You can recharge from an external USB battery if it's more convenient. I do that with my Light & Motion Urban 500 on long nighttime rides. I leave it hooked up to the bike, and just run a USB cord to a largish Jackery battery that can recharge all of my devices before the battery itself needs recharging -- iPhone, Cygolite Hotshot taillight, both small helmet lights, Ion Speed Pro video camera.

My hybrid bikes have small pouches Velcro-strapped across the handlebar to hold my USB battery and other stuff. On my road bike I just put the Jackery or other USB battery in my helmet, hanging from the handlebar, while I'm resting with friends for a snack, beer, coffee, whatever.

So, no big deal if your bike isn't parked near a computer, outlet or handy source for USB recharging. A Jackery or similar high capacity battery costs $15-$25. I have a few cheaper, smaller and lighter batteries but they're good for recharging one device, once a night. The larger batteries last longer between recharges. Get one with two or more ports and you can recharge all your devices at the same time while you're at work, wherever.