Rather than hack your bike wiring, or risk running down your starting battery, how about this $30 PowerBank from Amazon:
https://tinyurl.com/ybmcl37b I have one being delivered today. FOUR HOURS LATER, UPDATE:
GOODs: Unit is sturdy and the flashlight feature could come in handy on the road.
BADs: Unit is larger and heavier than any phone: 3.25" Wide, x 6.75" Long x 0.75" Thick.
"Carbon Fiber" cover is hard and slick, making it very hard to hold on to.
SPECS:
1.Portable USB charger with huge capacity:24000 mAh -your cellphone will have power for
days.
2.Charge three devices (Phone, Tablet, GPS) at the same time.
3.Digital remaining-charge display. Power Bank recharges overnight from AC.
4.Dual flashlights. Portable safety and enduring talk power. Never be in darkness or out of touch, ever.
5.Warranty:2 years.