I'm using an Arduino Uno with the Adafruit Data Logger Shield powered by a Voltaic 5V 2A battery pack. When writing to an SD card, the time stamp never changes (always reports the same now.unixtime). However, when powered from a laptop, the time stamp is correct and everything seems to be working.
The input voltage from the battery is confirmed to be just over 5V and not substantially lower the the laptop usb port.
Has anyone seen this before? Any ideas?