Battery Electric Vehicles (BEVs) are claimed to foster climate-neutral energy and transportation systems. However, the use of the BEVs shifts emissions only geographically. In addition, the charging of BEVs can lead to new problems, such as congestions in the electricity grid. Smart charging algorithms can avoid some of these problems. To do this, however, the BEV user must actively decide to make her temporal flexibility available. Feedback on possible CO2 savings could be a nudge that encourages BEV users to provide more temporal flexibility. We compare different charging events and algorithms to determine the savings in CO2 emissions of BEV charging. We find that temporal flexibility can on average save of CO2 emissions in the single-digit percentage range in Germany. The results can be used to implement a feedback nudge in smart charging systems to test its effectiveness on the provision of temporal charging flexibility of BEV users.