The RF-driven negative ion source testbed BATMAN upgrade is being developed at IPP Garching in the framework of the ion source development for ITER and DEMO neutral beam injection systems. The testbed has recently been enhanced to allow for steady state operation with a focus on beam optics studies. The previous titanium sublimation pumps and inertial calorimeter limited the beam pulse length to about 6 s every 3 min. The upgrade comprises a long-pulse compatible, actively cooled diagnostic calorimeter. This has been designed and is currently being manufactured to substitute the inertially cooled calorimeter that has limited diagnostic capabilities. The new diagnostic calorimeter consists of a copper plate with dimensions of 910 × 660 × 25 mm3 placed about 2 m from the ion source extraction grids, and through a novel solution, it will provide a 2D profile of beam power density with a 20 mm spatial resolution. Water flowing through cooling channels embedded in the copper plate will actively cool the calorimeter, which is loaded with about 160 kW beam power at ITER-relevant current density, but 45 kV acceleration. A fraction of the beam will pass through many small apertures (ø2 mm) positioned in the calorimeter plate and will be collected by thin (0.2 mm) copper foils attached to the calorimeter back side. Evaluation of power density will be performed by measuring the temperature of the heat flux foils with a high-resolution infrared camera observing the calorimeter from the back side and calibrated by thermocouples.