It's completely unfamiliar to me. I might have made the same guess about language evolution but I do NOT think this is an example. It's not a case of some proper technical term becoming shortened with popularity. (And I am in my sixties and have been using various kinds of rechargeable battery literally since I was a kid, since I was interested in electronics). As long as you're asking the question, can you find the exact title, author, and date of the book? (P. S. In the early days of cell phones saying "My cell" followed by a telephone number, I wonder if she was in prison or had joined a religious order!)

I've not heard this term. As you say, it's either an old term for what we now call simply a "rechargeable battery" or it could be a special battery which somehow recharges itself while you are using it e.g. like a car battery.

In a comment, anu says "here is a leaflet of the book from the publisher's site - http://www.expresspublishing.co.uk/leaflets/upstream-proficiency-international-leaflet/upstream_proficiency_leaflet.pdf The text is on page 11." It looks like a good book. There isn't any real company named "Supraphonic" but I can believe the text was copied from a real manual. But it is still a little strange. Quite seriously, my GUESS is that the phone was manufactured in a non-English-speaking country, and that the manual was written or translated by a very expert person--who, nevertheless, was not a native-English-speaking technical writer. Here is my GUESS. I have a digital camera with rechargeable batteries. I need to take the batteries out of the camera and put them in a charger to recharge them. This is a cordless telephone that usually is sitting in a base. I'm GUESSING that they mean, "You do not need to do anything to charge the battery. It is always being recharged automatically as long as the phone is sitting in its base."

It's definitely an old text. Another old word I've found is 'electronic display'... The text is meant to be an instruction manual though, so maybe I'm wrong.