What batteries does the Onn Universal Remote require and how do I install them?
The remote requires two (2) AAA Alkaline batteries, which are not included. To install them, slide the battery compartment cover down, match the (+) and (-) marks on the batteries to the marks inside the compartment, insert the batteries, and slide the cover back into place. Be sure to use fresh batteries for optimal performance.
From the official Onn Universal Remote manual, page 3
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