My hope is that this post will help someone who is experiencing the same loose triangle battery bag issue I was. After taking my ebike to work for the first time, I found I was constantly reaching down and pushing the battery back into the center of the bike frame, as the battery had a tendency to slide out, and the bag wasn't tight enough to prevent this movement. When I got home, I played around with the setup, holding the bag tight in certain places and testing the movement of the battery. I found that if I held the bottom of the bag tight against the frame, the battery would be secured and would not be able to slide left or right.
It seemed that if I could somehow lace the bottom of the bag to the frame, like a shoe, it would do the trick. I went to Home Depot and purchased a brass grommet kit: http://www.homedepot.com/p/General-Tool ... /100200416
I installed 4 grommets to each side of the bag as shown:
After the grommets had been installed, I mounted the battery bag to the bike.I highly recommend mounting your bag without the battery inside as you can more easily tighten the velcro straps to the frame.
I then loosely laced the grommets with some parachord (thin rope) I had laying around, and inserted the battery into the bag. Then it was just a matter of tightening the laces through the grommets.
The result is that the battery is now MUCH more secure, and is held firmly in place. This also narrowed the width of the bag as it's pulled tight, leaving more room for my knees as I pedal.
I hope someone finds this technique as helpful as I did!