If the whole circuit is the solar panel side of the diode, then there is no battery drain?
Are you absolutely sure that your panel doesn't have an intrinsic diode? To find out, cover your panel, hook your bench supply across the panel (+ to +, - to -), and crank the voltage up to ~14V. If your panel has an intrinsic diode, then no measurable current will flow from the supply into the panel. If your panel does not have an intrinsic diode, then a few mA will flow from the supply.
Only if the panel has no intrinsic diode would you have to add one (somewhere). You have a choice of four places where to put it, three of which I will discuss:
1. Between Panel and everything else. Keeps the panel from discharging the battery overnight. 560uA of TL431 current comes out of the battery overnight. Preserves excellent voltage regulation.
2. Between everything else and the battery positive pole. Keeps either the panel or the TL431 from discharging the battery. Screws up the voltage regulation as described below.
3. Place it inside the regulator at the big X. Prevents discharge by the panel, and reduces the overnight discharge by the regulator itself from 560uA to about 120uA (the voltage divider). This preserves the excellent voltage regulation.
In Case 2, the TL431 is sensing a voltage that is higher than the actual battery voltage by the forward drop of the diode. If you carefully set up the shunt regulator to switch on/off at 14.20V like I did in the simulation, at room temperature, the actual battery voltage would wind up about 0.25V lower (13.95V) if the diode is Schottky, or about 0.65V lower (13.55V) if the diode is Silicon. The forward drop of the diode and its internal resistance makes for a much more gradual turn on of Q1 as the battery voltage climbs. The forward drop of the diode is effected by temperature, so adding the diode makes for a less precise turn-on voltage where the regulator begins shunting current. If you put the diode there, you will have to use trial and error to get the correct float voltage for the battery.
If I was doing this, and started with a panel that had an intrinsic diode (every panel I have ever bought has one), then I would not use a secondary diode....