This subject has aired before and I was successful in my first RCT715 with an external battery pack linked to 1040 running on V19.22. This particular unit has battery issues (cannot hold charge) so the external battery was a good option. I had the chance to get another RCT so after updating both new RCT and Edge 1040 (I am on Beta Program so now at v20.13 and I see some new Betas had updates to Varia battery icon when on external power...early versions had an empty battery icon when connected to ext battery and "live" so I was hoping that as requested, Garmin had done an update to show a charging status for the Varia when on external power), I tried to use external battery on the new RCT.
Anyhoo, long story short, now 1040 on latest Beta v20.13 and when I connect the same external battery that worked before, the new RCT now just switches off, disconnects from the 1040 and flashing green light starts up to show its charging...I cannot get the unit to switch on and re-connect to the 1040 while the ext battery is plugged in...worked before on the older RCT and the 1040 on standard public firmware
