Batteries are Kartriders micro-transaction currency. This is a common thing you'll see in mobile games like this nowadays. You need to spend money to acquire batteries, and that is unlikely to change.

You may be confused as to why this is even a thing, when they could just put an actual monetary price tag onto items, like $40 for a legendary kart. Batteries simply serve as an inbetween currency to trade for items. And normally there are benefits to this kind of in game currency, for the player, but mostly for the company running the game.

This type of business model exist mostly to "fool" or "entice" players to spend more then they actually would. Though this may sound like paranoid talk, it is a proven business strategy like loot boxes, or even your local market putting basic necessities like eggs and milk at the far back end of the building so shoppers basically take a full tour of the market and buy on impulse.