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A V2G Communication Session always starts with the SessionSetupReq/Res message pair and always ends with the SessionStopReq/Res message pair.

The ISO 15118-2 says, as stated in the title, that : A V2G Communication Session always starts with the SessionSetupReq/Res message pair and always ends with the SessionStopReq/Res message pair.

Looking at Figures 103 and 104 it clearly looks like the SessionStopReq is only allowed if Charge Loop has previously started whereas in DIN 70121 it is possible to end the V2G communication session by sending SessionStopReq at almost any time. During tests with different EVCCs it looks like a valid scenario that EVCC resigns from HLC based charging by sending SessionStopReq after receiving i.e. ServiceDiscoveryRes or AuthorizationRes.

If we compare those scenarios in ISO 15118-2 and DIN 70121 I think that ISO 15118-2 should follow DIN 70121:

ISO 15118-2 [V2G2-563]The SECC shall respond with AuthorizationRes containing “ResponseCode = OK” and "EVSEProcessing=Finished" within V2G_SECC_Msg_Performance_Time according to Table 109, if the processing of the information is successfully passed and the authorization is finished. The allowed next request shall be ChargeParameterDiscoveryReq and the V2G_SECC_Sequence_Timeout is set according to Table 109.

DIN 70121 [V2G-DC-495]After the SECC has successfully processed a received ContractAuthenticationReq message, the SECC shall respond with a ContractAuthenticationRes message with ResponseCode equal to “OK” and EVSEProcessing equal to “Finished”, if the contract authentication has finished on EVSE side. The SECC shall then wait for a ChargeParameterDiscoveryReq message or a SessionStopReq message.

Not allowing SessionStopReq as a valid request in such cases results in necessity to end V2G communication session by timeouting/breaking the connection or (if the EVCC sends SessionStopReq) by sending SessionStopRes with ResponseCode 'FAILED_SequenceError'. I think that this is wrong and shall be changed.

I think that ISO 15118-2 requirements and figures 103 and 104 are missing the part in which SessionStopReq is allowed as the next request message at any time as in DIN 70121.