Bluetooth is soft blocked when running on battery

My bluetooth adapter gets soft blocked when my laptop is running on battery. Using AC everything is fine, but as soon as I unplug it, bluetooth gets blocked and cannot unblock it.

The bluetooth adapter in rfkill list is dell-bluetooth: Bluetooth. Running rfkill unblock all results in temporary appearance of another bluetooth adapter (hci0: Bluetooth) in rfkill list, which disappears in a few seconds.

Running rfkill unblock all 2-3 times unblocks it, but after a few second it gets blocked again.

How can I find what is blocking it?

after issuing rfkill unblock buetooth dmesg gives the following:

 2352.829492] usb 1-8: new full-speed USB device number 26 using xhci_hcd 
 2352.979134] usb 1-8: New USB device found, idVendor=8087, idProduct=0a2b, bcdDevice= 0.01 
 2352.979153] usb 1-8: New USB device strings: Mfr=0, Product=0, SerialNumber=0 
 2352.982885] Bluetooth: hci0: Bootloader revision 0.0 build 2 week 52 2014 
 2352.989924] Bluetooth: hci0: Device revision is 5 
 2352.989938] Bluetooth: hci0: Secure boot is enabled 
 2352.989942] Bluetooth: hci0: OTP lock is enabled 
 2352.989947] Bluetooth: hci0: API lock is enabled 
 2352.989950] Bluetooth: hci0: Debug lock is disabled 
 2352.989954] Bluetooth: hci0: Minimum firmware build 1 week 10 2014 
 2353.044130] Bluetooth: hci0: Found device firmware: intel/ibt-11-5.sfi 
 2353.145646] usb 1-8: USB disconnect, device number 26 
 2355.165633] Bluetooth: hci0: command 0xfc09 tx timeout 
 2363.326232] Bluetooth: hci0: Failed to send firmware data (-110) 
 2363.326381] Bluetooth: hci0: sending frame failed (-19) 
 2363.326431] Bluetooth: hci0: Intel reset sent to retry FW download

Finally (after several days of searching) I have found, that it was Laptop Mode Tools. Edited conf file and now everything is fine.