It will take years before the cells in a 4x18650 battery pack wear out. I bought some new battery packs this year for my MagicShine 808 lightheads, but the old ones still aren't dead enough to throw away, so I keep them around for household use. The lightheads have been going for as long as I can remember. USB by design, is limited to 5V x 2.5W power output. This is only sufficient to charge the smallest of 'be seen' headlights, and thus USB is mainly used to charge taillights and small ancillary devices that use the tiny internal Li-ion batteries.