A temporary phone charger or other accessory cord can easily be made using a two-wire trailer plug that fits into an on-board battery charger plug. You can feed it into your tank bag or even pocket if you remember to unplug before you get off and walk away.
With earphones you can keep your maps program going as long as it's needed and your phone will stay charged for when you arrive. I made mine with a female lighter/accessory plug so I didn't need to modify my phone charger wire.