I'm guessing the spec is really 450mA and not 450mAh?Do you have a link to this backpack? It sounds like a really neat product.
It is too powerfull for simple runing arduino dependig what yo wish arduino to do and how long time
woah there, if its a regulated 5 volt output then nothing will be in danger of being fried. and i think the conversion in mAh comes from the boost converter. to get 5 volts from 3.whatever you need to drop the current capacity, so more amp draw on the battery in exchange for more voltage. so long as the energy is the same (minus conversion losses) it works out.[edit: the input of 400mAh makes no sense however]