My tank stopped charging

. worked fine untill a couple days ago... Now, when I try to charge it, the screen displays the charge CH and red dot for a moment (like half a second) and then blanks and displays the CH with the green LED, and does not charge at all... (leaving it on with the green LED does not let it charge).
I am doing the following for charging (maybe I am doing something wrong?)
turn off tank.
turn off controller
middle switch bottom position
Put tank on charger
turn on controller.
No workie

. Any ideas?
L:F