Originally Posted by
noglider
On long rides, I keep a small external battery in my tool bag and run the charging cable to my phone which is on the handlebar. (My tool bag is under my saddle.) This past Sunday, I rode 60 miles. At the end, my phone's battery was still at 100%. Not only is it convenient not to discharge the phone's battery, it is good for the phone's battery's life. Discharging and recharging wears it down.
Actually high heat and high voltage (i.e., charging to, and storing at, 100%) are the most taxing to lithium ion batteries.
How to Prolong Lithium-based Batteries - Battery University
This triangulates with a military study on different cycling habits with 18650 cells -
clicky