I had one of those, I guess in the mid/late 70s! It came with a 'brief case' which housed two enormous spare batteries. IIRC, each battery provided 5-10 mins of 'talk time'!I knew someone once who had one of those pre-GSM BT mobile phones - it was about the size (and almost the weight) of a 6v motorcycle lead acid battery. He hung onto it well into the GSM era because it worked on underground trains.
Kind Regards, John