BPI-GSM has two power options, USB or external, and it can switch automatically.For external power option,you can choose AC-to-DC adapter(5V,2A) or battery, and the voltage range should be 4.5V - 5.5V, it might be unstable if voltage drops under 4.5V due to the 5V level can not be guaranteed on I/O pins. If the voltage exceeds 5.5V, power regulator may trigger thermal protection, more harm would be to BPI-GSM if over-voltage continues. So the recommended operational power is 4.5V-5.5V, 2A rating.