Switching on bluerov using battery

I tried to switch on my bluerov2 using bluerobotics battery. The pixhawk firmware installed was the latest version(3.5.4). The latest version of Qgroundcontrol was also downloaded and the ROV all worked fine. But each time, I switch on the ROV (when it gets power) suddenly all the lights blinked and thruster showed a slight movement. Then again it started working normally in the QGC. Can anyone explain the reason for this sudden blinking of light and sudden movement of thrusters (very very slight movement,not even half a rotation), when I connect the battery? Can I somehow change this by updating the pixhawk firmware??