Great, and a good day (it is really early in the morning for you)
assuming that you did not weld on your vehicle with the battery connected the following....

did you, measure the battery voltage with nothing on (contact off)
readout ?? does it drop ? if so, or the battery is broken, or some of the electric wiring what transport current/voltage before contact is switched on has a short. (possible rust)

contact on, does the voltage drops ? how much ?
after starting the engine, does the voltage raise ? how much ?

a battery with 6 cells if empty voltage should be 12 volts (2 volt per cell) if lower the battery is or in bad shape or on of the cells are broken (sulfate)
if the voltage does not raise after starting the engine, (suppose to go to 13/14 volts) probably the brushes are bad and need to be replaced. if the voltage goes way beyond the 14,5 volts you should check the regulator (inside the alternator)
in the FMS, you can find how to replace those parts.

I will hope this will bring you to an solution

Thomass