That information should have been covered in your assembly instructions . Most of the electrics that I have had the pleasure of assembling and flying ,the batteries fit snugly in a pocket and cant move . I don't have a lot of experience with electric planes as I only have three so I am glad that you have posted this information . I will be careful to watch for that type of situation . In fact I think I might have one now . I just bought a Ultra Polaris and I noticed that it has a large battery compartment with just one strap . I don't have a battery installed in it yet or the balance figured out .. Good tip . Thank you I think when I have mine balanced I will fill in around the battery with foam as well as the velcro