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.
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:
- electrode(metal hydride hydrogen);
- cathode(nickel oxide);
- 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 elements
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.
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 |
Nickel Cadmium NiCd |
Nickel- |
Lead acid |
|
| 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, |
2 times higher |
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:
- work individually with each battery in different modes,
- charge batteries in fast and slow mode,
- individual LCD display for each battery compartment,
- charge each battery independently,
- charge from one to four batteries of different capacities and sizes (AA or AAA),
- protect the battery from overheating,
- protect each battery from overcharging,
- determination of the end of charging by voltage drop,
- identify faulty batteries
- pre-discharge the battery to the residual voltage,
- restore old batteries (charge-discharge training),
- check battery capacity
- 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!
Research on nickel-metal hydride batteries began in the 1970s as an improvement on nickel-hydrogen batteries because the weight and volume of nickel-hydrogen batteries did not satisfy manufacturers (the hydrogen in these batteries was under high pressure, which required a strong and heavy steel case). The use of hydrogen in the form of metal hydrides made it possible to reduce the weight and volume of batteries, and the risk of battery explosion during overheating also decreased.
Since the 1980s, the production technology of NiMH batteries has been greatly improved and commercial use has begun in various fields. The success of NiNH batteries has been driven by increased capacity (up to 40% compared to NiCd), the use of recyclable materials ("environmentally friendly"), and a very long life, often exceeding that of NiCd batteries.
Advantages and disadvantages of NiMH batteries
Advantages
・ Higher capacity - 40% or more than conventional NiCd batteries
・ much less pronounced "memory" effect compared to nickel-cadmium batteries - battery maintenance cycles can be carried out 2-3 times less often
・ simple transportation option - airlines transport without any preconditions
・ environmentally friendly - recyclable
Flaws
・ limited battery life - usually about 500-700 full charge / discharge cycles (although depending on operating modes and internal device, there may be differences at times).
・ memory effect - NiMH batteries require periodic training (full discharge/charge cycle)
・ Relatively short battery life - usually no more than 3 years when stored in a discharged state, after which the main characteristics are lost. Storage in cool conditions with a partial charge of 40-60% slows down the aging process of batteries.
・ High battery self-discharge
・ Limited power capacity - If the allowable loads are exceeded, the battery life will be reduced.
・ A special charger with a staged charge algorithm is required, since a large amount of heat is generated during charging, and proho NiMH batteries can withstand overcharging.
・ Poor tolerance for high temperatures (over 25-30 Celsius)
The design of NiMH batteries and batteries
Modern nickel-metal hydride batteries have an internal design similar to that of nickel-cadmium batteries. Positive nickel oxide electrode, alkaline electrolyte and design hydrogen pressure are the same in both battery systems. Only negative electrodes are different: nickel-cadmium batteries have a cadmium electrode, nickel-metal hydride batteries have an electrode based on an alloy of hydrogen-absorbing metals.
Modern nickel-metal hydride batteries use a hydrogen-absorbing alloy composition of the AB2 and AB5 types. Other alloys of the AB or A2B type are not widely used. What do the mysterious letters A and B stand for in the composition of the alloy? - Under the symbol A, a metal (or a mixture of metals) is hidden, the formation of hydrides of which releases heat. Accordingly, the symbol B denotes a metal that reacts with hydrogen endothermally.
For negative electrodes of type AB5, a mixture of rare earth elements of the lanthanum group (component A) and nickel with impurities of other metals (cobalt, aluminum, manganese) - component B are used. For electrodes of type AB2, titanium and nickel with impurities of zirconium, vanadium, iron, manganese, chrome.
Nickel-metal hydride batteries with AB5 type electrodes are more common due to better cycle performance, despite the fact that batteries with AB2 type electrodes are cheaper, have a larger capacity and better power ratings.
In the process of cycling, the volume of the negative electrode fluctuates up to 15-25% of the initial one due to the absorption/release of hydrogen. As a result of volume fluctuations, a large number of microcracks appear in the electrode material. This phenomenon explains why a new nickel-metal hydride battery requires several "training" charge / discharge cycles to bring the power and capacity of the battery to nominal. Also, the formation of microcracks has a negative side - the surface area of the electrode increases, which undergoes corrosion with the consumption of electrolyte, which leads to a gradual increase in the internal resistance of the element and a decrease in capacitance. To reduce the rate of corrosion processes, it is recommended to store nickel-metal hydride batteries in a charged state.
The negative electrode has an excess capacitance relative to the positive one both in terms of overcharging and overdischarging to ensure an acceptable level of hydrogen evolution. Due to corrosion of the alloy, the capacity to recharge the negative electrode gradually decreases. As soon as the excess capacity for recharging is exhausted, a large amount of hydrogen will begin to be released at the negative electrode at the end of the charge, which will lead to the release of excess hydrogen through the cell valves, the electrolyte “boiling away” and the battery failure. Therefore, to charge nickel-metal hydride batteries, a special charger is required that takes into account the specific behavior of the battery in order to avoid the risk of self-destruction of the battery cell. When assembling the battery pack, keep the cells well ventilated and do not smoke near the high capacity NiMH battery being charged.
Over time, as a result of cycling, the self-discharge of the battery also increases due to the appearance of large pores in the separator material and the formation of an electrical connection between the electrode plates. This problem can be temporarily solved by deep discharging the battery several times and then fully recharging it.
Nickel-metal hydride batteries generate quite a lot of heat when charging, especially at the end of the charge, which is one of the signs that the charge needs to be completed. When assembling several battery cells into a battery, a battery parameter monitoring system (BMS) is required, as well as the presence of thermally open conductive connecting jumpers between a part of the battery cells. It is also desirable to connect the batteries in the battery by spot welding jumpers rather than soldering.
The discharge of nickel-metal hydride batteries at low temperatures is limited by the fact that this reaction is endothermic and water is formed at the negative electrode diluting the electrolyte, which leads to a high probability of freezing of the electrolyte. Therefore, the lower the ambient temperature, the lower the output power and battery capacity. On the contrary, at an elevated temperature during the discharge process, the discharge capacity of a nickel-metal hydride battery will be maximum.
Knowledge of the design and principles of operation will allow you to treat the operation of nickel-metal hydride batteries with greater understanding. I hope the information gleaned in this article will extend the life of your battery and avoid possible dangerous consequences due to a misunderstanding of the principles of the safe use of nickel-metal hydride batteries.
Discharge characteristics of NiMH batteries at various
discharge currents at an ambient temperature of 20 °C
image taken from www.compress.ru/Article.aspx?id=16846&iid=781
Duracell Nickel Metal Hydride Battery
image taken from www.3dnews.ru/digital/1battery/index8.htm
P.P.S.
Scheme of a promising direction for the creation of bipolar batteries
diagram taken from bipolar lead-acid batteries
Comparison table of parameters of various types of batteries
| NiCd | NiMH | lead acid | Li-ion | Li-ion polymer | Reusable Alkaline |
|
|---|---|---|---|---|---|---|
| Energy density (W*h/kg) | 45-80 | 60-120 | 30-50 | 110-160 | 100-130 | 80 (initial) |
| Internal resistance (including internal circuits), mΩ |
100-200 at 6V |
200-300 at 6V |
<100 at 12V |
150-250 at 7.2V |
200-300 at 7.2V |
200-2000 at 6V |
| Number of charge / discharge cycles (when reduced to 80% of the initial capacity) | 1500 | 300-500 | 200-300 | 500-1000 | 300-500 | 50 (up to 50%) |
| Fast charge time | 1 hour typical | 2-4 hours | 8-16 hours | 2-4 hours | 2-4 hours | 2-3 hours |
| Overcharge resistance | average | low | high | very low | low | average |
| Self-discharge / month (at room temperature) | 20% | 30% | 5% | 10% | ~10% | 0.3% |
| Cell voltage (nominal) | 1.25V | 1.25V | 2B | 3.6V | 3.6V | 1.5V |
| Load current - peak - optimal |
20C 1C |
5C 0.5C and below |
5C 0.2C |
>2C 1C and below |
>2C 1C and below |
0.5C 0.2C and below |
| Operating temperature (discharge only) | -40 to 60°C |
-20 to 60°C |
-20 to 60°C |
-20 to 60°C |
0 to 60°C |
0 to 65°C |
| Service requirements | After 30 - 60 days | After 60 - 90 days | After 3 - 6 months | Not required | Not required | Not required |
| Standard price (US$, for comparison only) |
$50 (7.2V) |
$60 (7.2V) |
$25 (6V) |
$100 (7.2V) |
$100 (7.2V) |
$5 (9V) |
| Price per cycle (US$) | $0.04 | $0.12 | $0.10 | $0.14 | $0.29 | $0.10-0.50 |
| Start of commercial use | 1950 | 1990 | 1970 | 1991 | 1999 | 1992 |
table taken from
Nimh batteries are power sources that are classified as alkaline batteries. They are similar to nickel-hydrogen batteries. But the level of their energy capacity is greater.
The internal composition of ni mh batteries is similar to the composition of nickel-cadmium power supplies. To prepare a positive output, such a chemical element, nickel, is used, and a negative one is an alloy that includes absorbing hydrogen metals.
There are several typical designs of nickel metal hydride batteries:
- Cylinder. To separate the conductive leads, a separator is used, which is given the shape of a cylinder. An emergency valve is concentrated on the cover, which opens slightly with a significant increase in pressure.
- Prism. In such a nickel metal hydride battery, the electrodes are concentrated alternately. A separator is used to separate them. To accommodate the main elements, a case prepared from plastic or a special alloy is used. To control the pressure, a valve or sensor is introduced into the lid.
Among the advantages of such a power source are:
- The specific energy parameters of the power source increase during operation.
- Cadmium is not used in the preparation of conductive elements. Therefore, there are no problems with battery disposal.
- No sort of "memory effect". Therefore, there is no need to increase the capacity.
- In order to cope with the discharge voltage (reduce it), specialists discharge the unit to 1 V 1-2 times a month.
Among the limitations that are relevant to nickel metal hydride batteries are:
- Compliance with the established interval of operating currents. Exceeding these indicators leads to a rapid discharge.
- Operation of this type of power supply in severe frosts is not allowed.
- Thermal fuses are introduced into the composition of the battery, with the help of which they determine the overheating of the unit, the increase in the temperature level to a critical indicator.
- Tendency to self-discharge.
Charging a nickel metal hydride battery
The charging process of nickel-metal hydride batteries is associated with certain chemical reactions. For their normal flow, part of the energy supplied by the charger is required from the network.
The efficiency of the charging process is the part of the energy received by the power supply that is stored. The value of this indicator may vary. But at the same time, it is impossible to get 100 percent efficiency.
Before charging metal hydride batteries, they study the main types, which depend on the magnitude of the current.
Drip charging
Use this type of charging for batteries with care, as it leads to a decrease in the period of operation. Since this type of charger is turned off manually, the process needs constant monitoring and regulation. In this case, the minimum current indicator is set (0.1 of the total capacity).
Since the maximum voltage is not set during such charging of ni mh batteries, they are guided only by the time indicator. To estimate the time interval, use the capacitance parameters that a discharged power source has.
The efficiency of a power source charged in this way is about 65-70 percent. Therefore, manufacturers do not advise the use of such chargers, as they affect the performance of the battery.
Quick recharge
When determining what current can charge ni mh batteries in fast mode, manufacturers' recommendations are taken into account. The current value is from 0.75 to 1 of the total capacity. It is not recommended to exceed the set interval, as the emergency valves turn on.
To charge nimh batteries in fast mode, the voltage is set from 0.8 to 8 volts.
The efficiency of fast charging ni mh power supplies reaches 90 percent. But this parameter decreases as soon as the charging time ends. If the charger is not turned off in a timely manner, then the pressure inside the battery will begin to increase, the temperature indicator will increase.
In order to charge ni mh batteries, perform the following actions:
- pre-charge
This mode is entered if the battery is completely discharged. At this stage, the current is between 0.1 and 0.3 of the capacitance. It is forbidden to use high currents. The time interval is about half an hour. As soon as the voltage parameter reaches 0.8 volts, the process stops.
- Switching to fast mode
The process of increasing the current is carried out within 3-5 minutes. During the entire time period, the temperature is controlled. If this parameter reaches a critical value, then the charger turns off.
When fast charging nickel-metal hydride batteries, the current is set at 1 of the total capacity. In this case, it is very important to quickly disconnect the charger, so as not to harm the battery.
To control the voltage, use a multimeter or voltmeter. This helps to eliminate false positives that adversely affect the performance of the device.
Some chargers for ni mh batteries do not work with direct, but with pulsed current. The current supply is carried out with a set frequency. The supply of pulsed current contributes to the uniform distribution of the electrolytic composition, active substances.
- Auxiliary and maintenance charging
To replenish the full charge of the ni mh battery at the last stage, the current indicator is reduced to 0.3 of the capacity. Duration - about 25-30 minutes. It is forbidden to increase this time interval, as this helps to minimize the period of operation of the battery.
Fast Charging
Some nickel-cadmium battery charger models are equipped with a boost charging mode. To do this, the charging current is limited by setting the parameters at the level of 9–10 from the capacity. You need to reduce the charge current as soon as the battery is charged to 70 percent.
If the battery is charged in accelerated mode for more than half an hour, then the structure of the conductive terminals is gradually destroyed. Experts recommend using such a charge if you have some experience.
How to properly charge power supplies, as well as eliminate the possibility of overcharging? To do this, follow these rules:
- Temperature control of ni mh batteries. Stop charging nimh batteries as soon as the temperature rises rapidly.
- nimh power supplies have time limits that allow you to control the process.
- It is necessary to discharge ni mh rechargeable batteries and charge them at a voltage of 0.98. If this parameter is significantly reduced, then the chargers are turned off.
Recovery of nickel metal hydride power supplies
The process of restoring ni mh batteries is to eliminate the consequences of the "memory effect", which are associated with loss of capacity. The likelihood of such an effect increases if the unit is often incompletely charged. The device fixes the lower limit, after which the capacitance decreases.
Before restoring the power source, the following items are prepared:
- Light bulb of the required power.
- Charger. Before use, it is important to clarify whether the charger can be used for discharging.
- Voltmeter or multimeter to determine the voltage.
A light bulb or a charger that is equipped with the appropriate mode is brought to the battery with their own hands in order to completely discharge it. After that, the charging mode is activated. The number of recovery cycles depends on how long the battery has not been used. The training process is recommended to be repeated 1-2 times a month. By the way, I restore in this way those sources that have lost 5-10 percent of the total capacity.
To calculate the lost capacity, a fairly simple method is used. So, the battery is fully charged, after which it is discharged and the capacity is measured.
This process is greatly simplified if you use a charger with which you can also control the voltage level. It is also advantageous to use such units because the probability of a deep discharge is reduced.
If the state of charge of nickel-metal hydride batteries is not established, then the light bulb must be approached carefully. Using a multimeter, the voltage level is controlled. This is the only way to prevent the possibility of a complete discharge.
Experienced specialists carry out both the restoration of one element and the whole block. During the charging period, the existing charge is equalized.
Restoring a power source that has been in operation for 2-3 years, when fully charged, discharged, does not always bring the expected result. This is because the electrolytic composition and conductive leads are gradually changing. Before using such devices, the electrolytic composition is restored.
Watch a video about restoring such a battery.
Nickel-Metal Hydride Battery Rules
The duration of operation of ni mh batteries largely depends on whether overheating or significant overcharging of the power source is allowed. Additionally, the masters are advised to consider the following rules:
- Regardless of how long the power sources will be stored, they must be charged. The charge percentage must be at least 50% of the total capacity. Only in this case there will be no problems during storage and maintenance.
- Batteries of this type are sensitive to overcharging, to excessive heat. These indicators adversely affect the duration of use, the magnitude of the current output. These power supplies require special chargers.
- Training cycles are optional for NiMH power supplies. With the help of a proven charger, the lost capacity is restored. The number of recovery cycles largely depends on the condition of the unit.
- Between recovery cycles, they must take breaks, and also learn how to charge the battery in operation. This time period is required for the unit to cool down, the temperature level drops to the required value.
- The recharging procedure or the training cycle is carried out only in an acceptable temperature regime: + 5- + 50 degrees. If this indicator is exceeded, then the probability of a rapid failure increases.
- When recharging, make sure that the voltage does not fall below 0.9 volts. After all, some chargers do not charge if this value is minimal. In such cases, it is allowed to connect an external source to restore power.
- Cyclic recovery is carried out provided that there is some experience. After all, not all chargers can be used to discharge the battery.
- The storage procedure includes a number of simple rules. Do not store the power supply outdoors or in rooms where the temperature drops to 0 degrees. This provokes the solidification of the electrolytic composition.
If not one, but several power sources are being charged at the same time, then the degree of charge is maintained at the set level. Therefore, inexperienced consumers carry out battery recovery separately.
Nimh batteries are efficient power sources that are actively used to complete various devices and assemblies. They are distinguished by certain advantages, features. Before using them, it is obligatory to take into account the basic rules of use.
Video about Nimh batteries
