Powering 3.3v-5V LED strip with 18650 batteries
I've done similar using a cheap ($3) "USB power bank module". These are designed to provide a constant 5 volts from an 18650 battery. I bought some with "134N3P program" which protect the battery by dropping out when the battery voltage goes down to 3.7v. You can even buy these things in an empty case to put your 18650 batteries in.