DRV8434: Stepper motor specifications and home run

Tool/software:

Hello experts.
I would like to ask you about a stepper motor specification.
The stepper motor specification given by the customer does not indicate the rated current of the motor.
Only the Rated voltage:12.8V(Range from 9V to 16V) is provided.
Coil resistance:8R
NO. OF PHASES:2PHASES
STEP ANGLE:15/STEP
How do I know the current of this motor?
This is a 2 phase 4 wire motor, do I need to X2 the current?
2. How can the stepper motor return to home position without sensor?

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