Casio C-801 batteries.....?

You need two SR1120 or type 391s.

The replacement batteries for a Casio C-801 are typically CR2032 lithium coin cell batteries. These batteries are commonly used in small electronic devices, such as watches, calculators, and remote controls.
To replace the batteries in a Casio C-801, you will need to follow these steps:

1. Remove the back cover: The back cover of the C-801 is typically held in place with screws or a spring-loaded latch. Use a small screwdriver or coin to remove the screws or release the latch.
2. Remove the old batteries: Once the back cover is removed, you will be able to access the batteries. Gently lift out the old batteries and discard them.
3. Install the new batteries: Carefully insert the new CR2032 batteries into the battery compartment, making sure to align the positive (+) and negative (-) ends correctly.
4. Replace the back cover: Secure the back cover to the device using the screws or latch. Make sure the cover is securely in place.