A 15 Volt, 12.55 Watt motor should be driven by rechargeable batteries.
Let's assume the batteries must be able to drive the motor 15 minutes.
Step 1. Required Voltage
One cell has a nominal voltage of 1.2 V. So we need 13 cells in series.
1.2*13=15.6V
Step 2. Required Current
12.5 Watt at 15 V requires a current of 0.837 A.
Step 3. Required Capacity
The required battery capacity is 0.837A during 15 minutes = 0.210 Ah = 210 mAh.
Step 4. Allowed Current
Assume the maximum current for the batteries is 3C, that is 3*210 mA = 0.63 A. This is less than the continuous motor current.
Step 5. Start and stall conditions
At start, this motor may cause a current of 5 times the nominal current, that is 4.185 A.
If 3C is the maximum current, a battery is required with a capacity of 4.185/3 = 1.395 Ah.
If other types like high drain batteries (10C) are applied, their capacity should be 4.135/10 = 413 mAh.
Conclusion
You need to take care about the inrush current at startup of motors (about five to eight. times the nominal motor current) , and is usually not an issue for big power supplies but for low capacity accumulators may be a problem. The 1100mAh battery it's on limit for the current required for that motor. So, if the motor use a 15V power supply, you will need to combine 15V/1.2V= 12-13 elements. If the motor will be frequently switched on and off you should consider using larger capacity batteries.