A single 18650 battery base. Search for "18650 lithium battery terminals, positive and negative terminals" on Pinduoduo and you can buy a bunch for just 10 yuan. There's wiring space inside the back cover, so just stick it together with glue and it's all good. In the future, for any projects that require battery replacement, you can just stick it directly to a flat surface. I've looked at many other battery cases, but they all feel flimsy. With this one, you can use it vigorously and it'll