Abstract Using lithium-ion energy storage to buffer energy between the electricity grid and a load can enable higher power loads to operate on lower power grid connections, thereby potentially avoiding costly electricity grid infrastructure upgrades. For such systems, an estimation of the lifetime of the lithium-ion battery is critical for calculating the system costs. The lithium-ion battery may however operate at elevated discharge rates where the lifetime cannot be easily estimated from datasheets. In this paper lithium-ion lifetime at two discharge rates is investigated experimentally. Two lithium-ion cells are cycled, one at the manufacturer recommended discharge rate of 1 C and one at the elevated discharge rate of 3 C. The capacity degradation is monitored and electrochemical impedance measurements are periodically taken to track internal cell resistance growth. It is found that after 400 cycles the cells discharged at 1 C and 3 C have capacities of 83.9 % and 81.4 % respectively of the maximum capacity recorded. Although this difference is not large, it should be noted that over these 400 cycles, 3260 A h had passed through the cell being cycled at 1 C while 10 % less charge (2930 A h) had passed though the cell being cycled at 3 C. Over the same cycles the electrochemical impedance results show the resistances of the cells discharged at 1 C and 3 C have increased by 34.7 % and 57.4 % respectively.