Originally Posted by al40
Avoid blocking diodes at all costs...
Nonsense. We've had this conversation before.
Your notion of a battery never reaching full charge is not correct and it presumes the battery with the diode is initially fitted in a partially discharged state. Fitting a fully charged battery and then using a diode to preserve the charge works fine. Even if you accept the voltage drop as 0.7v (which it usually isn't) you'll still have a charging voltage of approx 13.7v which is just fine for a float charge. If you're wise, you'll choose to use a schottky diode which has a voltage drop of approx 0.4v.
If you end up with a really difficult situation and you've partially discharged your backup battery too, all you need to do is ensure your switch is in the correct position to recharge it fully.
All these suggestions of switching....what a hassle.
I use a diode with complete success. Both my batteries are now over 4 years old and still work fine and they are starting a big diesel engine not a little outboard motor so I guess they're still delivering satisfactory current.
I have a 1-both-2 switch but I never switch it, it's always on both but I also have an isolating switch for the battery with the diode so it's the equivalent of always being on 1. During my boat build I felt it may be useful to have the switching facility but I've never used it. Some folk say it gives safety but I feel it's very little - after all, do you disconnect your car battery each time you park?
Originally Posted by BruceB
All the boat is to be run off the one battery with the second only for backup.
I think that is the way to go. Personally, I'd use two simple single switches; one as a main switch and one in series with the negative connection of the backup battery with the diode wired across it. Dead simple, no switching necessary in normal use and both batteries remain charged.