Aluminum hydride (AlH3) has been identified as a promising H-2 storage material for fuel cell systems and offers a significant weight savings over conventional Li-ion batteries, due its high H2 storage capacity and simple balance of plant requirements for H2 generation. This work reports on the development and characterization of a novel, wearable AIH(3) based PEM fuel cell system. System characterization revealed an unexpectedly low energy density value, 25% lower than anticipated, (436 Wh kg(-1) vs. 582 Wh kg(-1) for 1440 Wh) due in part to a previously unpublished phenomenon of incomplete alpha-AlH3 thermolysis. Based on literature reports, complete thermolysis was expected to occur at <180 degrees C, however this work reports on conditions where the full H2 yield cannot be obtained despite high temperature (>250 degrees C) exposure. This work provides an experimental characterization of this phenomenon and quantitatively describes it by developing a new model in the framework of the Avrami-Erofeev phase transformation model, which can be utilized for the optimum design of high energy density AlH3 cartridges. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.