Are you sure the battery can’t be replaced concurrently? Most of them can. If you are seeing performance degredation in performance data, then it is real, so it just depends on what your customer would rather risk, degraded performance or the risk of concurrent replacement.
We have had cache battery failures before but our users hardly notice degraded performance, but of course, they need to replaced sometime. The system will warn you about 90 days before they actually fail. From my experience, the system needs to be down for about 1 1/2 hours because our battery packs have never been concurrently maintainable. IBM recommends that you replace the batteries before you receive failure messages, so the way to view when you will get warning and then failure messages (and mark it on your calendar) is this:
Go into SST (STRSST) and take options 1, 4, 1, 2, 14 (Start a service tool, Display/Alter/Dump, Display/Alter storage, Licensed Internal Code (LIC) data, Advanced Analysis). Type a 1 to select BATTERYINFO and then enter -INFO -ALL on the Option parameter. It will show you the estimated number of days that the system will issue a warning followed by the estimated numbers of days until failure. Just below that it will tell you if your battery pack is concurrently maintainable.