The document discusses different types of batteries. It describes the zinc-carbon cell, which has a zinc negative electrode, manganese dioxide positive electrode, and ammonium chloride electrolyte. It provides 1.5V and is commonly used in devices like flashlights. The document also describes the nickel-cadmium cell, which has nickel and cadmium electrodes and a potassium hydroxide electrolyte. It supplies 1.2V and high currents, and must be recharged with a constant current supply due to its low internal resistance. The document explains how battery chargers work to recharge secondary cells by driving a current through the battery in the opposite direction of its normal output.