The BC850 was Tekin´s first charger after starting this brand in 1985. It seems a similar charger was sold by Delta before where Kevin Orton, founder of Tekin, was working as motor builder and electronic designer. This charger is using a peak detect circuitry for cut off, invented by Kevin around 1981-1982. As for charge current a soft pulse current is used, with the choice of fixed 3A or 4.5A depending on battery type.
As you can see from the pictures, two versions with slightly different frontplate design were sold.