry element | Concentration element | Air-zinc element | Normal Weston element
Electric batteries	Lead Acid | Silver-zinc | Nickel Cadmium | Nickel metal hydride | Nickel-zinc battery | Li-ion | Lithium Polymer | Lithium Iron Sulfide | Lithium Iron Phosphate | Lithium Titanate | Vanadium | Iron-nickel
fuel cells	Direct methanol | Solid oxide | Alkaline
Models

Nickel metal hydride batteries are a source of current based on a chemical reaction. Marked Ni-MH. Structurally, they are an analogue of the previously developed nickel-cadmium batteries (Ni-Cd), and in terms of the chemical reactions occurring, they are similar to nickel-hydrogen batteries. Belong to the category of alkaline food sources.

Historical digression

The need for rechargeable power supplies has been around for a long time. For various types of equipment, compact models with an increased charge storage capacity were very much needed. Thanks to the space program, a method has been developed to store hydrogen in batteries. These were the first nickel-hydrogen specimens.

Considering the design, the main elements stand out:

1. electrode(metal hydride hydrogen);
2. cathode(nickel oxide);
3. electrolyte(potassium hydroxide).

Previously used materials for the manufacture of electrodes were unstable. But constant experiments and studies led to the fact that the optimal composition was obtained. At the moment, lanthanum and nickel hydrite (La-Ni-CO) is used for the manufacture of electrodes. But various manufacturers also use other alloys, where nickel or part of it is replaced by aluminum, cobalt, manganese, which stabilize and activate the alloy.

Passing chemical reactions

When charging and discharging, chemical reactions occur inside the batteries associated with the absorption of hydrogen. The reactions can be written in the following form.

• During charging: Ni(OH)2+M→NiOOH+MH.
• During discharge: NiOOH+MH→Ni(OH)2+M.

The following reactions take place at the cathode with the release of free electrons:

• During charging: Ni(OH)2+OH→NiOOH+H2O+e.
• During discharge: NiOOH+ H2O+e →Ni(OH)2+OH.

On the anode:

• During charging: M+ H2O+e → MH+OH.
• During discharge: MH+OH →M+. H2O+e.

Battery design

The main production of nickel-metal hydride batteries is produced in two forms: prismatic and cylindrical.

Cylindrical Ni-MH cells

The design includes:

• cylindrical body;
• case cover;
• valve;
• valve cap;
• anode;
• anode collector;
• cathode;
• dielectric ring;
• separator;
• insulating material.

The anode and cathode are separated by a separator. This design is rolled up and placed in the battery case. Sealing is done with a lid and a gasket. The lid has a safety valve. It is designed so that when the pressure inside the battery rises to 4 MPa, when triggered, it releases excess volatile compounds formed during chemical reactions.

Many were encountered with wet or capped food sources. This is the result of the valve during recharging. Characteristics change and their further operation is impossible. In its absence, the batteries simply swell and completely lose their performance.

Prismatic Ni-MH elements

The design includes the following elements:

The prismatic design assumes alternate placement of anodes and cathodes with their separation by a separator. Assembled in this way into a block, they are placed in the case. The body is made of plastic or metal. The cover seals the structure. For safety and control over the state of the battery, a pressure sensor and a valve are placed on the cover.

Alkali is used as an electrolyte - a mixture of potassium hydroxide (KOH) and lithium hydroxide (LiOH).

For Ni-MH elements, polypropylene or non-woven polyamide acts as an insulator. The thickness of the material is 120–250 µm.

For the production of anodes, manufacturers use cermets. But recently, felt and foam polymers have been used to reduce the cost.

Various technologies are used in the production of cathodes:

Characteristics

Voltage. When idle, the internal circuit of the battery is open. And it's pretty hard to measure. Difficulties are caused by the equilibrium of potentials on the electrodes. But after a full charge after a day, the voltage on the element is 1.3–1.35V.

The discharge voltage at a current not exceeding 0.2A and an ambient temperature of 25°C is 1.2–1.25V. The minimum value is 1V.

Energy capacity, W∙h/kg:

• theoretical – 300;
• specific – 60–72.

Self-discharge depends on storage temperature. Storage at room temperature causes a capacity loss of up to 30% within the first month. Then the rate slows down to 7% in 30 days.

Other options:

• Electric driving force (EMF) - 1.25V.
• Energy density - 150 Wh/dm3.
• Operating temperature - from -60 to +55°С.
• Duration of operation - up to 500 cycles.

Correct charging and control

Chargers are used to store energy. The main task of inexpensive models is to supply a stabilized voltage. To recharge nickel-metal hydride batteries, a voltage of the order of 1.4-1.6V is required. In this case, the current strength should be 0.1 of the battery capacity.

For example, if the declared capacity is 1200 mAh, then the charging current should accordingly be selected close to or equal to 120 mA (0.12A).

Fast and accelerated charging are applied. The fast charging process is 1 hour. The accelerated process takes up to 5 hours. Such an intense process is controlled by changing the voltage and temperature.

The normal charging process lasts up to 16 hours. To reduce the duration of charging time, modern chargers are usually produced in three stages. The first stage is a fast charge with a current equal to the nominal capacity of the battery or higher. The second stage - a current of 0.1 capacitance. The third stage is with a current of 0.05–0.02 of the capacity.

The charging process must be monitored. Overcharging is detrimental to battery health. High gas formation will cause the safety valve to operate and the electrolyte will flow out.

Control is carried out according to the following methods:

Advantages and disadvantages inherent in Ni-MH cells

The latest generation batteries do not suffer from such a disease as the "memory effect". But after long-term storage (more than 10 days), it still needs to be completely discharged before starting charging. The likelihood of a memory effect comes from inaction.

Increased energy storage capacity

Environmental friendliness is provided by modern materials. The transition to them greatly facilitated the disposal of used elements.

As for the shortcomings, there are also a lot of them:

• high heat dissipation;
• the temperature range of operation is small (from -10 to + 40 ° C), although manufacturers claim other indicators;
• small interval of operating current;
• high self-discharge;
• non-observance of polarity disables the battery;
• store for a short time.

Selection by capacity and operation

Before you buy Ni-MH batteries, you should decide on their capacity. High performance is not a solution to the problem of lack of energy. The higher the capacity of the element, the more pronounced self-discharge.

Cylindrical nickel metal hydride cells are available in a large number of sizes, which are marked AA or AAA. Popularly nicknamed as finger - aaa and little finger - aa. You can buy them in all electrical stores and stores selling electronics.

As practice shows, batteries with a capacity of 1200-3000 mAh, having a size of aaa, are used in players, cameras and other electronic devices with high electricity consumption.

Batteries with a capacity of 300–1000 mAh, the usual size aa are used on devices with low power consumption or not immediately (walkie-talkie, flashlight, navigator).

The previously widely used metal hydride batteries were used in all portable devices. Single elements were installed in a box designed by the manufacturer for ease of installation. They usually had the EN marking. You can buy them only from official representatives of the manufacturer.

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 analogous 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 Batteries (NiMH)

Comparative characteristics of batteries

Options	Ni-Cd	Ni-H2	Ni-MH
Rated voltage, V	1.2	1.2	1.2
Specific energy: Wh/kg | Wh/l	20-40 60-120	40-55 60-80	50-80 100-270
Service life: years | cycles	1-5 500-1000	2-7 2000-3000	1-5 500-2000
Self-discharge, %	20-30 (for 28 days)	20-30 (for 1 day)	20-40 (for 28 days)
Working temperature, °С	-50 — +60	-20 — +30	-40 — +60

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

History of the 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 that were used in spacecraft. In the new element, hydrogen accumulated in alloys of certain metals. Alloys absorbing 1,000 times their own volume of hydrogen were discovered in the 1960s. These alloys are composed 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 can work at low hydrogen pressure and room temperature. Currently, work on the creation of new alloys and technologies for their processing continues throughout the world. Alloys of nickel with metals of the rare earth group can provide up to 2000 charge-discharge cycles of the battery with a decrease in the capacity of the negative electrode by no more than 30%. The first Ni-MH battery, using 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, nickel-metal hydride 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 batteries has been continuously improved in the direction of increasing their energy density. The replacement of the negative electrode made it possible to increase the load of active masses of the positive electrode by 1.3-2 times, which determines the capacity of the battery. Therefore, Ni-MH batteries have significantly higher specific energy characteristics compared to Ni-Cd batteries. The success of the distribution 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, like a nickel-cadmium battery, and a hydrogen-absorbing nickel-rare-earth alloy electrode instead of the negative cadmium electrode. On the positive nickel oxide electrode of the Ni-MH battery, the reaction proceeds:

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 reporting 70-80% of the capacity and during recharging, oxygen begins to be released on the oxide-nickel 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 capacitance of the metal hydride electrode is also provided due to the formation of the OH - group.

Construction of Ni-MH battery electrodes

Metal hydrogen electrode

The main material that determines the performance of a Ni-MH battery is a hydrogen-absorbing alloy that can absorb up to 1,000 times its own volume of hydrogen. The most widely used alloys are LaNi5, 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 manufacturers 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, there is an expansion and contraction of 15-25% of the crystal lattice of hydrogen-absorbing alloys 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 corroded 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 gives rise to electrolyte redistribution problems. 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 microencapsulation of 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 has found the widest application at present, consists in treating the surface of alloy particles in alkaline solutions with the formation of protective films permeable to hydrogen.

Nickel oxide electrode

Oxide-nickel electrodes in mass production are manufactured in the following design modifications: lamella, lamellaless sintered (metal-ceramic) and pressed, including pellets. In recent years, lamellaless felt and polymer foam electrodes have begun to be used.

Lamellar electrodes

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

Sintered (cermet) electrodes

electrodes of this type consist of a porous (with a porosity of at least 70%) cermet 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 carbamide (60-65% nickel, the rest is filler), is pressed, rolled or sprayed onto a steel or nickel mesh. Then the grid 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 the ammonium carbonate or carbamide decomposes and volatilizes, and the nickel is sintered. The substrates thus obtained have a thickness of 1-2.3 mm, a porosity of 80-85% and a pore radius of 5-20 µm. 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 induces 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 nickel nitrate solution. Due to the formation of hydrogen, the solution in the pores of the plate is alkalized, which leads to the deposition of oxides and hydroxides of nickel in the pores of the plate. Foil electrodes are classified as varieties of sintered electrodes. The electrodes are produced by applying on a thin (0.05 mm) perforated nickel tape on both sides, by spraying, an alcohol emulsion of nickel carbonyl powder containing binders, 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 under a pressure of 35-60 MPa of the active mass onto a mesh or a steel perforated 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 foundations is 95% or more. The felt electrode is made on the basis of nickel-plated polymer or 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 by different methods, depending on its density. Can be used instead of felt nickel foam obtained by nickel-plating polyurethane foam followed by annealing in a reducing environment. A paste containing nickel hydroxide and a binder are usually introduced into a highly porous medium by spreading. After that, the base with the paste is dried and rolled. Felt and foam polymer electrodes are characterized by high specific capacity and long service life.

Construction of Ni-MH batteries

Cylindrical Ni-MH batteries

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

Fig.1. The design of the nickel-metal hydride (Ni-MH) battery: 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.

Ni-MH Prismatic Batteries

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

Fig.2. Ni-MH battery structure: 1-body, 2-cap, 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. As a separator in Ni-MH batteries, non-woven polypropylene and polyamide 0.12-0.25 mm thick, treated with a wetting agent, are used.

positive electrode

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

Negative electrode

Five designs of a 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 the 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) on ​​an tensile nickel grid or copper grid; - sintered, when the alloy powder is pressed onto a nickel grid 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 Ur.c. 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 nickel oxidation, as well as the dependence of the equilibrium potential of the metal hydride electrode on the degree of hydrogen saturation. 24 hours after the battery is charged, the open circuit voltage of the charged Ni-MH battery is in the range of 1.30-1.35V.

Rated discharge voltage

Ur at a normalized discharge current Ir = 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; 4-3C

Battery capacity

With an increase in load (decrease in the discharge time) and with a decrease in temperature, the capacity of a Ni-MH battery decreases (Figure 4). The effect of temperature reduction on the capacitance is especially noticeable at high discharge rates and at temperatures below 0°C.

Fig.4. The dependence of the discharge capacity of Ni-MH battery on temperature at different discharge currents: 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 loss decreases to 3-7% per month. The self-discharge rate increases with increasing temperature (see Figure 5).

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

Charging a 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 an increase in the depth and speed of the discharge. The operating time depends on the speed of the charge and the method of controlling 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 depth of discharge of 80% and have a service life (on average) from 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 to the temperature regime. It is desirable to avoid overdischarges (below 1V) and short circuits. It is recommended to use Ni-MH batteries for their intended purpose, avoid mixing 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. Overcharging can lead to thermal runaway. Charging is usually carried out with a current of Iz \u003d 0.1C for 15 hours. Compensation charging is carried out with a current Iz = 0.01-0.03C for 30 hours or more. Accelerated (in 4 - 5 hours) and fast (in 1 hour) charges are possible for Ni-MH batteries with highly active electrodes. With such charges, the process is controlled by changes in temperature ΔТ and voltage ΔU and other parameters. Quick charge is used, for example, for Ni-MH batteries that power laptops, cell phones, and power tools, although laptops and cell phones now mostly use lithium-ion and lithium-polymer batteries. A three-stage charge method is also recommended: the first stage of a fast charge (1C and above), a charge at a rate of 0.1C for 0.5-1 h for the final recharge, and a charge at a rate of 0.05-0.02C as a compensation charge. 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. The charging voltage Uz at Iz=0.3-1C lies in the range of 1.4-1.5V. Due to the release of oxygen at the positive electrode, the amount of electricity delivered during charging (Qz) is greater than the discharge capacity (Cp). At the same time, the return on capacity (100 Ср/Qз) 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 batteries or chargers:

• charge termination method by 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;
• charge termination method by temperature change rate ΔT/Δt. With this method, the slope of the battery temperature curve is constantly monitored during the charging process, and when this parameter rises above a certain set value, the charge is interrupted;
• charge termination method by negative voltage delta -ΔU. At the end of the battery charge, during the oxygen cycle, its temperature begins to rise, leading to a decrease in voltage;
• charge termination method according to the maximum charge time t;
• method of termination of the charge by the maximum pressure Pmax. It is usually used in prismatic batteries of large sizes and capacities. The level of allowable pressure in a prismatic accumulator depends on its design and lies in the range of 0.05-0.8 MPa;
• method of termination of the charge by the maximum voltage Umax. It is used to disconnect the charge of batteries with high internal resistance, which appears at the end of the service life due to lack of electrolyte or at low temperature.

When using the Tmax method, the battery may be overcharged if the ambient temperature drops, or the battery may not be sufficiently charged if the ambient temperature rises significantly. The ΔT/Δt method can be used very effectively to terminate the charge at low ambient temperatures. But if only this method is used at higher temperatures, the batteries inside the batteries will be exposed to undesirably high temperatures before the ΔT/Δt value for shutdown can be reached. For a certain value of ΔT/Δt, a larger input capacitance can be obtained at a lower ambient temperature than at a higher temperature. At the beginning of a battery charge (as well as at the end of a charge), there is a rapid rise in temperature, which can lead to premature charge shutdown when using the ΔT/Δt method. To eliminate this, charger developers use timers for the initial sensor response delay with the ΔT / Δt method. The -ΔU method is effective for terminating the charge at low ambient temperatures rather than at elevated temperatures. In this sense, the method is similar to the ΔT/Δt method. In order to ensure that the charge is terminated in cases where unforeseen circumstances prevent the normal interruption of the charge, it is also recommended to use a timer control that regulates the duration of the charge operation (method t). Thus, to quickly charge batteries with rated currents of 0.5-1C at temperatures of 0-50 °C, it is advisable to simultaneously apply 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 a quick charge of the battery, chargers provide for switching them to recharge with a rated current of 0.1C - 0.2C for a certain time. Constant voltage charging is not recommended for Ni-MH batteries as "thermal failure" of the batteries can occur. This is because 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 charge rate should be reduced. Otherwise, oxygen will not have time to recombine, which will lead to an increase in pressure in the accumulator. 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. Moving away from cadmium also means moving towards cleaner production. The problem of recycling failed batteries is also easier to solve. These advantages of Ni-MH batteries determined the faster growth of their production volumes in all the world's leading battery companies compared to Ni-Cd batteries.

Ni-MH batteries don't have the "memory effect" that Ni-Cd batteries have due to the formation of nickelate in the negative cadmium electrode. However, the effects associated with the overcharging of the nickel oxide electrode remain. The decrease in the discharge voltage, observed with frequent and long recharges in the same way as 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 designed 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 operating temperature range: most of them are inoperable at temperatures below -10 °C and above +40 °C, although in some series of batteries, the adjustment of the recipes provided an expansion of temperature limits;
• during the charge of Ni-MH batteries, more heat is released than when charging Ni-Cd batteries, therefore, in order to prevent overheating of the battery 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 an increased self-discharge, which is determined by the inevitability of the reaction of hydrogen dissolved in the electrolyte with a positive oxide-nickel electrode (but, thanks to the use of special negative electrode alloys, it was possible to achieve a decrease in the self-discharge rate to values ​​close to those for Ni-Cd batteries );
• the risk 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 the battery parameters as a result of long cycling, so 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 discharging below 0 V is irreversible, which puts forward more stringent requirements for the selection of batteries in the battery and the control of the discharge process than in the case of using Ni-Cd batteries, as a rule, 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. In the charge-discharge cycle, the volume of the crystal lattice of the alloy changes, which leads to the formation of cracks and subsequent corrosion upon 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 improve the stability of its composition and structure. This forces battery manufacturers to be careful in choosing alloy suppliers, and battery consumers to be careful in choosing a manufacturer.

Based on the materials of the sites powerinfo.ru, "Chip and Dip"