[Al Skierkiewicz]

I agree, putting a fully charged battery in parallel with a discharged battery will cause significant current to flow. Think that the fully charged battery will try to supply charge current to the depleted one while also delivering current to the load. Schottky diodes are available for battery steering that have a lower forward drop. RV stores carry them and they are meant for much higher current than you might encounter. This would allow you to put a battery on line and take a battery off line without an issue.