My steppers don't have a marked voltage. I'd appreciate suggestions on how to proceed.
It is a six wire unipolar, with 17.5 ohms or resistance on each coil. Other than 1.8 degrees, there are no other markings.
It's a big honking motor. The spacing between the mounting screws on the flange is 57mm. It came out of a Mannesmann Tally printer circa 1980 or so.
I'd like to drive it through an Easydriver 4.4 with an Arduino Uno or Mega2560. I want to use a sealed lead acid battery to drive the motor, because I'm working in the field.
I've made electronic projects for decades, but I'm new to Arduino and steppers.
Try 6V, if that doesn't provide sufficient torque, try 9V, or 12V. As far as I know, steppers are quite flexible with the voltage. With a higher voltage, they will draw more current and provide more torque.
6V/17.5 Ohms = 340ma... likely won't cause a problem to start.
When you're slowly ramping up the voltage you should test the temperature of the motor; the primary limiting factor of any motor is how hot it can get before the insulation on the coils gives out. A safe test is that if you can grab the motor and hold it for a few seconds then you're not too hot.
47mm between the holes is standard for a NEMA 23 motor. 57mm is non-standard AFAIK; that would put it somewhere between 23 and 34. Assuming you didn't measure wrong then the EasyDriver might be a little too weak (amperage wise) for your motor; it's only designed for non-honking motors.