Metal hydrides (MHs) are promising candidates for hydrogen storage and applications in thermodynamic machines such as hydrogen compressors and heat pumps. These applications require thermal cycling between a lower and an upper working temperature to compress hydrogen and to absorb or release heat. The fundamental mechanism is desorption of hydrogen from the concentrated hydride phase (β) to the dilute phase (α) during heating and the reverse transformation during cooling. Thermal cycling causes degradation of the hydrogen storage alloy which results in loss of storage capacity and thus throughput of the machine. Literature reports of the rate of degradation for the same alloy vary widely. Quantifying degradation and understanding its mechanisms are therefore crucial to the commercial viability of MH-based machines. This paper presents a novel apparatus designed to thermally cycle six samples simultaneously under isochoric conditions that emulate service in a MH machine, along with a mathematical model for determining the required sample amount for all six samples to fully absorb and desorb between the same working temperatures. The capability of the apparatus was demonstrated by performing 1000 isochoric cycles on six samples of LaNi5 with differing masses, desorbed to differing degrees when heated, at a rate of approx. 1700 cycles per month, to explore the idea that time spent in the β phase at elevated temperatures determines the basic rate of degradation in the absence of other factors such as impurities in the hydrogen. Contrary to some literature reports and expectations, the samples suffered very little loss of hydrogen capacity (less than approx. 0.03 in the hydrogen-to-metal atomic ratio), suggesting that the service lifetime under isochoric conditions will be much longer than under isothermal conditions at elevated temperature. This work paves the way for systematic studies on fundamental degradation mechanisms in MHs using a repeatable protocol, leading to improved material design and selection for practical hydrogen storage and machine applications.