Electronic – constant current source circuit

Try this:

^{simulate this circuit – Schematic created using CircuitLab}

This circuit is capable of ~1% accuracy with good layout. If the battery is removed, the op-amp will current limit into the base, which should not hurt it. If you don't like that, or if you use another op-amp that doesn't current limit as well, add a small base resistor such as 100 ohms.

LM317HV, 50 ohm resistor, couple of caps, done.

^{simulate this circuit – Schematic created using CircuitLab}

The LM317 (or LMx17) in current source mode works as a current source since the LM317 makes the voltage at its Adj terminal 1.25 V less than the voltage at its Out terminal. So 1.25V = 50 ohms / 0.025 Amps. You need the HV part due to your 50V input range. The capacitors tend to keep the regulator happier - values can vary.

Per the TI datasheet suggested applications for current source mode, C1 could be 0.1uf and C2 could be omitted.

Per the moved goalposts, replace the 55v source with a 55V source consisting of a (probably multi-stage) boost converter to make 55V from 2-50V. Details of implementing that left to whoever wants to see where the goalposts move next. Presumably some sort of initial sense of the applied voltage will be required to determine where to connect the input to the stages of the booster, based on looking at input ranges for a few parts that might nearly do the job, but which have issues with the full range.