Everything you need to know about NiMH batteries

Everything you need to know about NiMH batteries

Nickel-metal hydride batteries are batteries composed of hydrogen ions and metallic nickel. The “metal” part of a nickel-metal hydride battery is actually a metal hydride, so the nickel-metal hydride battery is also called a nickel-metal hydride battery. The nickel-metal hydride battery has a voltage of C (1.2V), but because the rare earth alloy or alloy hydrogen storage material is used as the negative electrode active material, it replaces the carcinogen cadmium, which not only makes this new type of battery a green battery, but also The specific energy of the battery was increased by nearly 40%, and the battery was gradually industrialized in the early 1990s. Ni-MH batteries have 30% more electric capacity than Ni-Cd batteries, are lighter than Ni-Cd batteries, have a longer service life, and are non-polluting to the environment. The disadvantage of nickel-metal hydride batteries is that the price is much more expensive than that of nickel-cadmium batteries, and the performance is worse than that of lithium batteries. (If you’re interested in learning about quality lithium batteries, visit Tycorun Lithium Battery to buy.)

  1. Classification and structure of NiMH batteries

Nickel-metal hydride batteries can be divided into two categories according to the main metal elements: AB1 type, A is a mixture of rare earth elements (or) plus titanium (Ti), B is nickel (Ni), cobalt (Co), manganese ( Mn), (or) and aluminum (Al); while the “multi-component” electrodes of some high-capacity batteries are mainly composed of AB2, where A is titanium (Ti) or vanadium (V), and B is It’s zirconium (Zr) or nickel (Ni), plus some chromium (Cr), cobalt (Co), iron (Fe) and/or manganese (Mn). All these compounds (such as the most commonly used alloy LaNis) have the ability to reversibly absorb and desorb hydrogen and can reversibly form metal hydrides. When the battery is charged, the hydrogen ions in the potassium hydroxide (KOH) electrolyte are released, and these compounds absorb it, avoiding the formation of hydrogen gas to maintain the pressure and volume inside the battery. When the battery is discharged, these hydrogen ions return to their original positions through the reverse process.

  1. Structural composition of NiMH battery
NiMH batteries
NiMH batteries

The material of the positive plate of the nickel-hydrogen battery is Ni(OH)2, and the material of the negative plate is a hydrogen absorbing alloy. The electrolyte is usually 30% KOH aqueous solution, and a small amount of Ni(OH)2 is added. The diaphragm is made of porous vinylon non-woven fabric or nylon non-woven fabric. There are two types of NiMH batteries: cylindrical and square.

  1. Electrochemical principle of NiMH battery

(1) Charging
Positive reaction: Ni(OH)2+OH-→NiOOH+H2O+e-
Negative reaction: MHn→M+ n/2H2
Overall reaction: M+Ni(OH)2→MH+NiOOH
(2) Discharge
Positive electrode: NiOOH+H2O+e-→Ni(OH)2+OH-
Negative pole: M+n/2H2→MHn
Overall reaction: MH+NiOOH→M+Ni(OH)2
In the above formula, M is a hydrogen storage alloy; MH is a hydrogen storage alloy with hydrogen atoms adsorbed. Nickel-metal hydride batteries mainly use KOH as the electrolyte.

The biggest feature of the Ni-MH battery reaction: whether it is the positive electrode or the negative electrode, it is the reaction of hydrogen atoms entering the solid, which belongs to the solid phase transition mechanism. The intermediate product of any water-soluble metal ion, so the positive and negative electrodes of the battery have high structural stability. At the same time, there is no additional consumption of electrolyte components (KOH and HzO) during the battery operation, and the concentration of the electrolyte remains unchanged, so that the airtightness and maintenance-free of the Ni-MH battery can be realized. When the battery is overcharged, the oxygen precipitated on the positive electrode can be reduced to water on the surface of the hydride electrode through the diaphragm; and when overdischarged, the hydrogen gas precipitated on the positive electrode can be absorbed by the hydride electrode, so that the battery is overcharged. There are no problems such as gas accumulation, high pressure and release during discharge.

  1. Technical issues that need to be paid attention to when using NiMH batteries

(1) Charging If the nickel-hydrogen battery is charged in the trickle mode (charging with a small current for a long time) for a long time, the battery may be damaged. Therefore, in order to maintain the nickel-hydrogen battery for a long time, it is necessary to use the low-frequency pulse-high current charging method. Better battery life than using trickle charging.

Newly bought or unused NiMH batteries take a period of time to “activate” to restore the battery’s power. As a result, some new NiMH batteries require several charge-discharge cycles to reach their nominal capacity.

    When charging the battery, pay attention to the charging time. The theoretical charging time of a battery is equal to the capacity of the battery divided by the output current of the charger. Take a battery with a power of 800mA·h as an example, the output current of the charger is 500mA, then the charging time is equal to 800mA·h/500mA=1.6h. When the charger shows that the charging is complete, it is best to give the battery about half Recharge time of about an hour.

(2) Discharge Ni-MH batteries are periodically discharged and then fully charged, which is beneficial to maintain the capacity and quality of the battery. However, when several batteries are connected in series, it is necessary to avoid the battery being completely drained and the phenomenon of “reverse charging” occurs, because this will damage the battery. Usually the power generation system can detect the discharge voltage of the series connected battery, and when it drops to a certain level, it will automatically shut down to protect the battery. A single battery does not have the above dangers, it will only discharge until the voltage is zero, which will not cause damage to the battery.

NiMH batteries have a high self-discharge rate of about 30% per month or more, which is higher than the 20% per month self-discharge rate of NiCd batteries. The fuller the battery is charged, the higher the self-discharge rate; when the charge drops to a certain level, the self-discharge rate will decrease slightly. The temperature at which the battery is stored has a significant effect on the self-discharge rate. Because of this, NiMH batteries that are not used for a long time are best charged to 40%.

Generally, at room temperature, the NiMH battery can still store 70% to 85% of the power after one year of storage, and can be charged with a general NiMH battery charger. Some nickel-metal hydride batteries with low self-discharge effect have better discharge characteristics at low temperatures than alkaline and lithium-ion batteries.

(3) Capacity Batteries of different types (especially different volumes), the higher the capacity, the longer the service time. Putting aside the factors of volume and weight, of course, the higher the capacity, the better. But the same battery model has the same nominal capacity (such as 600mA·h), and the actual measured initial capacity is different: for example, one is 660mA·h and the other is 605mA·h, then the 660mA·h is higher than the 605mA·h ok? The actual situation may be that the high capacity is because there are more materials in the electrode material that increase the initial capacity, while the materials that stabilize the electrode are reduced. Low battery is still “strong”. Civilian AA nickel-metal hydride batteries (that is, AA batteries) generally have a capacity of 1400mA·h, but they also have an ultra-high capacity of 1600mA·h, which is the reason. The price of increasing the capacity is sacrificing the cycle life of the battery. Only by making efforts to modify the battery material can manufacturers truly “improve” the battery capacity.

(4) The rated voltage of each unit cell in the weight and volume nickel-metal hydride battery is 1.2V (same as the nickel-cadmium battery), but the rated voltage of each unit cell in the lithium battery is 3.6V, and the voltage of the lithium battery is the other two. 3 times that of the person. Lithium-ion batteries of the same type are almost the same in weight as nickel-cadmium batteries. Compared with these two kinds of batteries, nickel-metal hydride batteries are heavier in weight. It can be seen that in the case of outputting the same voltage, the number of lithium batteries can be significantly reduced when outputting a single battery combination, so the weight and volume of the formed lithium battery are much smaller than that of the nickel-metal hydride battery.

  1. Care and use

①Under normal circumstances, new NiMH batteries only contain a small amount of electricity, so they should be charged first after purchase, and then used. Because the newly bought NiMH battery generally needs to be charged and used 3 to 4 times before its performance can be brought into full play. However, if the battery has a short time from the factory and the power is sufficient, it is recommended to use it first and then charge it. Note that in order to prolong the life of the NiMH battery, recharge it after each use, and it is a one-time full charge, do not use it for a while, and then charge it again.

②When charging the battery, pay attention to the heat dissipation around the charger, so do not place too many sundries around the charger, let alone place the charger in a damp place. In addition, keep the battery clean, because keeping the contact points at both ends of the battery and the inside of the battery cover clean can avoid the loss of power.

③ If it is not used for a long time, in order to avoid short circuit of the battery, be sure to take the battery out of the battery compartment in time. Nickel-metal hydride batteries that have not been used for a long time will naturally enter a “sleep” state after being stored for several months, and the battery life will be greatly reduced. Due to the large self-discharge of NiMH batteries (10% to 15% per month), if the battery is not used for a long time, the self-discharge of the battery will cause the battery to overdischarge and damage the battery. The best condition for storage of Ni-MH battery is about 80% charged (self-check with a multimeter). Generally, the voltage of NiMH battery is below 1.2V before charging, and the normal voltage is about 1.4V after full charge.

④ When charging with a charger, it is better to use the slow charging method. Although the current during slow charging is small (usually around 160mA) and the charging time is long (it takes about 16h to charge a 1800mA·h NiMH battery), the charging will be sufficient and the battery will not be damaged. The fast charging current is usually above 400mA, and the charging time is significantly reduced (only 3 to 4 hours), but overcharging is prone to occur.

Slow charging does not hurt the battery, but the charging time is too long; fast charging can save time, but it is harmful to the battery. If you want to save time without harming the battery, it is best to buy a charger that can charge fast, and buy a charger that can charge slowly at the same time. When charging, first fast charge for a period of time (such as after 5 to 10 times), and then switch to slow charging once or twice, so that the performance of the battery can be restored to the best state. Good chargers come with protection against overcharging.

⑤ When many batteries are connected in series to form a battery pack, cells with similar parameters such as capacity and voltage should be connected in series to restore the performance of the batteries to a group, otherwise the entire battery pack will be affected due to the low voltage of one of the batteries. the output voltage. Therefore, in order to ensure the same battery capacity, it is best to choose batteries of the same brand and model, and the batteries purchased at the same time.

Nickel-metal hydride batteries have good low-temperature discharge characteristics. Even at an ambient temperature of 120°C, when discharged with a large current (at a 1C discharge rate), the discharged electricity can reach more than 85% of the nominal capacity. However, when the Ni-MH battery is at high temperature (above 40°C), the storage capacity will decrease by 5% to 10%. This capacity loss due to self-discharge (the higher the temperature, the greater the self-discharge rate) is reversible, and the maximum capacity can be recovered after a few charge-discharge cycles. The open circuit voltage of the Ni-MH battery is 1.2V, which is the same as that of the Ni-Cd battery.

⑥Ni-MH battery is more sensitive to high temperature. When charging and discharging NiMH batteries, generally do not allow the battery temperature to be higher than 45°C. When the battery is fully charged, the battery will heat up, and the temperature should be around 42°C when the battery is fully charged with a high current, and should not exceed 45°C, otherwise the lifespan will decrease quickly and the internal resistance of the battery will increase. If you find that the battery temperature is high after charging, you should wait for it to cool before charging it.

⑦When using it at ordinary times, pay attention to keep the packaging skin intact and not damaged, so as to avoid short circuit.

Read more: What are the polarities and basic requirements for solar modules?

Related Posts