Looks like you are using a battery tester like , which is good for determining overall battery condition. I would tend to agree with those that point to an alternator issue. A functional failure with the alternator has surfaced. The functional failure being defined as "inability to adequately provide/sustain charging capability to the electrical system". That failure mode could be caused by a number of faults, One could be simply a slipping belt condition, which is easily fixed. Take a close look at the drive belt to see if you see any signs of fatigue (stress cracks along the webbing). Change the belt if you finds this failure mode. Most direct alternator failures are addressed by a new alternator; rebuilt parts industry has made this somewhat economical to us DIYers. . You could remove the alternator & have it checked by a service shop that specializes in alternator & starter rebuilds. These shops have the capability to check the alternator under load. I've got one near to me that I've used occasionally. Good luck with your ongoing diagnosis & repair.