Battery electric vehicles (BEV) provide a clean mode of transportation since they operate free from local emissions. However, they challenge the energy system in causing load peaks that can result in grid congestion or high emissions by remote peak power plants. Such problems can be moderated if the BEV user provides charging flexibility, \ie enters how far the charging can be postponed compared to the fastest possible schedule without compromising her mobility needs. Following recent approaches in Green Information Systems research, we apply a digital nudge development process to adapt the interface design of smart charging systems in a way to nudging the user towards providing more charging flexibility. We present an expert survey to identify promising nudges and validated these with an online experiment with 164 subjects BEV users (most of them BEV owners) in different charging scenarios. The results suggest that goal framing on monetary benefits within the interface design leads to greater flexibility compared to a neutral interface. In contrast, hints on environmental impacts and social aspects showed no increase in charging flexibility.