I have a similar question, but it's different.
I have (2) deep cycle 200ah Interstate 31M-AGM House batteries in parallel and a dedicated Interstate SRM-24 Starter battery with a 2GM20F. The starter battery seems like it may be bad, although it is brand new. The slip I'm in has no shore power at the moment, and I am using a small 30W solar panel to trickle charge it. But, even after the panel shuts off the charge (fully charged), the battery doesn't have enough oomph (cranking AMPS) to turn over the motor. The House batteries start it just fine, these are also charged from a 100W different solar panel. Now the question is whether the battery is bad, or I also read about too much impedance in the cables. I tried using a jump-start battery pack that said it would start up to a 3L diesel motor to no avail. The Starter battery voltage after the solar panel kicks off is ~13.3V. So it seems the voltage is fine, but no/low current is coming out of the thing. After one attempt of trying to start (again, it won't turn the motor over), the voltage drops to ~12 volts, which again makes me think the battery is bad. I think the jump-starter battery isn't working because it perceives the battery as being fine because of the nominal voltage. It's a new boat to me, so I don't know the history, but the cables "look to have been replaced" not long ago. Although I can't be certain.
Any advice is appreciated.