Probably a fault but I don't have your equipment so don't take my word for it.

I'm concerned that the packs were dead after 3 months - is that right? This should not happen with Lipo's or Lion's which is what Duralites are. Some of the older ones were tadirans. In any case if the voltage is allowed to get too low they die. These chemistries have excellent charge retention over time - like a % or 2 loss per month. If they are dead after 3 months storage you may have a problem with the way things are wired - there is a current leak somewhere such as having the switch between the regulator and the Rx instead of between the battery and regulator. The regulator would draw some small amount of current even with no load on the output side. Flying and charging every week or two or three might be fine but aft a month or more the pack gets discharged to far.

Again, I don't use this stuff for Rx power. I do know a lot about batteries though - been flying electrics for a long time...