It will disable the cache if the battery is failed, but if it is not impacting performance, you can wait to replace it.   
I found one of the three disks (disk 0:2) status as "Foreign". I am not sure what I have to do.On front of this drive is steady green light but no activity. In OpenManage status of this drive is "Foreign" other two drives are "Online", system is working fine but I know I have to fix this ASAP. Please help. Thanks
RAID 5 - Foreign