Battery buses are a solution ahead of their time. Any old bus is better than car-based transportation, so shifting modes from car to bus is more important than changing the energy source of the bus. If shifting from car to bus is impossible because of built environment, that's the first priority. Buses powered by batteries should be way down at the end of the TODO list.

If you absolutely insist on having buses that aren't direct sources of CO2, fuel cells are a more mature technology that makes more sense than batteries.