IBM System Storage Subsystem DS5020 - Manual (Page 147)

If caching is enabled and the Cache Active LED never comes on during I/O activity,
it indicates one of the following conditions:
v The cache memory from either controller A or controller B has failed.
v The cache sizes in controller A and controller B are not the same.
v The battery has failed. (In this case, one or both of the green Battery LEDs will
be off and one or both amber Battery Needs Attention LEDs will be lit.)
Note: Always use the DS Storage Manager client to check your cache memory
settings before assuming a hardware failure.
Figure 73 shows the locations of the Cache Active LEDs on the front of the RAID
controllers.
Subsystem cache battery
The backup battery unit provides power to back up the cache memory of each
controller onto flash drives in the event of a power failure. Each battery unit
contains a sealed, rechargeable SMART lithium ion battery. The battery unit
contains enough charge to back up the cached data in each controller to a flash
drive in the event of a power failure.
Cache
active
LED
Cache
active
LED
Figure 73. Cache Active LEDs
Chapter 4. Operating the storage subsystem 117