For the technicians (a brief explanation of the circuitry of regenerative braking): The transistors in the motor controllers short-circut the motor and the the current and the magnetic field in the motor rise. Upon opening the "short-circuit" the field energy is rectified over a diode and returned to the battery. The execution of this " short shot " takes over the motor control. The controlling of the process takes place rate-dependently as programmed into the electronics of the recuperation.
Such a recuperation particularly for the City-el was developed by JME in Heidelberg (I was working there at that time). At the moment, Roger Stoll in Switzerland manufactures and sells the "Reku-Kit", contact him at CH 6030 Ebikon, Tel #41 41 44 01417.
Alternatively the company Curtis Instruments (which made the original motor controller for the City-El -RRY) manufactures a Controller with recuperation, however to my knowledge this Controller starts from 400A rated current, which for the City-El is excessive. (This controller is no longer in production. Peter Senkowsky of EV Transport uses one on his vehicle with an Advanced DC motor, and an 84 Volt system -RRY)
Recuperation is worthwhile if one lives in a hilly area, and thus daily up and down must go. The recuperation returns going downward approx. 20% to the energy which one used upward. A further very important aspect is that one preserves the brakes. In particular, during long downward travels it restrains with the recuperation very pleasantly. One needs the foot brake only on the last meters before the traffic light. A side effect of the JME recuperation installation is that the vehicles with the "speed kit" installed (all vehicles from Pacific Electric Vehicles/Sacramento Municipal Utilities District, and probably all US vehicles - RRY) get more starting torque, since under 30 km/h the "speed resistance" is bridged.