You can recharge portals as you encounter them or remotely, by holding their Portal Key 
and being near enough according to your Access Level. Walking to the portal is an easy way because you also get XM in the process to compensate what you lose. Recharging portals gains you some AP and badges, and also allows you to consume some of that XM which you collect while walking but would otherwise waste if you're full.
Recharging Attacked Portals
Recharging portals remotely may need some planning:
- if the portal is of high level, you will probably need power cubes to recharge yourself in case of attack;
- if the attacker is powerful, you may need to predict their arrival before they attack or you won't charge quickly enough;
- in any case the alerts will give you a quick link to the portal recharge (you only need to wait for a fix to the portal), but that's useful only if you have a key.
Finding Portals to Recharge
The easiest way known so far is to click OPS > inventory > [menu] portal keys, then browse the keys and, when you see one for a portal of your faction, check whether the energy bars of the respective resonators are full. If not, just tap twice and you'll be able to recharge. This process is rather tedious but comprehensive.
Medal
- Main article: Recharger
Most players will agree that Recharger is the easiest Black medal to achieve, as agents typically receive a surplus of Power Cubes from hacking.