Here's how I'd do it in your situation. Grab a Forumslader, if you have the technical skills, make it up into a PVC tube the size of an old school bike pump with 18650 batteries. Get some SKS pump clips to store it on the down tube but easily take it into your tent. Otherwise get Jens to make it up with 18650s in heatshrink like he usually does, that can go in your handlebar bag. Get some waterproof 2.1mm dc connectors from ebay to connect it to the bike wiring. Grab a USB boost charger so you can charge the forumslader from AC, use it as a powerbank. Get the IQ-X AC to DC circuit and fit that to your light so you can run the light of the 12V lighting circuit of the Forumslader so you can have lighting and nearly full charging (less the current running the light)