To keep your V3i kit working its best, be sure to change your cartomizers often, even when refilling. You should change to a new cart every 5 days or so.
Also, when charging, be sure not to leave the battery for your V3i charging longer than is needed, such as overnight, as this will negatively impact the battery's lifespan.
You may also want to clean the connections on the battery and charger, to prevent a buildup which will eventually stop a solid connection being made. You can do this by running a pin around the inside of the connection, as shown in the video below. While doing this, you can also gently raise the connection pin on the battery, to ensure that it always has a good connection with your carts or charger.