How did you get a plain dumb charger to cut back that far?

It's called electrical theory. Even the dumbest of dumb chargers have
to abide by it.

The dumbest of dumb chargers use rectified sine waves. Without a
voltage regulator, the state of charge will have little effect on current.

Really? The state of charge has a direct effect on the output of the
charger. Just what world do you live in?