All Powermax Treadmill needs 230V single phase ac power supply to run the treadmill. There is a convertor in treadmill which converts input ac power to dc power as motor inside the treadmill works at 180V DC.
Now lets assume you have Powermax Treadmill model TD-N1 which use 1hp (745Watts) dc motor. 745 Watt motor means it will run at its rated speed when it consumes 745 Watt , that means the rated current of the motor is (745/180)= 4.13 A , so to run the motor you will have to ensure that the battery is capable of supplying that much amount of current or the current capacity of the battery should be atleast 4.13A. So if the capacity of battery is less than the rated current of the motor , motor will not run properly it will get less current since battery itself is not able to supply that much amount of current. Say If you have 12 Volt 6 A battery (6Ah) , than you should connect 15 such batteries (15X12 =180V) in series so that Voltage is 180 V.
So 15 such batteries connected will run your 1Hp (745watts) motor treadmill for one hour minimum.
So the invertor you have selected should provide 230V ac with 6A output as its maximum nominal output current.