There is provided electron beam welding apparatus comprising an electron beam gun (50) associated with a welding chamber (54) configured for welding of a battery array (33) to a bus bar (32) and beam adjustment devices (56, 58) operative in response to a controller (60) to modify beam characteristics and position of an electron beam (52) generated by the electron beam gun (50), wherein at least two subsidiary chambers (70, 72) are disposed on opposing sides of the welding chamber (54), all chambers being evacuable to be under a vacuum, and a beam detector (61) is positioned proximal the welding chamber (54) to generate data relating to beam characteristics and position, the controller (60) configured to respond to data from the beam detector (61) to control synchronously the beam adjustment devices (56, 58) and to create a consistent welding penetration depth for welds formed between a bus bar (32) and a battery array (33) regardless of angle of incidence of an electron beam. An associated welding method is also provided.