If you purchased the BBU from CenturyLink, CenturyLink’s warranty covers the replacement of the battery in the event it stops working within the first year. When the BBU is ordered from CenturyLink, it is provided by APC until April 15th, 2016. After that date, it will be provided by Alpha.
More information regarding these units is available here:
Replacement batteries may also be purchased at major battery distributors, including those listed below. Please use this product description when securing the replacement battery: 12 Volt 12 AMP Ah Valve-Regulated (sometimes referred to as "sealed") Lead Acid Battery, Approximately 6" wide by 3 1/2" deep by 4" tall, with quick-disconnect tab posts.
Some batteries may be covered under the BBU equipment supplier's warranty. CenturyLink does not warranty BBU equipment not purchased directly from CenturyLink. If a battery failure occurs, contact the supplier of the BBU to determine whether your battery is under any warranty.