The Rad batteries use a proprietary cradle, so we do not have any spare batteries that are directly compatible. We do however offer an adapter that lets you plug in an aftermarket battery: https://boltonebikes.com/collections/electrical-accessories/products/radpower-battery-adapter
This will allow you to replace the existing battery entirely, or mount the spare battery in a separate location, such as a rear rack and swap back and forth between them.
We would recommend this style of battery for The Mini. Note that we have NOT tested 52v batteries on the stock components, but they are compatible if you have one of our upgraded controller kits. https://boltonebikes.com/collections/batteries/products/48v-12-8ah-battery-free-us-shipping