ThisDellEmailWillSelfDistructInOneMonth, The steps are outlined on page 119 in the T410 manual found here. You were close with the steps you listed, but you would actually need to remove the power cables after powering down. Once the battery is installed you just plug in and power on. Hope this helps.
Dell PowerEdge T410. Perc 6/i RAID battery (NU209) died. Can't seem to find any info on the STEPS to replace. It does come with a manual, but all it says is the typical bureaucratic BS "NEVER THROW THE BATTERY PACK INTO FIRE" <-- Yes, it actually says that on the very first part in the "MANUAL". NOWHERE does it actually say how to install or replace, and I'm not about to assume ANYTHING. Will I loose my RAID config? Will I only loose what hasn't been written to disk? Knowing these things are important! Sure, I could just turn off the server, leave it plugged in, and replace the battery and that would be fine and dandy -- and who knows, maybe those are the exact instructions that I can see how it'd cost too much money to print that one-liner on the "MANUAL". The problem is, I cannot find anything online to settle my nerves. I call Dell and get the typical ROBOT replies, and am directed here. I ask "so you don't have any documentation on how to replace the Dell NU209, Dell?". They provide a ROBOT response to which I hang up, go create yet another fake & temporary email account just to ask this one question. Does anyone know the correct process of replacing the Dell NU209? Dell doesn't. Thanks and I hope I was able to give at least one person a chuckle,    
Dell NU209 replacement steps - HOW TO REPLACE