When you are worried about running out of battery life on your phone, on a cordless drill, or anything else, you keep a backup battery handy. When your drill runs out of power, you grab the backup battery and plug in the dead battery. This way, you keep moving on your project while the extra battery recharges. Having multiple kanbans means you never run out of product at the wrong time. When you reach the bottom of a kanban, you start working on the next one while the empty kanban is refilled.