How to use center tapped transformer's one connection?
You can use a 15-0-15 secondary like this: -



It works slightly better than a non-centre-tapped 15v secondary at producing the correct voltage. A single 15 volt secondary will produce 21.21 volts minus two forward diode drops = ~19.8 volts whereas the split secondary will produce 21.21 volts minus one diode drop = ~20.5 volts.

The other problem you may have relates to load current - the VA rating of the split secondary needs to be twice as much because each half of the winding is only supplying half wave rectified current.

Looking at the circuit you linked, I can't see where the battery-to-be-charged is meant to be connected so I'm guessing (possibly incorrectly) that the so-called "charge controller circuit" is just a control circuit to activate a remote battery charger? If that is true then the transformer VA rating will be fine but please do check.