Make sure the firmware is up to date on the Perc 5/i controller. Go to this site: http://www.dell.com/support/drivers/us/en/19/Product/poweredge-2950 and select your OS, and then go down SAS RAID and select the Perc 5/i firmware update. You will need to power down the server, replace the battery, and then power it back up. Here is a link to the User Guide, page 74-75 will show you how to replace the battery:
ftp://ftp.dell.com/Manuals/all-products/esuprt_ser_stor_net/esuprt_poweredge/poweredge-2950_owner%27s%20manual_en-us.pdf
Here is a link to the replacement battery: http://accessories.us.dell.com/sna/productdetail.aspx?c=us&l=en&cs=19&sku=312-0448&baynote_bnrank=0&baynote_irrank=1&~ck=dellSearch
Regards,