"Rick" wrote

I agree with carrying two batteries. Go to west marine and buy a
battery combiner. With one of these you can keep the spare battery
fresh and fully charged. When the starting battery dies move the
spare into full time duty and buy a new spare.


For a big engine large starting loads or no other way to start it,
sure. On a power boat where there is no other propulsion, absolutely.

The 20 hp diesel in my boat is a very small electrical load. People
have even started them with a handful of flashlight batteries in a
pinch. With both batteries tied together and doing everything, the
drawdown cycle from ship's service loads will only be half the
discharge depth for both batteries. Since these will be expensive
AGM's on my boat, that's significant in battery life.

I make it a practice to always start and warm up the engine before
sailing (no choice in our marine dock) even if sailing off a mooring
or anchor. Then I know it will be ready to go in an emergency. With
the engine as an auxiliary, it makes sense for me to swap the
redundancy of keeping a dedicated starter battery for the longer life
of smaller discharge depth.

Plus, I have a dead simple system.

--

Roger Long