You can recharge any portal that isn't at a full charge that is held by your faction if you have a key to it and some spare XM. Open up your inventory (tap OPS in the top right, then tap Inventory in the top menu). Tap on the bottom left button, then tap "Portal Keys". Tap a portal owned by your faction that has at least one resonator that isn't at a full charge, tap "Recharge" and tap "Recharge" again. You get 10 AP for recharging all resonators once (by tapping "Recharge All") or 10 AP if you recharge a specific resonator (tap a resonator, then tap "Recharge").