The one hole at the center between potential + and - is one of the wire to include when balancing.
So balancing would Required 3 Wire minimum for Balancing this battery,
the + - and center hole.
the center hole is a bridge between the 1st and 2nd cell
which mean it is + and - wire for both Cells
it's like this
you must have the connector for balancing that had 3 wires for it.
for the (+) (-+) (-).
Hope this will help you