When I inserted the next set of batteries the green led remained on. I let the unit operate this way for several hours expecting the red led to eventually come on. It did not. I assume that when the green led is on, only trickle current is flowing to the battery. I had to unplug the charger to get the green led to turn off and the red led to come on for a new set of batteries.  Is this normal operation?

Thank you in advance for your help. Gary

This is normal. As long as you leave the charger plugged in it will remain in trickle mode when the green lights are on. You must unplug the unit to reset the 7 hour timing control. This starts a new 7 hour charging cycle.