Results of recent tests of an 8-cell, AMTEC ground-demo are analyzed and the performance of individual cells compared. The ground-demo produced a peak electric power of 27 W_e at an output voltage of 16 V, when tested at hot and cold side temperatures of 1123 K and 553 K. The electric power output and terminal voltage of the individual cells, however, differed by as much as 25%, from 2.94 to 3.76 W_e, and 1.73 to 2.21 V, respectively. These variations were attributed to differences in: (a) contact resistance between electrode/BASE/current collector; (b) current (or electrons) leakage between anode and cathode electrodes through the metal-ceramic braze joint between BASE tubes and support plate; and (c) the charge-exchange polarization losses. Model's predictions compared very well with measured voltage and electric power output of individual cells and of the ground-demo. At the operating conditions for the Pluto/Express spacecraft (T_(hot) ～1200 K, T_(cd) ～573 K), the best performing ground-demo cell would have delivered 5 W_e at an output voltage of 3 V. These values, however, are still significantly lower than those needed to meet the Pluto/Express mission power requirements (8.2 W_e at 3.5 V, per cell).
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