The RadPower 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 replaced 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 a Radmini. 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