Midnight Blues wrote
When booting, device is using more power than in normal work.
If battery cell has little to high internal resistance (result of aging) than voltage drops under certain limit and device resets.
Final diagnosis could be made with voltmeter connected to battery at booting and see if voltage drops too much (even for moment), but anyway it is (almost) certain reason for your problem.
It's a totally accurate description of your problem. So either install a new Battery, or just accept it as it is and continue using it on external power