There are two types of battery mods. The first is normal, and charges while attached to your phone. The second charges both normally like the first, or wirelessly. Unlike a battery case, which is bad because it lowers the recharge cycles of your phone by charging your phone when it is activated and not stopping, the mod is attached, and the phone draws power from the mod, and after the mod has no more power to give, the phone utilizes it's own battery. That is if you attach the mod to the phone when it has sufficient battery life (16% and up) or utilize said mod 24/7. If your phone's battery is at what is considered to be low (15% and below) the mod will automatically charge the phone's battery. So it is like a hybrid car using an electric motor for a certain extent, then switching to its petrol engine.