That particular vest is made by activheat. They have their own website you can order from. The one you are looking at from brookstone has a awfully small battery pack for 5 hours. My battery pack is larger but I believe was advertised for 10 hours use and I know it will work consistantly for 6 to 8 hours. It does not state on your site but mine came with 2 batteries and a 2 chargers (1 wall A/C and 1 Auto 12volt) for around $165.
If you don't understand the technology which you depend on, you will soon be a victim of that technology.