Here's what the FAA recommend :
a. Class 1 EFB Hardware. Class 1 EFB hardware may:
• Be used on the ground and during flight
• Connect to ship’s power through a certified power source
• Recharge batteries onboard the aircraft
• Require quick-disconnect from power and/or data sources for egress
• Have read-only data connectivity to other aircraft systems
• Have receive/transmit data connectivity for AAC only
(1) The operator should provide evidence to the PI demonstrating that the Class 1 EFB is properly stowed or mounted for takeoff and landing.
(2) Certificate holders should document EFB non-interference compliance in accordance with the guidance in the current version of AC 91.21-1, Use of Portable Electronic Devices Aboard Aircraft.