Short detection voltage limit

This is a belated answer so hopefully it can be of use to folks who search for and find this question.

Normally when searching for shorts, we should inject a lower voltage directly on VCC_MAIN rather than a higher voltage on BATT_VCC. The reason for doing this is that injecting a high voltage may overcome some of the built-in over-voltage protection circuits on the logic board. The problem is that most of the IC's used in an iPhone are of proprietary design so we don't have access to the spec sheets. With no way of knowing what the the limits are on voltage, we can easily cause massive damage to the logic board.