Differences between Ni-Cd and Ni-Mh batteries. Nickel metal hydride batteries Nickel metal hydride batteries

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 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°C.
• Duration of operation - up to 500 cycles.

Correct charging and control

Used to store energy charging device. 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 generation will trigger safety valve and the electrolyte will leak 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.

Rechargeable batteries have become the main source of power for modern electronic devices. Ni-MH batteries are considered the most popular, as they are practical, durable and can have an increased capacity. But for safety specifications during the entire service life, you should learn some features of the operation of drives of this class, as well as the correct charging conditions.

Standard Ni-MH batteries

How to properly charge Ni-MH batteries

When you start charging any autonomous drive, be it a battery of a simple smartphone or a high-capacity battery of a truck, a series of chemical processes begin in it, due to which the accumulation of electrical energy occurs. The energy received by the drive does not disappear, part of it goes to charge, and a certain percentage goes to heat.

The parameter by which the efficiency of battery charging is determined is called the coefficient useful action offline storage. Efficiency allows you to determine how the ratio useful work and its unnecessary losses, leaving for heating. And in this parameter, nickel-metal hydride batteries and batteries are much inferior to Ni-Cd drives, since too much of the energy spent on charging them is also spent on heating.

Nickel-metal hydride drive can be repaired by yourself

In order to quickly and correctly charge a NiMH battery, the correct current must be set. This value is determined based on such a parameter as the capacity of an autonomous power source. You can increase the current, but this should be done at certain stages of charging.

Specifically for nickel-metal hydride batteries, 3 types of charging are defined:

• Drip. It flows to the detriment of battery longevity, does not stop even after reaching 100% charge. But with drip charging, a minimal amount of heat is released.
• Fast. Following the name, we can say that this type of charging proceeds a little faster, due to this input voltage within 0.8 Volts. At the same time, the efficiency level rises to 90%, which is considered a very good indicator.
• recharge mode. Required to charge the drive to its full capacity. This mode is carried out using a small current for 30-40 minutes.

This is where the charge features end, now we should consider each mode in more detail.

Features of drip charging

The main feature of drip charging of NiZn, as well as Ni-MH batteries, is the reduction of its heating during the entire process, which can last until the full capacity of the drive is restored.

Standard charger for Ni-MH batteries

What is remarkable about this type of charging:

• A small current, respectively - the absence of a clear framework for the potential difference. The charge voltage can reach its maximum without any negative impact on the lifetime of the drive.
• Efficiency within 70%. Of course, this indicator is lower than the others, and the time required for full recovery of capacity increases. But this reduces the heating of the battery.

The above indicators can be classified as positive. Now you should pay attention to the negative qualities of drip charging.

• The drip recovery process does not stop even after the restoration of full capacity. Constant exposure to even a small current, when the battery is fully charged, quickly renders it unusable.
• It is necessary to calculate the charge time based on factors such as current, voltage and. Not very convenient and may take too long for some users.

Modern nickel-metal hydride power supplies don't take drip charge as negatively as older models. But manufacturers of chargers are gradually abandoning the use of such a restoration of battery capacity.

Fast charge mode for Ni-MH batteries

The nominal charge rates for nickel-metal hydride batteries are:

• Current strength within 1 A.
• Voltage from 0.8 V.

Those data from which it is necessary to build on are given. For a fast charge mode, it is best to set the current to 0.75 A. This is quite enough to restore the drive in a short period of time without reducing its service life. If you raise the current more than 1 A, then the consequence may be emergency reset pressure at which the release valve opens.

Memory with accurate current readings

In order for the fast charging mode not to harm the battery, it is necessary to monitor the end of the process itself. The efficiency of fast recovery of capacity is about 90%, which is considered a very good indicator. But at the end of the charging process, the efficiency drops sharply, and the consequence of such a drop is not only the release of a large amount of heat, but also a sharp increase in pressure. Of course, such indicators negatively affect the durability of the drive.

The fast charge process consists of several steps, which should be considered in more detail.

Confirming the availability of charge indicators

Process sequence:

1. A preliminary current is supplied to the storage poles, which is no more than 0.1 A.
2. The charge voltage is within 1.8 V. At higher rates, fast charging of the battery will not start.

Nickel-Metal Hydride Medium Capacity Cell

The logic circuit in the chargers is programmed for no battery. This means that if the output voltage is more than 1.8 V, then the charger will perceive such an indicator as the absence of a power source. A high potential difference also occurs when the battery is damaged.

Power supply capacity diagnostics

Before starting the recovery of capacity, the memory must determine the level of charge of the power supply, so the fast recovery process cannot begin if it is completely discharged and the potential difference is less than 0.8 V.

To restore the partial capacity of the nickel-metal hydride drive, an additional mode is provided - pre-charge. This is a gentle mode that allows the battery to “wake up”. It is used not only after full recovery of capacity, but also during long-term storage of the battery.

It should be remembered that in order to preserve the operational life of nickel-metal hydride power supplies, they must not be completely discharged. Or, if there is no other way out, then do it as little as possible.

What is pre-charge? Process Features

To know how to properly charge a battery, you need to understand the pre-charge process.

The main feature of the pre-capacity recovery mode is that a certain period of time is allotted for it, no more than 30 minutes. The current strength is set in the range from 0.1 A to 0.3 A. With these parameters, there is no unwanted heating, and the battery can calmly “wake up”. If the potential difference exceeds more than 0.8 V, the pre-charge is automatically turned off and the next stage of capacity recovery begins.

Variety of Nickel Metal Hydride Products

If after 30 minutes the power supply voltage has not reached 0.8 V, this mode is terminated, as the charger detects the power supply as faulty.

Quick battery charge

This stage is the very fast charging of the power source. It proceeds with the obligatory observance of several basic parameters:

• Control over the current strength, which should be in the range of 0.5-1 A.
• Time control.
• Continuous comparison of potential differences. Disable the recovery process if this indicator drops by 30 mV.

It is very important to monitor the change in voltage parameters, since at the end of fast charging, the battery begins to heat up quickly. Therefore, the memory includes separate nodes responsible for controlling the voltage of the power source. For this, the voltage delta control method is specially used. But some memory manufacturers use modern developments that turn off the device if there is no change in the potential difference for a long time.

A more expensive option is to install a temperature controller. For example, when the temperature of the Ni-MH drive rises, the fast capacity recovery mode is automatically disabled. This requires expensive temperature sensors or electronic circuits, respectively, the price of the charger itself also increases.

Recharging

This stage is very similar to the pre-charge of the battery, in which the current is set within 0.1-0.3 A, and the whole process takes no more than 30 minutes. Recharging is necessary, since it is it that allows you to equalize the electronic charges in the power source, and increase its operational life. But with a longer recovery, on the contrary, there is an accelerated destruction of the battery.

Super Fast Charging Features

There is another important concept of restoring the capacity of Ni-MH batteries - ultra-fast charging. Which not only quickly restores the power source, but also extends its service life. This is due to one interesting feature of Ni-MH batteries.

Metal hydride power supplies can be charged with increased currents, but only after reaching 70% capacity. If you skip this moment, then an overestimated current strength parameter will only lead to the rapid destruction of the battery. Unfortunately, charger manufacturers consider it too costly to install such control nodes on their products, and use simpler fast charging.

Convenient finger-type power supplies

Ultra-fast charging should only be carried out on new batteries. High currents lead to rapid heating, the next stage of which is the opening of the pressure shut-off valve. Once the shut-off valve is opened, the nickel battery cannot be recovered.

Choosing a charger for Ni-MH batteries

Some charger manufacturers are leaning towards products made specifically for charging Ni-MH batteries. And this is understandable, since these power sources are the largest in many electronic devices.

It is necessary to consider in more detail the functionality of chargers designed specifically to restore the capacity of nickel-metal hydride batteries.

• Mandatory presence of several protective functions, which are formed by a certain combination of some radio elements.
• The presence of manual or automatic mode for adjusting the current strength. Only in this way it will be possible to set the various stages of charging. The potential difference is usually taken constant.
• Automatic recharge of the battery, even after reaching 100% capacity. This allows you to constantly maintain the main parameters of the power source, without compromising the service life.
• Recognition of current sources operating in a different way. Highly important parameter, since some types of batteries, with too much charge current, may explode.

The last function also belongs to the category of special ones and requires the installation of a special algorithm. Therefore, many manufacturers prefer to abandon it.

Ni-MH power supplies are widely popular due to their durability, ease of use, and affordable price. Many users have appreciated positive traits these products.

The main types of batteries:

• Ni-Cd Nickel Cadmium Batteries
• Ni-MH Nickel-metal hydride batteries
• Li-Ion Lithium-ion batteries

Ni-Cd Nickel Cadmium Batteries

For cordless tools, nickel-cadmium batteries are the de facto standard. Engineers are well aware of their advantages and disadvantages, in particular Ni-Cd Nickel-cadmium batteries contain cadmium - a heavy metal of increased toxicity.

Nickel-cadmium batteries have a so-called "memory effect", the essence of which boils down to the fact that when charging an incompletely discharged battery, its new discharge is possible only to the level from which it was charged. In other words, the battery "remembers" the level of residual charge from which it was fully charged.

So, when charging an incompletely discharged Ni-Cd battery, its capacity decreases.

There are several ways to deal with this phenomenon. We will describe only the simplest and most reliable way.

When using a cordless tool with Ni-Cd batteries, there is a simple rule to follow: only charge fully discharged batteries.

Pros of Ni-Cd Nickel Cadmium Batteries

• Low Price Ni-Cd Nickel Cadmium Batteries
• Ability to deliver the highest load current
• Ability to quickly charge the battery
• Maintain high battery capacity down to -20°C
• A large number of charge-discharge cycles. With proper operation, such batteries work perfectly and allow up to 1000 charge-discharge cycles or more.

Cons of Ni-Cd Nickel Cadmium Batteries

• Relatively high level of self-discharge - Ni-Cd Nickel-cadmium battery loses about 8-10% of its capacity in the first day after a full charge.
• During storage Ni-Cd Nickel Cadmium battery loses about 8-10% charge every month
• After long-term storage, the capacity of the Ni-Cd Nickel-Cadmium battery is restored after 5 charge-discharge cycles.
• To prolong the life of the Ni-Cd Ni-Cd battery, it is recommended to completely discharge it each time to prevent the “memory effect”

Ni-MH Nickel-metal hydride batteries

These batteries are offered on the market as less toxic (compared to Ni-Cd Nickel Cadmium batteries) and more environmentally friendly, both in production and disposal.

In practice, Ni-MH Nickel-Metal Hydride batteries do show a very large capacity with dimensions and weight somewhat smaller than standard Ni-Cd Nickel-Cadmium batteries.

Due to the almost complete rejection of the use of toxic heavy metals in the design of Ni-MH Nickel-metal hydride batteries, after use, the latter can be disposed of quite safely and without environmental consequences after use.

Nickel-metal hydride batteries have a slightly reduced "memory effect". In practice, the "memory effect" is almost invisible due to the high self-discharge of these batteries.

When using Ni-MH Nickel-Metal Hydride batteries, it is desirable to not fully discharge them during operation.

Store Ni-MH NiMH batteries in a charged state. For long (more than a month) interruptions in operation, the batteries should be recharged.

Pros of Ni-MH Nickel-Metal Hydride Batteries

• Non-toxic batteries
• Less "memory effect"
• Good performance at low temperature
• Large capacity compared to Ni-Cd Ni-Cad batteries

Cons of Ni-MH Nickel-Metal Hydride Batteries

• More expensive battery type
• The self-discharge rate is about 1.5 times higher than Ni-Cd Ni-Cad batteries
• After 200-300 charge-discharge cycles, the working capacity of Ni-MH Ni-MH batteries decreases slightly
• Ni-MH Nickel-Metal Hydride batteries have a limited lifespan

Li-Ion Lithium-ion batteries

The undoubted advantage of lithium-ion batteries is the almost imperceptible "memory effect".

Thanks to this remarkable property, the Li-Ion battery can be charged or recharged as needed, based on needs. For example, you can recharge a partially discharged lithium-ion battery before important, demanding or long work.

Unfortunately, these batteries are the most expensive batteries. In addition, lithium-ion batteries have a limited service life, independent of the number of charge-discharge cycles.

In summary, we can assume that lithium-ion batteries are best suited for cases of constant intensive use of cordless tools.

Pros of Li-Ion Lithium-Ion Batteries

• There is no "memory effect" and therefore it is possible to charge and recharge the battery as needed
• High Capacity Li-Ion Lithium Ion Batteries
• Light Weight Li-Ion Lithium-Ion Batteries
• Record low level of self-discharge - no more than 5% per month
• Ability to quickly charge Li-Ion Lithium-ion batteries

Cons of Li-Ion Lithium-Ion Batteries

• The high cost of Li-Ion Li-ion batteries
• Reduced operating time at temperatures below zero degrees Celsius
• Limited service life

Note

From the practice of operating Li-Ion Lithium-ion batteries in phones, cameras, etc. it can be noted that these batteries serve an average of 4 to 6 years and withstand about 250-300 discharge-charge cycles during this time. At the same time, it was absolutely definitely noticed: more discharge-charge cycles - shorter service life of Li-Ion Lithium-ion batteries!

All these types of batteries have such an important parameter as capacity. The capacity of the battery indicates how long it will be able to power the load connected to it. The radio's battery capacity is measured in milliamp-hours. This characteristic is usually indicated on the battery itself.

For example, let's take the Alpha 80 radio station and its 2800 mAh battery. With a work cycle of 5/5/90, where 5% of the radio station's operating time is for transmission, 5% of the work is for reception, 90% of the time is standby mode - the radio station's operating time will be at least 15 hours. The lower this parameter is for the battery, the less it will be able to work.

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Research into 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 service 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 - typically around 500-700 full charge/discharge cycles (although depending on operating modes and internal device there may be significant differences).
・ 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 higher 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 capacity in relation 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.

Discharge of nickel-metal hydride batteries at low temperatures is limited by the fact that this reaction is endothermic and water forms at the negative electrode, diluting the electrolyte, resulting in 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

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:

1. Temperature control of ni mh batteries. Stop charging nimh batteries as soon as the temperature rises rapidly.
2. nimh power supplies have time limits that allow you to control the process.
3. 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, battery 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 study 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 units. 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