A "Backup Battery Low" message is an indication that the internal battery is running down in the TD-20 module. The internal battery is designed to save your custom/user settings when the power is turned off.
The battery is a model 2032 (watch/calculator type) and can be found at most all electronic supply stores that have a battery section.
The internal battery is not considered to be "user" replaceable since it is mounted on the main circuit board and the TD-20 would need to be disassembled to access it. Due to safety reasons, we recommend having it changed by an authorized technician to avoid any damage to the TD-20.
You can use our Service Center Locator at http://www.rolandus.com/support/service_centers/ to locate an authorized Roland Service Center in your area.
Note: While it will not cause harm or permanent damage to the TD-20 if the battery runs down, ALL User settings will be lost if the battery runs completely down or when replacing the battery. Be sure to backup any user settings that you wish to keep to a Compact Flash card before replacing the battery.