So how can I find the time to put an alkaline battery in Royce's back
And at the same time put juice in mine?

from Bad Meets Evil (Ft. Slaughterhouse) – Loud Noises Lyrics on Rap Genius

Meaning

Alkaline batteries are a type of primary battery or rechargeable battery. So eminem and Royce can keep going, and going, and going and going.

Em is also referring back to his battling days when MC Juice was the only one to beat him after 6 rounds of battling, therefore, “So how can I find the time to put an alkaline battery in Royce’s BACK and at the same time put JUICE in mine…” as in put that battle with Juice BEHIND (back) him.

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