The best way is to solder a short on the two middle USB pins (on the LARGE USB connector) inside the charger. I have also done it externally with a small bit of aluminium foil. Again, you want to short out the two center pins in the large connector. Carefully glue the small piece of foil over the two center pins in the charger. Plug the cable into the charger so that the two center pins are shorted. It helps to use an app like "battery monitor". It should state that you are plugged into and AC charger, not USB.