You should enter more details about the exact Apple TV model you have. They usually need triple A batteries, brand is not problem but stay away from the cheap ones.
Apple company uses a CR2032 lithium 3. 0V battery which is very commonly used in all the electronic devices which you can buy it from any electronic store. When the battery power is low you can see the icon in the computer. That's the time to change the battery. Locate the battery compartment and replace with the original.
When the display screen of the Apple shows a TV remote display, its time to replace the battery. The battery number is CR2032 lithium 3. OV coin battery. You can get them at any electronic store. Radio Shack is a good start. They can be pricey for such a small battery.
The Apple TV remote requires a CR2032 lithium 3. 0 V coin battery. Coin batteries are used in things like hearing aides, watches, calculators, etc. You can find this type of battery at just about any grocery store, Target, Walmart, Kmart, Walgreens, etc. They run about $11 to $12 a piece.
The Apple Remote uses the CR2032 lithium 3. 0 V coin battery. This battery is a commonly used battery in small electronic devices. You can find this battery at all most any electronics stores, most drug stores, Walmart, and also at Radio Shack.