The amount of power that a reader will need depends primarily on the antenna. Many antennas will operate between 1 and 2 amps with a few a little more than that. Antennas over 3 amps may cause the reader damage due to overheating.
The datalogger constantly measures and records the amperage so we recommend setting up a system using a portable battery. Once the system is built and the current is known, the appropriate power system can be designed to provide it.
HDX readers use a charge/listen cycle and so the amperage is constantly changing. The datalogger also calculates the effective amperage which takes into account the duration of the charge and listen periods for the overall power consumption.
With the multiple antenna reader, each antenna may have a different power level. The datalogger calculates the effective amperage for all antennas that are running. Because only one antenna is on at a time the consumption is still under 3 amps.