  The LISA Pathfinder charge management device was responsible for neutralising
the cosmic ray induced electric charge that inevitably accumulated on the
free-falling test masses at the heart of the experiment. We present
measurements made on ground and in-flight that quantify the performance of this
contactless discharge system which was based on photo-emission under UV
illumination. In addition, a two-part simulation is described that was
developed alongside the hardware. Modelling of the absorbed UV light within the
Pathfinder sensor was carried out with the GEANT4 software toolkit and a
separate MATLAB charge transfer model calculated the net photocurrent between
the test masses and surrounding housing in the presence of AC and DC electric
fields. We confront the results of these models with observations and draw
conclusions for the design of discharge systems for future experiments like
LISA that will also employ free-falling test masses.
