What I am talking about switch off the system is not press the power button once and leave the screen goes to sleep. On Windows OS, goto Start/Power/Shut Down. On Android OS, press the power button till the pop up ask me if I am going to power it off then touch YES.
After shut down the tablet, sufficient power for Pixhawk PX4 and its telemetry Tx/Rx up and running from the USB ports. I connected the the FC and telemetry at the same time to the usb ports only to confirm both the usb port still have current after shutting down.
And, I am not using USB keyboard but the docking one from Chuwi. So, it makes me think if the keyboard drain up the power.
The charger is the stock one for Galaxy Tab 3 which supply 2A current.