There is no exemption for Electric vehicles in paying the Dart charge for the Essex to Kent Dartford crossing toll and, like all other road users, it is still free to use the crossing between 10pm and 6am. There are exemptions for those with a disability badge. Also reduced rates for local residents.
The Dart charge is not related to the London Congestion charge despite their proximity and both using the letter ‘C’ to indicate the boundary.
ecarclub information about charges for electric cars around London
Other charges / Tolls in the UK
There is no payment exemption for Electric cars on the M6Toll motorway.