Interesting article that was linked to,, although their assertation of a 1/2 volt drop was somewhat extreme, a more common voltage drop, with short cables, (if they're of good size,) would be ~ 1/10 of a volt,, perhaps a bit more.
Anyway, ran two batts in my old square-body for years with no issues, and the batts were off diff ages and capacity, and never had any issues.
Lots of larger boats have 2>6 batts connected together, and go for years of service.
It sounds like there is something not quite right with the 2nd batt that is beyond a simple CCA or date mismatch,, that alone would not cause two batts, (that were well charged,) to "go dead".
That article was directed at the motorhome community, which in many cases, might have batteries mounted several feet from one another,, like one next to the engine, and another maybe 10 feet or more away,, like some guy with a big stereo system that puts a second batt in the trunk of the car,, it works, but to keep the voltage drop down, (within 1/10 volt,)you need huge cables, like big welding cables.
The use of a solenoid or a manual switch is a good idea in any case.
Anyway, ran two batts in my old square-body for years with no issues, and the batts were off diff ages and capacity, and never had any issues.
Lots of larger boats have 2>6 batts connected together, and go for years of service.
It sounds like there is something not quite right with the 2nd batt that is beyond a simple CCA or date mismatch,, that alone would not cause two batts, (that were well charged,) to "go dead".
That article was directed at the motorhome community, which in many cases, might have batteries mounted several feet from one another,, like one next to the engine, and another maybe 10 feet or more away,, like some guy with a big stereo system that puts a second batt in the trunk of the car,, it works, but to keep the voltage drop down, (within 1/10 volt,)you need huge cables, like big welding cables.
The use of a solenoid or a manual switch is a good idea in any case.
