Battery/Electrical help requested

[SailorChlud]

Good Day:

Requesting help determining if my battery is bad, or my Blue Seas ACR module is bad, or what?

This weekend I took out my Seafari after it sat for 3 weeks unused, and noticed that my 2nd battery was dead. It is 3 years old, and had been functioning properly up until now. Upon testing, the voltage read 8.8 volts DC. My starting battery was fully charged (I keep that one on a solar trickle charge).

The boat is a single outboard with two batteries - starting and auxiliary, and is wired according to the attached diagram, with a Blue Seas Automatic Charge Relay installed between them

Before I buy a new battery, it occurred to me that I should test that my outboard is outputting the correct charge, and the Blue Seas ACR is sending the proper current to both batteries. Does anyone know how to test that? I have ammeters and voltmeters.