There have been a handful of cases of the battery cradle working itself loose on the DJ Fat Bike. While it is a good practice to keep the bolts tight, I wondered if there was something else that could be done to stabilize the battery cradle.
The Hailong style battery cradle is intended to mount to a set of bottle cage bolts. Due to the large weight of the battery, the cradle has a metal insert for higher strength. I have noticed that even when I tighten the bolts, the battery still remained a little bit wobbly, there was also gap that was present underneath the cradle. When I looked closer at the underside of the cradle, I noticed that the water bottle bosses were compressing the plastic around the area of the mounting holes. It seems that between the water bottle mounts and the metal insert, the plastic on the cradle was being the weak link in the chain.
To do this mod, you will need to completely remove the battery cradle, detach the power connector/controller, and slide out the metal insert.
You'll need to remove some of the plastic around the mounting holes, so that the cradle fits through the water bottle mounts, and is supported by the surface of the downtube, instead of the tiny surface on the face of the mounts. The mounts are 11mm in diameter. I removed a little bit of plastic using a rotary tool on low speed.
When the mounting holes are properly clearanced, none of the plastic gets in the way of mounting bosses. With a direct metal to metal interface, the mounting bolts can be torqued a little tighter than normal, as you don't have to worry about damaging any plastic. When the cradle is bolted in place, the gap underneath it disappears too.
Remember to put some blue thread sealant on the mounting screws.
This small mod certainly does stiffen the battery cradle. I'm happy with the difference it makes. Even though this does improve things, it's still a good idea to check that the mounting bolts are tight whenever you do routine maintenance on the bike. Another thing you can do to improve the security of the battery, is to use a tie down strap to keep the battery from moving. Something like this is cheap insurance for preventing mishaps with the battery: