Both the PurePro2000 and PurePro3000 inverters need to be connected to batteries to operate, even in standby mode.
It can't work from a power supply, such as the power supply mode on BatteryCharge25, as this can't supply enough current to charge up the internal capacitors inside the inverter.
This is considered normal behaviour, as you would not want to be supply an AC load if the inverter was not ready to support it.