The nickel-iron battery (NiFe battery) is a storage battery having a nickel(III) oxide-hydroxide cathode and an iron anode, with an electrolyte of potassium hydroxide. The active materials are held in nickel-plated steel tubes or perforated pockets. It is a very robust battery which is tolerant of abuse, (overcharge, overdischarge, and short-circuiting) and can have very long life even if so treated. [6] It is often used in backup situations where it can be continuously charged and can last for more than 40 years. Due to its high cost of manufacture, other types of rechargeable batteries have displaced the nickel-iron battery in most applications. Because of their long life NiFe batteries are ideal for backing up renewable energy applications. The reason for their disappearance in the North American market is largely due to the Exide Corporation's decision to abandon the technology in 1975 after purchasing it from the Edison Storage Battery company for several million dollars. The reason for acquiring the manufacturing process to make NiFe batteries and then simply abandoning the technology is unknown. Exide remains the second largest manufacturer of lead acid batteries in the world.
(If anyone knows more about why Nickel Iron batteries went out of production in North America please contact us and we will update this website.)
Charging Parameters
The proper float voltage is 1.45 volts per cell. If 10 cells were used, the proper charge voltage would be 14.5 volts.
The charge voltage can vary from 1.46 to 1.55 volts per cell. Unlike other battery designs, the exact charge voltage is unimportant. A higher voltage will result in quicker charges but more water loss that will necessitate more frequent topping up with distilled water. Since the cells can withstand overcharge there is debate over what constitutes a proper charge voltage. The higher you go the quicker water will disappear from the batteries. At voltages greater than 1.5 volts/cell the batteries will store approximately 15% more power than they are rated for. If 10 cells were used, the charge voltage could range from 14.6 volts to 15.5 volts. It is probably better to use the 1.46 volts / cell level of charge in order to minimize water loss if the battery will be unattended for months at a time. Regenerative catalytic caps are available to combine the h2 and o2 back into water if unattended maintenance is required. There are also auto watering systems that are available.
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The proper equalization voltage is 1.65 volts per cell. If 10 cells were used, the proper equalization voltage would be 16.5 volts. This equalization charge is applied for 8 hours using at least C/10 current. According to Edison's original manual from 1914, it is best to completely discharge the batteries from time to time before applying the equalization charge. Edison also recommends a 1.7 volt equalization charge and he recommends changing the electrolyte every 5-10 years.
This will all come as a surprise for lead acid battery users. In contrast to lead acid, the NiFe battery can be overcharged for decades at a time without damage and can be left discharged for years at a time and will still work perfectly when needed.
Durability
The ability of these batteries to survive frequent cycling is due to the low solubility of the reactants in the electrolyte. The formation of metallic iron during charge is slow because of the low solubility of the Fe3O4. While the slow formation of iron crystals preserves the electrodes, it also limits the high rate performance: these cells charge slowly, and are only able to discharge slowly.Nickel-iron cells should not be charged from a constant voltage supply since they can be damaged by thermal runaway; the cell internal voltage drops as gassing begins, raising temperature, which increases current drawn and so further increases gassing and temperature.
Nickel-iron batteries have long been used in European mining operations because of their ability to withstand vibrations, high temperatures and other physical stress. They are being examined again for use in wind and solar power systems and for modern electric vehicle applications.
Electrochemistry
The half-cell reaction at the cathode:
2 NiOOH + 2 H2O + 2 e− ↔ 2 Ni(OH)2 + 2 OH−
and at the anode:
Fe + 2 OH− ↔ Fe(OH)2 + 2 e−
(Discharging is read left to right, charging is from right to left.)
The open-circuit voltage is 1.4 volts, dropping to 1.2 volts during discharge.The electrolyte mixture of potassium hydroxide and lithium hydroxide is not consumed in charging or discharging, so unlike a lead-acid battery the electrolyte specific gravity does not indicate state of charge. Lithium hydroxide improves the performance of the cell. the voltage required to charge the cells is between 1.6 and 1.7 volts. Most people use 1.65 volts.
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