The MAC 600 will only operate if the rechargeable battery is inserted in the ECG machine, even if you are using the power cable, and in most situations, you will not need to remove it. 

The rechargeable battery compartment for the MAC 600 is located on the base of the machine towards the rear. When working with the battery compartment, please ensure that the machine is upside down to enable easy access to the compartment. 

The battery needs to be inserted with the label facing downwards, and the terminal contacts are going into the machine.