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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?

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Where are you brining the power onto the Arduino board? If you bring it in through the external power connector you need to account for the voltage needed to run the regulator. You might be running into problems with low voltage and some part of your system that is sensitive to it.

You could try bring the power in on the 5 V pin, but then you are taking responsibility for the quality of the power supply.

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The problem was indeed a question of voltage.

I was using the battery to supply 5 volts to the DC power port, which as suggested by the comments needs 7 volts or more. The problem is fixed by using the battery, which supplies a stable 5V, to give power to the USB port (or, presumably the 5V pin, though that is not available in my application).

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If you are putting just over 5V into the Vin, it goes through the on-board regulator and needs a 7v minimum per https://www.arduino.cc/en/Main/ArduinoBoardUno The Adafruit Data Logger Shield doc at https://learn.adafruit.com/downloads/pdf/adafruit-data-logger-shield.pdf shows a 9V battery pack.

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