I agree that a couple of diodes are the way to go to place the
batteries in parallel without allowing one battery to cause problems
with the other. However the key is the voltage drop which is typically
0.6vdc for a power diode. Do you have a specific diode number that you
have been using?

I am using a 302. I could have a separate battery for recorder backup
but I would rather come up with a more generic solution for everyone
that powers everything.