NiMH batteries
Research in the field of nickel-metal hydride batteries began in the 1970s as improving 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 durable 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 explosion of the battery during overheating also decreased.
Since the 1980s, NiMH battery technology has been significantly improved and commercial use has begun in various fields. The success of NiNH batteries has been enhanced by increased capacity (by 40% compared to NiCd), the use of recyclable materials (“friendly” to the natural environment), and a very long service life, often exceeding NiCd batteries.
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More capacity - 40% and more than ordinary NiCd batteries
・ Much less pronounced effect of “memory” compared to nickel-cadmium batteries - battery maintenance cycles can be performed 2-3 times less
・ Easy transportation - airlines are transporting without any preconditions
・ Ecologically safe - recycling possible
・ Limited battery life time - usually about 500-700 full charge / discharge cycles (although there may be differences at times depending on the operating modes and internal device).
・ Memory effect - NiMH batteries require periodic training (full discharge / battery cycle)
・ Relatively short storage life of batteries - 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% slow down the aging process of batteries.
・ High self-discharge of batteries
・ Limited capacity - when the permissible loads are exceeded, the battery lifetime is reduced.
・ A special charger with a step-by-step charge algorithm is required, since a large amount of heat is released during charging and nickel-metal hydride batteries pass overcharging.
・ Poor tolerance to high temperatures (over 25-30 centigrade)
Modern nickel-metal hydride batteries have an internal structure similar to nickel-cadmium batteries. The positive nickel oxide electrode, the alkaline electrolyte, and the calculated hydrogen pressure are the same in both battery systems. Only negative electrodes are different: in nickel-cadmium batteries, a cadmium electrode, in nickel-metal hydride, an electrode based on an alloy of hydrogen-absorbing metals.
In modern nickel-metal hydride batteries, the composition of a hydrogen adsorbing alloy of the type AB2 and AB5 is used. Other alloys of the type AB or A2B are not widespread. What do the mysterious letters A and B in the composition of the alloy mean? - Under the symbol A there is a metal (or a mixture of metals), during the formation of which the hydrides generate heat. Accordingly, the symbol B denotes a metal that reacts endothermically with hydrogen.
For AB5 type negative electrodes, a mixture of rare-earth elements of the lanthanum group (component A) and nickel with impurities of other metals (cobalt, aluminum, manganese) - component B is used. For AB2 electrodes, titanium and nickel with impurities of zirconium, vanadium, iron, manganese, chromium.
Nickel-metal hydride batteries with AB5 type electrodes are more prevalent because of the best cycling characteristics, despite the fact that batteries with AB2 type electrodes are cheaper, have higher capacity and better power characteristics.
In the process of cycling, the volume of the negative electrode oscillates up to 15-25% of the initial one due to absorption / release of hydrogen. As a result of volume fluctuations, a large number of microcracks occur in the electrode material. This phenomenon explains why for the new nickel-metal hydride battery, it is necessary to perform several “training” charge / discharge cycles to bring the values of the power and capacity of the battery to the nominal. Also, the formation of microcracks has a negative side - the surface area of the electrode increases, which is subjected to 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 corrosive processes, it is recommended to store nickel-metal hydride batteries in a charged state.
The negative electrode has an excess capacity with respect to the positive one both in overcharge and in overdischarge to provide an acceptable level of hydrogen evolution. Due to corrosion of the alloy, the capacity for recharging the negative electrode gradually decreases. As soon as the excess capacity is exhausted by recharging, a large amount of hydrogen will start 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, boiling of the electrolyte and battery failure. Therefore, to charge nickel-metal hydride batteries, a special charging device is needed, taking into account the specific behavior of the battery to avoid the danger of self-destruction of the battery cell. When collecting batteries, batteries, it is necessary to provide for good ventilation of the cells and not to smoke near the rechargeable nickel-metal hydride battery of high capacity.
Over time, as a result of cycling, battery self-discharge increases due to the appearance of large pores in the separator material and the formation of an electrical connection between the plates of the electrodes. This problem can be temporarily solved by several cycles of deep discharge of the battery, followed by a full charge.
When charging NiMH batteries, a rather large amount of heat is released, especially at the end of the charge, which is one of the signs that the charge needs to be completed. When collecting several battery cells into a battery, a system for monitoring battery parameters (BMS) is required, as well as the presence of thermally disconnected conductive connecting bridges between a portion of the battery cells. It is also desirable to combine 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 on the negative electrode, which dilutes the electrolyte, which leads to a high probability of electrolyte freezing. 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 a more understanding of the process of operation of nickel-metal hydride batteries. I hope the information gathered in the article will allow you to extend the life of your battery and avoid possible dangerous consequences due to a misunderstanding of the principles of safe use of nickel-metal hydride batteries.
PS youROCK advised to insert several graphs and pictures, did not want to do this because of copyright considerations, but I will try to insert them with reference to the source
Depends on the characteristics of the 6V NiMH rechargeable battery on cycling
capacity and self-discharge are shown as a percentage of nominal
image taken from batteryuniversity.com/parttwo-36.htm
The discharge characteristics of NiMH-batteries with different
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
Pps
Diagram of the promising direction of creating bipolar batteries
scheme taken with bipolar lead acid batteries
Comparative table of parameters of various types of batteries
The table is taken from www.ixbt.com/mobile/review/batacademy.shtml
Since the 1980s, NiMH battery technology has been significantly improved and commercial use has begun in various fields. The success of NiNH batteries has been enhanced by increased capacity (by 40% compared to NiCd), the use of recyclable materials (“friendly” to the natural environment), and a very long service life, often exceeding NiCd batteries.
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Advantages and disadvantages of NiMH batteries
Benefits
More capacity - 40% and more than ordinary NiCd batteries
・ Much less pronounced effect of “memory” compared to nickel-cadmium batteries - battery maintenance cycles can be performed 2-3 times less
・ Easy transportation - airlines are transporting without any preconditions
・ Ecologically safe - recycling possible
disadvantages
・ Limited battery life time - usually about 500-700 full charge / discharge cycles (although there may be differences at times depending on the operating modes and internal device).
・ Memory effect - NiMH batteries require periodic training (full discharge / battery cycle)
・ Relatively short storage life of batteries - 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% slow down the aging process of batteries.
・ High self-discharge of batteries
・ Limited capacity - when the permissible loads are exceeded, the battery lifetime is reduced.
・ A special charger with a step-by-step charge algorithm is required, since a large amount of heat is released during charging and nickel-metal hydride batteries pass overcharging.
・ Poor tolerance to high temperatures (over 25-30 centigrade)
NiMH battery and battery design
Modern nickel-metal hydride batteries have an internal structure similar to nickel-cadmium batteries. The positive nickel oxide electrode, the alkaline electrolyte, and the calculated hydrogen pressure are the same in both battery systems. Only negative electrodes are different: in nickel-cadmium batteries, a cadmium electrode, in nickel-metal hydride, an electrode based on an alloy of hydrogen-absorbing metals.
In modern nickel-metal hydride batteries, the composition of a hydrogen adsorbing alloy of the type AB2 and AB5 is used. Other alloys of the type AB or A2B are not widespread. What do the mysterious letters A and B in the composition of the alloy mean? - Under the symbol A there is a metal (or a mixture of metals), during the formation of which the hydrides generate heat. Accordingly, the symbol B denotes a metal that reacts endothermically with hydrogen.
For AB5 type negative electrodes, a mixture of rare-earth elements of the lanthanum group (component A) and nickel with impurities of other metals (cobalt, aluminum, manganese) - component B is used. For AB2 electrodes, titanium and nickel with impurities of zirconium, vanadium, iron, manganese, chromium.
Nickel-metal hydride batteries with AB5 type electrodes are more prevalent because of the best cycling characteristics, despite the fact that batteries with AB2 type electrodes are cheaper, have higher capacity and better power characteristics.
In the process of cycling, the volume of the negative electrode oscillates up to 15-25% of the initial one due to absorption / release of hydrogen. As a result of volume fluctuations, a large number of microcracks occur in the electrode material. This phenomenon explains why for the new nickel-metal hydride battery, it is necessary to perform several “training” charge / discharge cycles to bring the values of the power and capacity of the battery to the nominal. Also, the formation of microcracks has a negative side - the surface area of the electrode increases, which is subjected to 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 corrosive processes, it is recommended to store nickel-metal hydride batteries in a charged state.
The negative electrode has an excess capacity with respect to the positive one both in overcharge and in overdischarge to provide an acceptable level of hydrogen evolution. Due to corrosion of the alloy, the capacity for recharging the negative electrode gradually decreases. As soon as the excess capacity is exhausted by recharging, a large amount of hydrogen will start 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, boiling of the electrolyte and battery failure. Therefore, to charge nickel-metal hydride batteries, a special charging device is needed, taking into account the specific behavior of the battery to avoid the danger of self-destruction of the battery cell. When collecting batteries, batteries, it is necessary to provide for good ventilation of the cells and not to smoke near the rechargeable nickel-metal hydride battery of high capacity.
Over time, as a result of cycling, battery self-discharge increases due to the appearance of large pores in the separator material and the formation of an electrical connection between the plates of the electrodes. This problem can be temporarily solved by several cycles of deep discharge of the battery, followed by a full charge.
When charging NiMH batteries, a rather large amount of heat is released, especially at the end of the charge, which is one of the signs that the charge needs to be completed. When collecting several battery cells into a battery, a system for monitoring battery parameters (BMS) is required, as well as the presence of thermally disconnected conductive connecting bridges between a portion of the battery cells. It is also desirable to combine 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 on the negative electrode, which dilutes the electrolyte, which leads to a high probability of electrolyte freezing. 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 a more understanding of the process of operation of nickel-metal hydride batteries. I hope the information gathered in the article will allow you to extend the life of your battery and avoid possible dangerous consequences due to a misunderstanding of the principles of safe use of nickel-metal hydride batteries.
PS youROCK advised to insert several graphs and pictures, did not want to do this because of copyright considerations, but I will try to insert them with reference to the source
Depends on the characteristics of the 6V NiMH rechargeable battery on cycling
capacity and self-discharge are shown as a percentage of nominal
image taken from batteryuniversity.com/parttwo-36.htm
The discharge characteristics of NiMH-batteries with different
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
Pps
Diagram of the promising direction of creating bipolar batteries
scheme taken with bipolar lead acid batteries
Comparative 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), MoM | 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 |
| The number of charge / discharge cycles (with a decrease 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 |
| Resistance to recharge | average | low | high | very low | low | average |
| Self-discharge / month (at room temperature) | 20% | thirty% | five% | ten% | ~ 10% | 0.3% |
| Element voltage (nominal) | 1.25V | 1.25V | 2B | 3.6B | 3.6B | 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 | In 3-6 months | Not required | Not required | Not required |
| Typical price (US $, for comparison only) | $ 50 (7.2B) | $ 60 (7.2B) | $ 25 (6B) | $ 100 (7.2B) | $ 100 (7.2B) | $ 5 (9B) |
| 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 |
The table is taken from www.ixbt.com/mobile/review/batacademy.shtml
Source: https://habr.com/ru/post/53879/
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