When your Nike+ FuelBand displays the image below, it's time to charge the device.
When the battery is only partially drained, you can charge your FuelBand using any method (computer, car charger, wall adaptor, etc.).
If the battery is completely drained, please only charge the FuelBand by plugging the device into your computer’s USB port. When your FuelBand loses all battery power, the clock stops. By plugging it into your computer, you automatically reset the clock by syncing it to your computer’s time and date settings.
The FuelBand battery will fully charge within 3-4 hours. The battery icon on the FuelBand will be filled when the FuelBand is fully charged. The expected battery life is 1-4 days.