What is the safety of Ni-MH batteries?

Ni-MH batteries have six main characteristics: performance charging characteristics and discharge characteristics, self-discharging characteristics and long-term storage characteristics, and comprehensive cycle life characteristics and safety characteristics. They are all determined by the structure of the battery, which is manifested in the environment in which it is located. The remarkable feature is that it is greatly affected by temperature and current.

1. Charging characteristics: When the charging current of the nickel-hydrogen battery increases and/or the charging temperature decreases, the charging voltage of the battery rises. Generally, a constant current charging of not more than 1 C is used at an ambient temperature between 0 ° C and 40 ° C, and charging is possible between 10 ° C and 30 ° C to obtain a higher charging efficiency.

If the battery is often charged in a high temperature or low temperature environment, the performance of the power battery will be degraded. For fast charging above 0.3C, charging control measures are indispensable. Repeated overcharging will also degrade the performance of the battery, so the protection measures for high and low temperature and high current charging must be in place.

2. Discharge characteristics: The discharge platform of nickel-hydrogen battery is 1.2V. The higher the current, the lower the temperature, the lower the discharge voltage and discharge efficiency of the battery, and the maximum continuous discharge current of the battery is 3C.

The discharge cut-off voltage of the battery is generally set at 0.9V, and the IEC standard charge and discharge mode is set to 1.0V, because a stable current can be generally supplied below 1.0V, and a slightly smaller current can be supplied below 0.9V. The discharge cut-off voltage of Ni-MH batteries can be regarded as a voltage range of 0.9V~1.0V. And some batteries can be subscripted to 0.8V. Under normal circumstances, if the cut-off voltage is set too high, the battery capacity cannot be fully utilized, and conversely, the battery is easily over-discharged.

3. Self-discharge characteristics: It refers to the phenomenon of capacity loss when the battery is fully charged and open. The self-discharge characteristics are mainly affected by the ambient temperature. The higher the temperature is, the greater the battery storage capacity loss.

4, long-term storage characteristics: mainly refers to the power recovery capacity of nickel-metal hydride batteries. After a long period of time (such as one year), the battery capacity may be smaller than the capacity before storage, but after several charge and discharge cycles, the battery can be restored to the capacity before storage.

5. Cycle life characteristics: The cycle life of Ni-MH batteries is affected by the charge and discharge system, temperature and usage. When charging and discharging according to the IEC standard, a full charge and discharge is the charging cycle of the nickel-hydrogen battery. The multiple charge cycles constitute the cycle life, and the charge-discharge cycle of the nickel-hydrogen battery can exceed 500 times.

6. Safety: The safety performance of nickel-metal hydride batteries is better in battery design, which is of course related to the material materials used, and also inseparable from its structure. During use, if the battery is improperly used, causing overcharge, over discharge, and short circuit, and causing the internal pressure of the battery to rise, then a recoverable safety valve will be opened to reduce internal pressure and prevent The role of battery explosion.

Nickel-metal hydride batteries are a good performance battery. Nickel-metal hydride batteries are classified into high-voltage nickel-hydrogen batteries and low-voltage nickel-hydrogen batteries. The positive active material of the nickel-hydrogen battery is Ni(OH)2 (called NiO electrode), the negative active material is metal hydride, also called hydrogen storage alloy (electrode called hydrogen storage electrode), and the electrolyte is 6 mol/L potassium hydroxide solution. Nickel-hydrogen batteries have become more and more important as an important direction for hydrogen energy applications.

Since fossil fuels have become less and less in the case of large-scale development and utilization of human beings, the development and utilization of hydrogen energy has received increasing attention in recent years. Nickel-hydrogen batteries have become more and more important as an important direction for hydrogen energy applications. Although nickel-metal hydride batteries are indeed a good performance battery,special nickel-metal hydride batteries are high-voltage nickel-hydrogen batteries (hydrogen pressure up to 3.92 MPA, or 40kg/cm²). Such high-pressure hydrogen is stored in thin-walled containers. It is easy to explode, and nickel-metal hydride batteries also need precious metals as catalysts, making its cost very expensive, which is difficult to accept for civilian use. Therefore, foreign low-voltage nickel-hydrogen batteries have been explored since the 1970s. Nickel-metal hydride batteries are classified into high-voltage nickel-hydrogen batteries and low-voltage nickel-hydrogen batteries. High-voltage nickel-metal hydride batteries were first developed in the early 1970s by M. Klein and JF Stockel of the United States. The trend of replacing nickel-hydrogen batteries with nickel-hydrogen batteries and applying them to variousspecials has been formed.

The positive active material of the nickel-hydrogen battery is Ni(OH)2 (called NiO electrode), the negative active material is metal hydride, also called hydrogen storage alloy (electrode called hydrogen storage electrode), and the electrolyte is 6 mol/L potassium hydroxide solution. The process of forming the electrode sheet of the active material mainly includes sintering type, slurry type, foam nickel type, fiber nickel type and infiltration type. The electrodes prepared by different processes have large differences in capacity and large current discharge performance. The battery is produced according to a process using different conditions. Most of the consumer batteries such as communication use a slurry-type negative electrode and a foamed nickel-type positive electrode to form a battery. The charge-discharge chemical reaction is as follows [1]:

Positive electrode: Ni(OH)2+OH-=NiOOH+H2O+e-

Negative electrode: M+H2O+e-=MHab+OH-

Total reaction: Ni(OH)2+M=NiOOH+MH

Note: M: hydrogen alloy; Hab: adsorption of hydrogen; the process of the reaction from left to right is the charging process; the process of the reaction from right to left is the discharge process.

When charging, the positive Ni(OH)2 and OH- react to form NiOOH and H2O, and release e- together to form MH and OH-. The total reaction is Ni(OH)2 and M to form NiOOH, hydrogen storage alloy hydrogen storage; discharge In contrast, MHab releases H+, H+ and OH- generate H2O and e-, NiOOH, H2O and e- regenerate Ni(OH)2 and OH-. The standard electromotive force of the battery is 1.319V.

Nickel-metal hydride batteries are classified into high-voltage nickel-hydrogen batteries and low-voltage nickel-hydrogen batteries.

The low-voltage nickel-hydrogen battery has the following characteristics: (1) the battery voltage is 1.2~1.3V, which is equivalent to the cadmium-nickel battery; (2) the energy density is high, 1.5 times or more of the cadmium-nickel battery; (3) the rapid charge and discharge, low temperature Good performance; (4) sealable, strong resistance to overcharge and discharge; (5) no dendritic crystal formation, can prevent short circuit inside the battery; (6) safe and reliable, no pollution to the environment, no memory effect. [1]

High-voltage nickel-metal hydride batteries have the following characteristics: (1) High reliability. It has better over-discharge and over-charge protection, and can withstand high charge and discharge rates and no dendrite formation. Has a good ratio characteristic. Its mass specific capacity is 60A·h/kg, which is five times that of cadmium nickel batteries. (2) The cycle life is long, up to thousands of times. (3) Fully sealed and less maintenance. (4) The low temperature performance is excellent, and the capacity does not change significantly at -10 °C.

Ni-MH batteries should be maintained during use.

(1) Avoid using the process of charging. Within the cycle life, the use process should not overcharge, because overcharge easily causes the positive and negative electrodes to swell, causing active material shedding and diaphragm damage, conductive network damage and battery OHMIC polarization to become larger.

(2) Prevent deterioration of the electrolyte. During the cycle life of nickel-hydrogen batteries, hydrogen evolution should be suppressed.

(3) Storage of nickel-metal hydride batteries. The nickel-hydrogen battery should be stored in a fully charged state. If the battery is stored for a long period of time without storing electrical energy in the battery, the function of the battery hydrogen storage alloy will be weakened and the battery life will be shortened.

(4) Charging after the battery is exhausted. Nickel-metal hydride batteries and nickel-cadmium batteries have the same "memory effect", that is, if the battery is repeatedly charged while the battery is still in the middle of the discharge, the battery will soon be unavailable.

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