Previous works have identified that EV charging can be weaponised to attack the power grid. As a case study, we consider the newest charging protocol ISO 15118-20, which provides a high-level communication protocol for EV charging. We first highlight fundamental issues in ISO 15118-20 which prevent the development of security features within the existing standard: We show that an attacker can perform a downgrade attack on ISO 15118-20, and propose modifications to the standard to prevent this. We show how this can be used to enable the development of additional security features within the modified protocol. A proof of concept is developed to prove functionality, determine interoperability between various parties, verify that it meets the original standard's timing requirements, and does not impact charging speed or the length of a charging session.