Need more Power!
https://www.amazon.com/mophie-Power-...for+cell+phone
these devices come in all sizes & prices. This one would probably boost/charge the ereader several times before it needs a re-charge! I have one that will Fully charged can charge my Kobo ereader .... cell phone & 2 Fires before it needs to be charged back up!! How long the battery in any device with lights lasts depends on how high/bright the lights are.