Thanks very much! Today, I have just heard back from Nikon and they say that a rapidly flashing light indicates either a faulty charger or faulty battery. They would need to see both in their repair shop. On advice from Chris I took off the RRS L-bracket and popped the EN-EL 15a battery that had been in drawer for 8 months into the camera. Shock of shocks it showed 94% charged and "new" on the age scale!!! So the charger may have had a hissy fit because I tried to charge an almost full battery. Very annoying because both charger and battery have had very little use since new. For months I just rotated my 2 grip batteries. NOTE TO CHRIS - I removed the 2 triangular strap supports from my D850. I then inserted a screw with the most tiny alan key you have ever seen. It is this that prevents me whipping off my grip. It's a most maddening fiddly job. However it makes the L-bracket rock solid. I'll run without the internal battery for a while, and will send the battery and charger off to Nikon to see what they say. I don't have a USB charger option to try. I suspect a faulty charger. Thanks guys for such a prompt response with great advice and suggestions!Just as a side note, sometimes chargers can be also too clever. I had the rapid flashing on the Nikon charger with one of my EN-EL15 as well. I didn't know what it means according to documentation, but it was obvious that the battery wasn't charging. I then decided to put this battery in my simple USB charger that I got for travelling (because it allows easy charging with any USB charging port, be it a hotel, train, car, a power bank or even a solar panel). This charger didn't care at all, just took it as an empty battery and charged it. After this exercise the Nikon charger decided to accept this battery again as a working member of the family
