The decarbonization of urban bus fleets can be made by their electrification as in-motion-charging (IMC) buses which can run as trolleybuses or in battery mode. The benefit is that IMC buses can use the existing trolleygrid infrastructure where their route overlaps with it to charge the battery and operate in battery mode outside of it. Presently, the IMC battery charging power is set conservatively to the minimum of all the spare capacities of the traction substations (SSs) found along the bus route. This can render most electrification projects techno/economically infeasible as not enough energy is picked up for the battery-mode operation and long charging times at bus terminals are required. This article proposes then an adaptive charging approach that uses the locally available spare capacity under any traction SS, taking into account the limitations of the maximum SS power and the minimum line voltage. The method is proven here both theoretically and in a case study over one full year of operation of four electrified diesel/compressed natural gas (CNG) bus lines in Arnhem, The Netherlands, using comprehensive and verified trolleybus and trolleygrid models. The proposed adaptive charging method, as opposed to the present conservative method (here, Regular Charging), is shown to make one bus electrification project completely feasible and reduce the extra terminal charging time for the other lines by up to 64%.