The RAID adapter may report the following message:
"Current capacity of the battery is below threshold" indicating that maybe the battery needs to be replaced. This may be fixed by "relearning" the battery. This will drain the cache battery and charge it a few times to relearn it back up to full charge."
The command takes a day or two to complete but is fairly simple. Change the adapter number as needed to match the line in /var/log/messages.
The command is:
megacli -adpbbucmd -bbulearn -a1 for adapter 1.
Current capacity of the battery is below threshold . OR
2019-07-29T19:30:01-05:00 hostname_here disk_subsystem: snlog: sn="24:6e:96:90:85:a7" id="DS" m="23" c="6" event="DISK_STATUS" category="HARDWARE" ip="10.2.1.53" model="R730xd" msg="Adapter 1; seqNum: 0x00019bb5; Time: Mon Jul 29 20:26:52 2019; Event Description: Current capacity of the battery is below threshold; "
The batteries need to be drained and recharged periodically. This is not common.
megacli -adpbbucmd -bbulearn -a1 command for adapter 1. Change the adapter number to match the line in /var/log/messages.