As the use of plug-in electric vehicles (PEVs) further increases in the coming decades, a growing stream of batteries will reach the end of their service lives. Here we study the potential of those batteries to be used in second-life applications to enable the expansion of intermittent renewable electricity supply in California through the year 2050. We develop and apply a parametric life-cycle system model integrating battery supply, degradation, logistics, and second-life use. We calculate and compare several metrics of second-life system performance, including cumulative electricity delivered, energy balance, greenhouse gas (GHG) balance, and energy stored on invested. We find that second-life use of retired PEV batteries may play a modest, though not insignificant, role in California's future energy system. The electricity delivered by second-life batteries in 2050 under base-case modeling conditions is 15 TWh per year, about 5% of total current and projected electricity use in California. If used instead of natural gas-fired electricity generation, this electricity would reduce GHG emissions by about 7 million metric tons of CO2e per year in 2050.