Dynamic contactless charging is an emerging technology for charging electric vehicles (EV) on the move. For efficient charging and for proper billing, dynamic charging requires secure communication between the charging infrastructure and the EVs that supports very frequent real-time message exchange for EV authentication. In this paper we propose Portunes, an authentication protocol for charging pads to authenticate an EV's identity. Portunes uses pseudonyms to provide location privacy, allows EVs to roam between different charging sections and receive a single bill, and achieves fast authentication by relying on symmetric keys and on the spatio-temporal location of the EV. We have implemented Portunes on RaspberryPi Model B with 700 MHz CPU and 512 MB RAM. Portunes allows the EV to generate authentication information within 0.3 ms, and allows charging pads to verify the information within 0.5 ms. In comparison, ECDSA signature generation and verification take over 25 ms and over 40 ms respectively.