Linux server box with LSI MegaRAID controller got degraded. But after some time RAID status changed to Optimal.

Adapter 0 -- Virtual Drive Information:
Virtual Drive: 0 (Target Id: 0)
Name                :
RAID Level          : Primary-1, Secondary-0, RAID Level Qualifier-0
Size                : 2.727 TB
Mirror Data         : 2.727 TB
State               : Optimal
Strip Size          : 256 KB
Number Of Drives per span:2
Span Depth          : 3
Default Cache Policy: WriteBack, ReadAdaptive, Cached, No Write Cache if Bad BBU
Current Cache Policy: WriteThrough, ReadAdaptive, Cached, No Write Cache if Bad BBU
Default Access Policy: Read/Write
Current Access Policy: Read/Write
Disk Cache Policy   : Disk's Default
Encryption Type     : None
Is VD Cached: No

But now I'm getting RAID BIOS POST message:

Your battery is either charging, bad or missing, and you have VDs configured for write-back mode. Because the battery is not currently usable, these VDs willl actually run in write-through mode until the battery is fully charged or replaced if it is bad or missing.

(Image: http://cl.ly/image/1h1O093b1i2d)

So may it be battery issue caused problem ?

I get information about battery:

BatteryType: iBBU
Voltage: 4001 mV
Current: 0 mA
Temperature: 22 C
Battery State     : Operational

BBU Firmware Status:

  Charging Status              : None
  Voltage                                 : OK
  Temperature                             : OK
  Learn Cycle Requested                   : No
  Learn Cycle Active                      : No
  Learn Cycle Status                      : OK
  Learn Cycle Timeout                     : No
  I2c Errors Detected                     : No
  Battery Pack Missing                    : No
  Battery Replacement required            : No
  Remaining Capacity Low                  : No
  Periodic Learn Required                 : No
  Transparent Learn                       : No
  No space to cache offload               : No
  Pack is about to fail & should be replaced : No
  Cache Offload premium feature required  : No
  Module microcode update required        : No

Where can be problem ? I'm disabled alarms, but get them if enabled. But don't know how find root of problem.