After about one month discussion and tests, we got a new idea that we could make it happen. We reshaped the battery to allow them stay in a tube. Why we want to try this resolution, because we need a flashlight shape tool.
Power Bank is not a tool shape. When people are using power bank, they normally do not consider them as a tool. What is a tool? A tool that fits human hands the best, is the tool we are looking for. So we got our second prototype draft.