T&T: Is it time to replace our 4D AGM Start Battery?

[Pete Balkus]

The 4D AGM "start battery" on our boat is ~7 years old now. 

This single 4D AGM "Start battery" provides power for the: 
    - Engine starter - Cummins 330 hp diesel
    - Engine Pre-heat
    - Bow Thruster
    - Windless

I am beginning to notice that when I use the bow thruster either leaving the
dock or when returning (pre-heat now off and battery fully charged) I am
seeing the Cummin's panel volt meter drop and getting a "low voltage" alarm
from the Cummin's panel. Power connections on the Bow thruster seem fin and
the motor was rebuilt (new brushes) in 2006.

Note - I have no problems starting the engine, although after starting and
running at 600-700 rpms the battery remains voltage dipped (no alarm) until
the pre-heat shuts off (~10+ min) or I bring the rpms up to ~1000 rpm to
allow the alternator to output some current.

While I do not have access to a Battery Tester that can verify the raw
amperage that the Start Battery can still deliver it would seem to me that
given the amperage required to run the Bow Thruster, a battery of such age
may no longer be capable of supply this kind of current, (possibly due to
increased internal resistance?).

Our House Bank with (2) 4D's in parallel seems fine since the load there is
much lighter (e.g. no high amperage devices) 

So with this background in mind I would like to inquire:

- What is reasonable life expectancy of a 4D AGM battery used under loads
like our Start Battery (just curious)?

- Given the symptoms described does this seem like a weak/aging Start
battery?

- When replacing a 4D AGM Battery, any recommendations on brands to either
go with or stay away from?

     I have been looking at a Lifeline GPL-4DA as a possible replacement
candidate...

As always, advice and insights from other who have experience in this area
will be appreciated.

Pete

Braveheart