A not so small Battery is there inside loco to provide power in case of emergency (in cab light, flasher in case of loco breakdown) and to provide power to a small compressor called as 'baby compressor'. This baby compressor builds up the requisite pressure required to raise the panto (6 kg/cm^2). Once the contact is made and there is no issue with line (no flashing etc) then the VCB is opened and the next steps to energize the loco may begin.
Battery rating: 110 v, 199 Ah (Ni-Cd, unlike the Li-ion or what is more commonly and wrongly called as Li-Po battery used in our mobiles)