A programming session can keep the ignition on long enough for the battery voltage to fall. A battery support unit holds the supply at a set voltage and provides the current the vehicle draws, which is separate from the programmer that reads or writes the control unit.
Match the unit to the system voltage and to the highest current expected with the ignition on. A 12 V car and a 24 V truck do not use the same setting. Follow the manufacturer’s connection order, and do not use an expert power-supply mode unless the voltage and current limits are known for that job.