On the schematic you have to power with more that 15Volts. 2950-5 needs at least 0.45 volts at the input above from 5V. If your battery is discharged and request more current, the transformer might drop the voltage bellow 15V.
The circuit is quite economical but I am afraid that is quite a compromise too.
I have attached my idea in a hand schetch (sorry, no CAD here!).
Ioannis
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