Is this a float charger or trickle charger?
When no battery is connected the circuit operates as a constant-voltage source having an output voltage of \$\mathrm{V_o=11\cdot 1.25V = 13.75VDC}\$ with zero current flow.

When a battery is connected, if the battery voltage is less than, say, 12V then the circuit will work as a constant-current source having a constant output current of \$\mathrm{I_o[A]=V_{BE-BC547}[mV]/470\approx1.5[A]}\$. 

During the battery is charging the battery voltage will rise up to a point that the D1 gets reverse biased (Note: Charge current may drop as the battery voltage increases, but I'm not sure about that). So the circuit will switch its operating mode from constant current to constant voltage with zero current flow even if the battery is not removed.

So, this circuit fits in your "float charger" definition:


  float charger will only turn on and charge your battery when it has self-discharged below a certain level


PS: The circuit may not work with 15V source since the LM317 requires a minimum difference of 3V between the input and output. For a source voltage of 18VDC and an output current of 1.5A, the LM317 will dissipate at least 7.5W. I'm sure you know what that means.