Question

why voltage remains constant during discharge of nickel metal halide

Answer 1

NiMH (Nickel Metal Halide) batteries have a relatively low internal resistance due to wound construction, enhanced contacts and large surface area of the electrodes. The low battery internal resistance allows NiMH batteries to have excellent high rate performance. The internal resistance of fresh, fully charged NiMH batteries is typically less than 50 milliohms. During discharge, the battery internal resistance will stay relatively constant until near the end of life where it will rise sharply. Low internal resistance allows NiMH cells to deliver a nearly constant voltage until they are almost completely discharged. Again, small variations from room temperature do not appreciably affect the NIMH battery voltage profile. Though similar to lithium AA primary batteries, the NIMH battery exhibits a sharp knee at the end of the discharge where the voltage drops quickly.