The problem with the 2300c running on battery may not be the Duo, but the battery. Type 3 batteries (I don't know about the BTI type 4) are notorious for draining down to the point that the Duo would not run with one inserted or power off in you inserted it after the computer was running. I suspect they appear to the system as a dead short and a protection circuit kicks in.

Back in the day, I had a little luck with reviving such batteries using a Lind SuperCharger II. These days, I'd put long odds on being able to revive the battery past the point where the Duo would at least run with it inserted.

John