Driving Stepper motor using 3.7V Li-Ion

[electroguy]

Hi,

Has anyone here had success (or failure?!) driving steppers using 3.7V Li-Ion?
Need some advice for driving a stepper such as this:
http://www.pololu.com/catalog/product/1204
I realise that i won't be driving it to it's full potential, given that it can run up to 3.9V and single cell battery is usually less than this, but will i still get the majority of the power gm/cm (oz-in) listed?
Motor won't be going overly fast, maybe 1 or 2 rps (60 to 120 RPM).

I know there are some great stepper chips out there with current limit, etc... such as TI DRV8811 or Allegro A4988, but chips like these require a minimum Vin for motor to be about 8v (they use chopper current limiting, etc...)
Although they *might* work with a lower Vin, I don't want to do this as it is out of spec according to the data sheet, so i am thinking of using something like this:
http://www.pololu.com/catalog/product/2135

Does anyone have any advice or experience for driving steppers from single cell 3.7V li-ion?

[Kremmen]

You should be aware that the "nominal" voltage of a stepper has precious little to do with the actual voltage of the optimal power supply. The nominal voltage is the one that drives the nominal DC current through the motor stator winding through its DC resistance.
The situation changes completely when you actually start stepping the motor. The instantaneous winding current is determined by the resistance and inductance of the winding, the applied voltage and elapsed time. When a DC voltage is applied to an ideal inductor, the current rises linearly from zero as a function of time. The slope is determined byt the inductance and applied voltage. So, the coil current does not rise instantaneously. If you supply only the nominal voltage then at even moderate step rates the coil current hasn't got time to raise to any useful level for torque production and you motor will either stall or never get to speed in the first place.
The solution is to raise the supply voltage and limit the current at slow step ratesusing some PWM scheme. A popular mfg of stepper drives - Gecko - suggests the supply voltage should be 32 * sqrt(L) where L is the coil inductance in mH. In your motor the value is 1.7 mH resulting in recommended supply of 41 V. Quite far above 3.7 V.
The fact is that you just won't get anything useful from the motor at such low supply voltages unless you are satisfied with dead slow motion.

[electroguy]

Thanks. Looks like im going to have to step up the voltage...