Walkera Runner 250 Advance Battery Stop

This adds support for both the very weak battery tray and battery stop that comes with the Walkera Runner 250 Advance quadcopter. The very first time my Runner randomly flipped and crashed from only about 10 feet, the battery broke right through the cheap battery stop and smashed the OSD module and the pins so I can't easily replace module.

This support is designed for 2 things. First you'll notice from the picture with the battery that it extends towards the rear past the battery stop. Thi