Not charging

[Ramgarza155]

I have a Mahindra 3525 that is not charging new battery I installed the battery it would turn on had it running for hour then the next day it was dead I had to put jumper cables for it can start when the battery is at 12 volts the volt slowly go down when running then at 5 volts that’s when it dies I put the voltmeter while it’s running the voltage doesn’t go up I bring the rpm to 1500 still doesn’t go up I put a new alternator too but still doing the same thing check the fuses there good has a new ground wire too I checked the voltage at the back of the alternator it’s the same voltage as the battery the red light on the instrument cluster only turns on when I’m jumping the battery what can be the problem? And also the ksb relay might be bad bc I did a ohm test on the relay get no reading idk if that matters with the alternator it runs with the relay disconnect