Basic knowledge of nickel metal hydride batteries

Basic knowledge of nickel metal hydride batteries

①The cost and characteristics of electrode materials for nickel metal hydride batteries
The cost and characteristics of the electrode material will affect the price and life of the battery. The amount of material is strictly dependent on the application and its power capacity requirements. High power and high energy density nickel metal hydride batteries are most suitable for heavy cars such as buses and trucks. About 10 years ago, Energy Conversion Devices Inc. manufactured nickel metal hydride batteries for electric and hybrid cars. The weight and size of the battery depend on the number of primary batteries, the density of the materials used, and the power output capacity of the battery. Compared with other battery technologies, nickel metal hydride batteries significantly reduce weight and size. As nickel-metal hydride batteries have higher energy density than nickel-cadmium and lead-acid batteries, they increase the number of passengers in buses and cars, expand the payload of trucks, and provide military electric cars with higher stealth and offensive capabilities. In addition, high reliability, maintenance-free operation, and the ability to accept peak regenerative currents make nickel metal hydride batteries an ideal solution for heavy commercial and military cars.

Regarding the cost of cathode materials, it is difficult to get an accurate quotation. The price per pound or kilogram of material depends on the purity of the material, the amount of material to be purchased, and the transaction time. Table 1.6 summarizes the quotations based on preliminary price surveys of various cathode materials.

These prices are based on small batch prices. The price of manganese is based on unrefined ore. The price of manganese will increase according to the percentage of purification. Similarly, the price of refined nickel and cobalt will be higher.

②The influence of temperature on the discharge capacity of nickel metal hydride battery
The designers of nickel metal hydride batteries suggest that using these batteries outside the temperature range of 0-40°C will not achieve the best performance. In this temperature range, a discharge capacity better than 90% can be achieved. Storing this battery at room temperature will not cause permanent capacity loss. The experiment conducted by the supplier shows that continuous exposure to 45°C can reduce the cycle life of the battery by about 60%. Although nickel metal hydride batteries can maintain high discharge currents, repeated charging and discharging at high current levels will shorten the battery life. The best performance during the life cycle can be achieved when the temperature change rate is 0.2 ~ 0.5℃.

③Charging process of nickel metal hydride battery
The battery designer pointed out that due to the sensitivity of nickel metal hydride batteries to charging conditions, charging is the most critical step in determining the electrical performance and overall life of nickel metal hydride batteries. Therefore, great attention must be paid to the charging rate, temperature range, and effective techniques for indicating the end of charging. Nickel metal hydride batteries must be charged at a constant current to improve battery performance and extend cycle life. All batteries designed more than 10 years ago suffer from memory loss. But the latest nickel metal hydride batteries do not have memory loss. The charging current level must be limited to avoid overheating and incomplete oxygen recombination. These two conditions significantly affect the performance and life of the battery. Battery tests have shown that the charging process of nickel metal hydride batteries is exothermic (formed by exothermic heat), while nickel-cadmium batteries are heat-absorbing (formed by absorbed heat).

④ Deterioration factors of nickel metal hydride battery performance
·Sensitivity: The sensitivity of the battery depends on the storage temperature. At higher temperatures and longer storage periods, the loss of remaining battery capacity is faster. After 30 days of storage, the capacity drops to 20%.
·Voltage drop: The voltage drop rate is higher at low temperatures.
·Voltage platform: only suitable for low-rate charging.
·Temperature cut-off: This method stops charging when the temperature reaches the preset limit and indicates overcharging.
·Rising rate: Measure the rate of temperature rise according to time, and stop charging when it reaches a predetermined value of 1°C/min. This is the preferred method to prevent high-speed charging because it ensures battery life or extends battery cycle life. If overcharged repeatedly, the nickel metal hydride battery will be damaged.