I just installed a battery monitor. The dedicated shore power charger has always maintained both batteries at around 13.3 volts (when I would happen to check them with a voltmeter for any reason). Just had a big weekend of sailing, and noticed that one of the batteries was at 12.6 for a couple of days on the charger, though, it finally came up to the 13 volt norm. Does a battery taking longer to charge indicate it's weak? I'll check the specific gravity asap. Seems like it should have charged overnight, if not motoring back into the Marina. Maybe it's always done this, but I didn't realize it until I had the new toy. Sometimes ignorance IS bliss!
Just measured the Specific Gravity of the battery in question. It's 1.3, the upper level of the good range on the guage. I have no reason to suspect the battery is bad, as it still seems to be functioning fine.
In addition to the symptoms mentioned above, when I sailed last:
1.The battery monitor's alarm (over/under acceptable voltage) started sounding when I keyed the mic on the VHF radio. It would stay on until I turned the monitor off.
2. I had the boat's FM radio tuned to a talk show. The alarm would sound, ONLY when someone was speaking on the show. So, if someone on the radio said "What time did he leave?", the alarm (which usually is a continuous high pitched Beep) would sound; Beep Beep Bip Bip Beep, in cadence with the words being spoken.
When I started investigating, I found a major crack in the batteries positive terminal cable clamp (on the battery that had been showing low voltage in the OP). Fixed it, and Presto! - The battery is showing voltage in the 13's and all the other symptoms are gone! So, the new battery monitor has already pointed out a problem I would not have known existed (exactly why I wanted one!).
I assume the battery wasn't getting a full charge due to the poor Pos. connection (the shore power charger tied into the same connection).
I know a VHF uses more power when transmitting so I guess I understand why the alarm sounded when I keyed the mic. However, I do not understand why the alarm was responding to the voices on the FM. I assume a FM radio demands a certain amount of power to receive a FM signal and doubt the requirement fluctuates because people are talking.