If you want to keep control over the portals you capture, you will need to recharge their resonators. When deployed, resonators will lose 15% of their max XM every day. Seven days without recharging will cause all fields, links, and resonators to decay and the portal will become neutral again. Fortunately there are 3 different ways to recharge resonators:
Recharge Resonators on Portal Info Screen
The most common way to recharge resonators when you are in the action range of a portal is by using the Recharge button in the portal info screen. It will switch you to the portal recharge screen where it will show you all resonators on the portal and a bar with each of their current xm levels. By clicking the recharge all button you will evenly disperse the 1000XM between each of the resonators not currently charged. You may also click on a single resonator (if one resonator is lower than the other or if an enemy is hitting one harder than the rest due to their attack vector) and the entire 1000XM will be given to that resonator (if it takes less than the 1000XM to charge the resonator fully, you will be refunded the excess XM).
This is useful when you have upgraded a damaged portal and are recharging the remaining damaged resonators which were previously deployed.
Remote Resonators Recharge:
The most convenient way to recharge resonators is with the remote recharge. To use it you will need to have a Portal Key for the portal you wish to recharge, and over 1000 XM. The cost is the same as recharging by physically being at the portal, but it’s less effective depending on the agent’s distance from the portal. When you use a portal key, it will show you the map view of that portal and the option to recharge will be active if the portal is in possession by your faction and if the portal’s resonators need to be recharged. The difference between regular and remote recharging is range efficiency. When you are near the portal your range efficiency is always 100 % and it’s not displayed. When you remote recharge using a key, your recharge will not be as efficient. If you are remote recharging within your own town or city, the efficiency is typically over 95%. This is very handy for maintaining long distance links and defending your portals from attacks, though you will need a lot of XM to support huge numbers of portals in this way. This is currently the only active defensive strategy in Ingress.
Remote Recharge Efficiency Formula:
Efficiency (%) = 100 – (distance in km)/(5 * Player Level)
The base cost for this action is 1000 XM but if a portal or resonator that has been recharged needs less XM, what’s left will be returned to your scanner.
For every recharge you will get same amount of AP:
Practical Reasons for recharging:
The circle of XM around a portal that forms around a portal every 20 minutes, is equivalent to 10% of the current energy in the portal. So letting a portal go uncharged for a few days will mean that you are able to claim less xm from it.
Recharging is a great way to defend your portal and to gain achievements. For some very important portals you will always want to have a key with you so you can perform a remote recharge.
If you want to gain the most AP per XM from recharging, use the single resonator recharge. This is very useful if you are a low level agent. Here is one scenario: You are L1 and you created first L1 portal with 8 resonators. The next day, because Resonators decay, every resonator has 15% less XM. That is 150 * 8 = 1200 XM missing, you can do 2 recharges and gain 20 AP and use 1200 XM. But if you go and recharge the resonators one by one you will gain 80AP for 8 recharges and still use same amount of XM.