You can change the battery if you wish, but it may be unnecessary.  I have several 2850's (2004-2006) and 2650's (2002-2004), most of which are running on their original batteries.  If it is bulging or you have recently seen battery messages on your LCD or from OMSA, then by all means, replace it.  If you want to be absolutely sure it is fixed, then get the memory (95% chance), battery (4% chance), and the riser (1% chance, but it happens that the memory slot is bad) ... if your LCD panel is blue with no error message.
I have a PowerEdge 2850 with Windows 2000 Raid 1.  here is my diary of grief: Day 1. Application failed and OS hung.  Reboot produced a blue screen with suggestion that I ran chkdsk /r  from the recovery console which I did for it to fail with the message that the drive appeared to be corrupted.-  I did a bit of googling and decided it would be a good idea to replace the MRAID2K driver using recovery console. Which I did. I should point out that this and every subsequent boot had the message that "Memory/Battery problems were detected, the adapter has recovered etc.."  The next boot into windows went a bit further, but it started to check the disk and 17% through checking the indexes it failed. Day 2. Checking the hardware. Opened up the bonnet, blew out the dust, removed the Riser Card re-seated the memory, checked the battery connection, then using the Dell utility (F10) I checked everything including a full surface scan of the drives everything passed OK. There was no message in the LCD panel (blue) and the 2 drives had a single continuous green LED which I took to being a good thing. Next the PERC/CERC Bios Configuration Utility (Ctrl M) . Viewed config, both drives Green and ONLINE, so good so far. Decided to check the Consistency and it hung on 0% so I powered of the server and went out for a beer Day 3.  Misery. Now I can do nothing as the boot sequence stops before the Setup or option to boot to CD. I am thinking of changing the Raid Kit (in particular the Memory)  What do you guys think?
PowerEdge 2850 was working, then OS failed, now will not even get to setup 
utility