A "BATT Low" message is an indication that the internal battery is running down in the DR-5. 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 DR-5 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 DR-5.
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 DR-5 if the battery runs down, ALL User settings will be lost when replacing the battery. Be sure to backup any user settings you wish to keep before replacing the battery.