The nickel-cadmium battery was invented in 1899 and used commercially in the early 20th century. Its cost is more expensive than lead-acid batteries. Nickel-cadmium batteries usually have two types: sealed and open type. Smaller batteries are often sealed, while larger open-type batteries will be used in independent power supply systems.
The self-discharge rate of nickel-cadmium batteries is higher than that of lead-acid batteries. Generally, the monthly self-discharge rate of nickel-cadmium batteries is about 10% at 20°C, and the monthly self-discharge rate can be as high as 20% at higher temperatures.
Compared with lead-acid batteries, open nickel-cadmium batteries are not subject to electrical damage (including fast charging and fast discharging), and are very sturdy. They have a longer life (depending on the type, up to 20 years or more), and can be extreme Operate at temperature (-40~70℃).
Nickel-cadmium batteries can also be sealed and are often used for charging equipment, such as in radios, tape players, calculators, razors, mobile phones, etc. (now this type of application is being used by lithium ion, nickel metal hydride, etc. Battery replacement).
In general, cost issues limit its use in renewable energy systems. These batteries must recycle cadmium at the end of their useful life. Therefore, the cost of recovery must be considered.
Unlike lead-acid batteries, the electrolyte (alkaline potassium hydroxide aqueous solution) of nickel-cadmium batteries only serves as an ion transport medium, and its chemical properties remain basically unchanged during charging or discharging. Regardless of the state of the rechargeable battery, its electrolysis The specific gravity of the liquid remains relatively constant. The positive plate is made of nickel active material (on a nickel plate), while the negative plate is a nickel plate impregnated with cadmium active material.
Each unit of nickel-cadmium battery produces 1.2V voltage, so a 12V battery pack requires 10 battery cells in series, and a 24V battery pack requires 20. Similarly, a 48V battery pack requires 40.
The chemical reaction of the discharge process on the negative electrode (cadmium) is
Cd+2OH–→Cd(OH)2+2e–
The chemical reaction on the positive electrode (nickel) is
2NiO(OH)+2H2O+2e–→2Ni(OH)2+2OH–
The total reaction during the discharge is
2NiO(OH)+Cd+2H2O+2e–→2Ni(OH)2+Cd(OH)2
Alkaline electrolyte (usually KOH) is not consumed in this reaction, so the electrolyte specific gravity of nickel-cadmium batteries cannot reflect its state of charge, which is different from lead-acid batteries.
When discharging, the voltage of the nickel-cadmium battery remains relatively constant (1.2V). Therefore, it is difficult to accurately know whether the battery has been discharged based on the battery voltage. The constant characteristic of this voltage is not conducive to the voltage monitoring of inverters and other devices (such as load control of solar controllers) for disconnecting the load (and discharging) in the low charge state. However, the open nickel-cadmium battery will not damage the battery like a lead-acid battery when it is over-discharged.
When recharging, the reaction in the above equation will proceed from right to left.
Unlike lead-acid batteries, sealed nickel-cadmium batteries can be charged at a high speed (such as charging rate C1), that is, a nickel-cadmium battery with a capacity of 10A·h can be charged at 10A at C1 speed. During the charging process, the voltage will rise from 1.2V to
1.45V, when the battery is close to full charge, this rate of rise is greater. The terminal voltage of lead-acid batteries will decrease as the temperature rises. Some chargers will monitor the temperature of the battery and disconnect the charging at a specified temperature.
The sealed nickel-cadmium battery is technically in a pressure vessel with a safety valve. Therefore, the oxygen and hydrogen generated during charging will not escape, and will recombine into water in the pressure vessel of the battery. If the battery is overcharged and the pressure increases to a certain level, the safety valve opens and oxygen and hydrogen will escape. The amount of battery electrolyte is related to the battery capacity, so the loss of water will result in a loss of capacity. Gas is generated during rapid charging and is lost during overcharging. Therefore, the battery charger must have the ability to detect overcharge.
Note: If the sealed battery is discharged quickly, it can produce oxygen and hydrogen.
Open nickel-cadmium batteries are used in occasions that require higher capacity and dischargers (such as battery storage systems connected to the grid). This type of battery has a control safety valve to release oxygen and hydrogen during rapid discharge or charging. Like lead-acid batteries, distilled water must be added regularly to supplement the loss of water. Depending on the charging and discharging cycle, the interval for this maintenance work to add moisture ranges from a few months to a year. Since batteries are not currently a pressure vessel, they are actually lighter and safer. The battery will not be damaged during overcharging or discharging.
During charging, the open nickel-cadmium battery cell voltage can reach 1.55V, and the voltage rises rapidly at the end of charging. The preferred charging rate is C1, but when renewable energy is used as the only charging source, the charging rate of C1 may not be reached.
The battery needs to be charged for 4 hours in a balanced state of charge. During this charging period, the battery cell voltage can reach 1.6V, but should not be lower than 1.55V or greater than 1.7V. The purpose of overcharging is to remove the gas on the plates, including hydrogen on the negative (cathode) plate and oxygen on the positive plate (anode).