Dell PowerEdge 1950 RAID Battery – what kind

You may end up with a singularity forming in your server closet if you don't replace it.

Just kidding.

What happens is the PERC controller will stop caching data and your disk performance may drop. That way if there's a power outage, it should prevent you from losing data (or minimize the chances of losing data.

It shouldn't cause any immediate issues if you have to wait a few days to get the new battery. You turn off the server, replace the battery as per the documentation, and power it back up. The server will run fine until you get it replaced.

Another thing to consider is what type of drives you have connected to the controller. If you've connected consumer grade drives you should be aware that the PERC is expecting a FAST response (as in NOW not 2 or 3 seconds) on a write.