Pixhawk rebooting repeatedly

I am using a Pixhawk with PX4 1.8.1 firmware on my quadcopter. I am powering the Pixhawk using a power module connected to 4S LiPo battery. I want to connect the Pixhawk to an onboard computer using a USB wire. Since it is suggested to not connect two power sources, I cut the +5 V wire and connected to the Pixhawk. But when I do this, the Pixhawk keeps rebooting. How can I solve the issue?