Arduino not transmitting when connected to LiPo

Hi, I am running a sketch with nrf24l01 which runs (transmits data) when I have the USB connected but does not work when I supply power from an external 7.4V LiPo.

When I connect the LiPo the board gets powered on as the On green light is lit and the L light (orange) flashes and then goes off (like usual). The tx light never flashes!

I have the same LiPO battery connected to the Rx side and it is working fine. I also changed the battery on the tx side with a fresh battery but it is still not working.

Bad connections? How do you connect the LiPo?

Tried connecting LiPo using the Vin pin as well as 2.1mm jack. I measured the Voltage across Vin and ground pin it measures 6.5V with battery connected.

sagargarg:
Tried connecting LiPo using the Vin pin as well as 2.1mm jack. I measured the Voltage across Vin and ground pin it measures 6.5V with battery connected.

With the LiPo connected as above, measure the voltage between the +5V pin and ground. Maybe the regulator is faulty.

Edit: Which Arduino board, out of interest?

OldSteve:
With the LiPo connected as above, measure the voltage between the +5V pin and ground. Maybe the regulator is faulty.

Edit: Which Arduino board, out of interest?

Arduino Uno R3. It measures 5V! don't understand what is the problem.

sagargarg:
Arduino Uno R3. It measures 5V! don't understand what is the problem.

Neither do I. I was betting on a dead regulator.

It is not power supply but Pin 13/12 issue. This thread can be closed.