Perform an Alternator Output test on a Cat Skid-steer 

While this test is done on a 12v system on a skid steer, the operation will be much the same for a 12v commercial vehicle as well as a 24v piece of mobile equipment. Make sure to lookup manufacturer specifications and watch the amount of load that is placed on the battery to keep the vehicle from stalling.
This test is helpful if the vehicle regularly needs a boost and when voltage is checked it is good but the battery never seems to charge.
Keep in mind that a shorted rectification diode will cause a 2/3 current loss while an open diode will reduce the output by 1/3. If your loss does fit into one of those 2 losses, look to the regulator or a failing rotor.