Originally Posted by
Skipjacks
I keep something like this
https://www.amazon.com/HAME-11000mAh-Metal-Portable-External/dp/B00NTWQE2O/ref=sr_1_15?ie=UTF8&qid=1537534469&sr=8-15&keywords=15000%2Bmah%2Bbattery&th=1
in a top tube bag. (These comes in all shapes and sizes and weights. From a couple ounces for enough battery to charge a light to a 2 lbs brick that will charge a cell phone and all your lights twice.)
If a light goes down while I'm riding I keep a USB word with me and just plug it in right on the bike while I'm riding.
The only catch is this only works if 1) The light works while charging. I know Cygolites do. and 2) The USB port is accessible while mounted. My Cygolite Dash and Hotshot can be plugged in while mounted. My Cygolite Hotrods have the USB port on the back of the unit that pressed up against the handlebars. They can't be charged while mounted to the bike.
The whole idea of the USB lights is to have clean lines without the external battery. I don't have a problem with external batteries but why spend the extra money for the USB's cleaner lines?