[Lynn Eades]

First,I dont know anything about a ferret charging system.Does it have a generator or an alternator?
If it has an alternator generally speaking an alternator produces ac current which is normally converted to dc via a "bridge rectifier".
The bridge rectifier normally has six diodes in it, and only requires one to leak, to slowly run your batteries down.
This said, your isolating switch will stop this happening if it is fitted correctly.
Here is something that might not be relevant to your ferret, but might help someone somewhere. You shouldnt "tap off' 12 volts from a 24 volt system, for instance, to run a 12v radio. In this situation, one battery ends up half flat, while the other gets the guts boiled out of it.