Consider the difference between the nickel iron and nickel cadmium batteries.

The nickel–cadmium battery (NiCd battery) is a type of rech-argeable battery using nickel oxide hydroxide and metallic cadmium as electrodes. A NiCd battery has a terminal voltage during disc-harge of around 1.2 volts, which decreases little until nearly the end of discharge. NiCd batteries are made in a wide range of sizes and capacities, from portable sealed types inter-changeable with carbon-zinc dry cells, to large ventilated cells used for standby power and motive power. Compared with other types of rechargeable cells they offer good cycle life and performance at low temperatures with a fair capacity but their significant advantage is the ability to deliver practically their full rated capacity at high discharge rates (dischar-ging in one hour or less). Charging. Ni–Cd batteries can be charged at several different rates, depending on how the cell was manufac-tured. The charge rate is measured based on the percentage of the amp-hour capacity the battery is fed as a steady current over the du-ration of the charge. Regardless of the charge speed, more energy must be supplied to the battery than its actual capacity, to account for energy loss during charging, with faster charges being more ef-ficient. Overcharging. Sealed Ni–Cd cells consist of a pressure vessel that is supposed to contain any generation of oxygen and hydrogen gases until they can recombine back to water. Such gene-ration typically occurs during rapid charge and discharge and excee-dingly at overcharge condition. If the pressure exceeds the limit of the safety valve, water in the form of gas is lost. Since the vessel is designed to contain an exact amount of electrolyte this loss will ra-pidly affect the capacity of the cell and its ability to receive and de--liver current. Electrochemistry. A fully charged NiCd cell con-tains: а nickel (III) oxide-hydroxide positive electrode plate; а cadmium negative electrode plate; а separator; аn alkaline electrolyte (potassium hydroxide). The chemical reactions at the cadmium electrode during discharge are: Cd + 2OH^– = Cd(OH)₂ + 2e^–.Cd+ = Cd(OH)₂ + The reactions at the nickel oxide electrode are: 2NiOOH + 2H₂O + 2e^– 2Ni(OH)₂ + 2OH^–.2NiO(OH)+2H₂O+ = 2Ni(OH)₂ + The net reaction during discharge is 2NiOOH + Cd + 2H₂O 2Ni(OH)₂ + Cd(OH)₂. 2NiO(OH)+Cd+2H₂O = 2Ni(OH)₂ + Cd(OH)₂

The nickel–iron battery (NiFe battery)is a rechargeable battery having nickel(III) oxide-hydroxide positive plates and iron negative plates, with an electrolyte of potassium hydroxide. The active mate-rials are held in nickel-plated steel tubes or perforated pockets. It is a very robust battery which is tolerant of abuse, (overcharge, overdis-charge, and short-circuiting) and can have very long life even if so treated. It is often used in backup situations where it can be conti-nuously charged and can last for more than 20 years. Due to its low specific energy, poor charge retention, and high cost of manufacture, other types of rechargeable batteries have displaced the nickel–iron battery in most applications. Durability. The ability of these batte-ries 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 ferrous hydroxi-de. While the slow formation of iron crystals preserves the electro-des, it also limits the high rate performance: these cells charge slowly, and are only able to discharge slowly. Electrochemistry.The half-cell reaction at the positive plate: 2NiOOH + 2H₂O + 2e^– 2Ni(OH)₂ + 2OH^– and at the negative plate: Fe + 2OH^– Fe(OH)₂ + 2e–. Charge. Charge/discharge involves the transfer of oxygen from one electrode to the other (from one group of plates to the other). Hence this type of cell is sometimes called an oxygenlift cell. In a charged cell the active material of the positive plates is superoxidized, and that of the negative plates is in a spongy or reduced state. If the nor-mal capacity of the cell is insufficient, short intermediate high rate charges can be given provided that the temperature of the electrolyte does not exceed 115 F / 46 C. These short charges are very effici-ent and cause no injury. Fully charging a NiFe cell consists of seven hours at the normal cell rate. In service the amount of charge given is governed by the extent of the previous discharge. Discharge. Un-der discharge the positive plates are reduced ("deoxidized"); the oxygen, with its natural affinity for iron, goes to the negative plates, oxidizing them. It is permissible to discharge continuously at any rate up to 25% above normal, and for short periods at up to six times normal. When the normal discharge rate exceeds this value, abnormal voltage drops will occur.