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What make/model of OB do you have?
In terms of charging deep cycle batteries from the OB - their chemistry is no different from that of starting batteries - they just have larger, denser plates meaning that they can resist being deeply discharged in multiple cycles without dying. (I'm assuming you've got standard wet deep cycle batteries rather than AGM or Gel batteries).
So given that your OB already has a rectifier and regulator suitable for charging a starting battery, that same system will also work on a deep cycle battery.
If your OB really doesn't have a rope start (some have the rope hidden under the lid) then you are right to worry about keeping a redundant electric source for starting the engine if the house batteries are drained. In this instance, yes you will be able to use the OB to charge both banks, but you'd need some sort of isolating diode to allow charging of the banks, but not allow draining of the starter battery when running the house circuits.
Basically you'd be looking at a charging system every bit as complex as that on a large boat with an inboard - there are plenty of threads on that line....
p.s. I'd be really surprised if there was no way to start your OB manually.
Last edited by BritAbroad; 08-19-2007 at 11:31 PM.