It finally occurred to me why that Battery Minder is dead. Last summer - before harvest, so June~July, we had a power surge. The neutral line in from the pole broke. The lights went bright then dim. I turned everything off, then measured 70 to 140 volts at outlets. No damage to computers (on surge protectors) or appliances. The clock radio in the house and a similar one out in the barn's shop were the only things smoked. And .. that likely took down this Battery Minder. The tractor where I use it is run intermittently and was charging itself sufficiently, so I never noticed the Battery Minder on it has been doing nothing, for months now.
A note re the lights on the Battery Minder, the manual says the red light is on any time the unit is plugged in. No red light showing on this dead unit. My green light shows when its connected to a battery regardless if the unit is plugged into the wall or not. That fooled me into thinking the battery was charged. Only recently did I think to test the battery with a voltmeter - I found 12.3 volts.
Now that I've concluded that the power surge killed my Battery Minder I'll buy another.