mm writes:

I sort of think the opposite. Even a bad battery will accept current
through it. That makes me think there is so much dirt between the
battery cable and the battery post that no current ccan get through.

Not necessarily. If the battery has been overcharged for a long period,
so that one or more cells is virtually dry, the battery will be an open
circuit. Or if it's been left discharged for a long time, the plates
will sulfate, and then it won't draw any significant current from a
charger. The latter is more likely in this case.

Apparently, sulfated batteries can be restored to operation in some
cases.

Dave