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If it's raining though, I don't want it connected. I want it all safely packed in a bag with the weather cover over the USB port on my Garmin. My plan was to charge the battery during the day and then disconnect and use only the light at night with the battery alone powering the Garmin.
hippy-
I've used a bit of home silicone sealant around the usb cable entry port of the garmin (with the cable connected), because it won't be leaving the bike (its also secured with a lanyard to avoid anyone trying to grab-and-run). Its easy to peel it off when the ride ends, and I don't have to plug again the usb cable into the Garmin under heavy rain (worst scenarios happen more often than they should)
As far as I know the D1 is waterproof, but @acharnley recommends that you don't let water sit too much on the usb connectors, just to avoid corrosion. So pointing the connector downwards would be enough for the rain (in my case the D1 was fixed to the frame with the velcro straps of the framebag, but pointed it upwards to made easier the connecting/disconnecting of the usb cables.
I also charged the power-bank during the day and used alone at night, but still there where some high-speed long descents at night when there was more than enough power for lights & D1. Just so you don't have to test it... supernova + D1 operating at the same time at night, in a 2-5% climb... you could see enough to ride without problems :D, but don't recommend it :)
bertocq
Pass-through charge is an obligatory feature in my opinion... so you don't have to care about how much charge gps/mobile/power-bank have... just leave it all connected, and only disconnect when you get to a climb that really hurts and don't want to think about the little watts you must be losing with the dynamo dragging.... or when night comes and lights should get all the dynamo power. A switch for those two moments would be awesome :), but disconnecting/connecting a usb cable is not so bad one-handed.