I'm heading out on a ride tomorrow, and seem to always have cell phone battery issues with GPS trackers, especially if I keep the screen on for any period of time.

My solar charger is a bit awkward, and seems to work better as an external battery pack than an actual solar device.

So, one of my headlamp batteries also has a USB port. I thought I'd check, and it seems to be putting out about 8.38v (no load) at the USB port. The pack would be extremely easy to tie to the handlebars.

However, that is quite a bit more than the typical 5V that USB is rated for.

Safe or not? Currently I have a couple of older Samsung phones, but I'd also rather not fry them, or their batteries.