From memory I believe the voltage as being 550V DC and as trolleybuses shared the overhead on a few occasions -using a skate on the tram track as return - I guess it was the same. The BCT had their own electricity generating facilities until 1947 when the MEB was formed.
As far as motors were concerned it appears that the older - inherited from City of Birmingham Tramways (a private company) - cars has 25HP and 35Hp motors. Later some were upgraded to 40HP and newer cars had 40 HP with uprates to 63 HP motors. A few had 70HP motors. Motors were usually by Dick Kerr, Brush and GEC.
The early trolleybuses haD 35 and 45HP motors, the next batch 65HP and the newer buses (1937/40) had 80HP motors. The post 1934 fleets had GEC motors, earlier buses and demonstrators BTH and English Electric ones.
Regarding the Fischer bow collectors, yes they did not suffer from many de-wirings as did the pole fitted cars. However they were principally fitted, in 1926, to the cars on the 32 Lodge Road, affectionately known as 'the big dipper', after an accident and the great number of curves. The 8 Alum Rock and 10 Washwood Heath routes also got cars, built new in 1928, fitted with the bow collectors. It was, as far as I know, due to there being a few wiring junctions on the routes. The principal disadvantage was, it seems, that lubrication was needed for successful use and failing that there became excessive wear on the overhead. For this reason bow collectors were never used across the rest of the system. Trams with poles were used, but only at peak times due to these junctions being designed for bow collection.