hey marcos,
ok that makes sense, yes like brewski said the 24v signal your reading is just the power supply for the display itself. The BDI monitors the actual battery voltage through a different circuit and displays it through the monitor display.
They explanation you gave sounded like you were checking the battery voltage at the bdi and it was 24v.
Now since the battery is 36v as you say you should check that voltage at the battery and see what it is, a 36v battery charged should ready 37.5 to 39v fully charged, depending on the condition of the battery acid and age of the battery, Older batteries tend to read on the lower side.
In either case you can try to lower the reset voltage on the BDI controller and it should reset, as batteries do get older it may be necessary to adjust that reset voltage to compensate for lower voltages on older batteries because they do not reach the higher voltage they once did when they were new.

I'm sure since you may be calling a tech out he may have to resort to this to make the BDI operate correctly for that battery or hopefully it's just a poor connection in the harness somewhere keeping the BDI from reading the battery voltage correctly.