How many batteries do I need to get a pet?

For constellation pets, you'll need about 270 to 320 Star Gems. Since you also need Transcendence Cakes and Awakening Cards, it's best to start with around 5,000 to 6,000 batteries. The Transcendence Cake is a rare item given only when you consume 5,000 batteries in a consumption event, and Awakening Cards are items you can get by purchasing them from the shop or opening the Time Rift 400 times (1200 batteries based on the shop price).