Having a Portal Key in your inventory allows you to remotely recharge Resonators deployed on that Portal. Portal Keys can be obtained by hacking Portals.
The efficiency of a remote recharge (the XM cost) depends on your physical distance from the Resonators you're attempting to recharge. The closer you physically are to a Portal and its Resonators, the less XM it will cost to recharge. Also, as you attain higher Agent Levels in Ingress, your ability to recharge Resonators at larger distances also improves.
To remotely recharge Resonators:
- Navigate to INVENTORY and select Portal Keys.
- Select the Portal Key for the Portal you’d like to recharge.
- Tap CHARGE to power up Resonators remotely.