It may sound bad but I don't know?
Is there a battery in the slave unit? I've never seen the slave boards.
I got a brand new "old" AA set still in the box.(G scale) I figure I should charge it since it's been in the box for years. 2004!
http://mthtrains.com/70-2007-1
I was going to use my external charger to keep the unit un-run. Then it hit me. What about the trailing A???