Its definetely NOT TIMER BASED. It will have an indicator while charging (red led) which goes off once the battery is charged. You can test by using a near depleted cell and a full one. The full one will not have its led lighting implying it is fully charged and not charging more.
I think 2 bays are linked or something like that.
Here is my original post.
And a link to another post contained in my thread (REVIEWED BY ADMIN)