I have a BC250D and have never heard of an external cradle charger. Last time I was in my 250 I recalled that the battery pack used a small connector on a short pigtail lead. This is not the typical configuration a manufacture would make an external charger for. Not that one does not exist, just that I have not seen one.
Good Luck with your search.
PS - When I get home tonight I will have to check the battery compartment just to make sure my recollection is correct.
I just recalled that the previous scanner (245?) had a battery pack with surface mounted contacts that could be externally charged, but again I never came across one.
The BC235 came with a cradle that you plugged the battery into to charge. It also has a pigtail connector that allowed you to operate the scanner, or charge a battery inside the radio at the same time.