I've had to replace many of these and honestly - if I could pass along any single tip it would be this one - please check the firmware of your Hitachi FC drives first.
If you read enough IBM release notes, this text will be familiar to you:
Customer with IBM branded Hitachi FC drives should verify that these drives are at drive firmware FQ8, or later, before doing a controller firmware or ESM firmware download. It has been observed by IBM Support that customer's with older versions of the Hitachi FC drive firmware have experienced failure of these Hitachi drives during controller firmware and ESM firmware downloads. Upgrading the drive firmware to FQ8 will prevent this problem.
The way this bug was described to me was - if you have Hitachi drives with older firmware than FQ8 - as soon as you reseat your controller - all of your Hitachi drives will show up as failed. I would not wish this on anyone. So. Check your firmware. And upgrade if necessary. Sometimes IBM will check this for you, but ultimately you as the customer are responsible.
That out of the way, you should have the replacement battery on-site. And we're ready to begin.
First you'll need to manually fail the controller. This is done through the Storage Manager Client. You'll select the controller. Then select Advanced > Recovery > Place Controller > Offline.
In this example, failing Controller B will fail over all of its associated array groups to Controller A. You're counting on MPIO/RDAC here to be successful on the host side, so definitely something to check and consider in a non-disruptive maintenance. Once Controller B is failed. It can be removed.
When removing the fiber cables, keep them identified so that you have some way of reseating them in the correct order later on. Whichever is your preference. I usually label them prior to removal.
To remove, push down the black latch and pull on the levers. Controller slides right out.
To open the Controller, place it lever-side-down and use a Philips head screwdriver to remove the screws holding the bottom panel to the sides of the Controller. Three on each side. The bottom panel then slides out and can be tilted up and away.
The final screw on the battery access panel will allow access to the failed battery.
You must connect the new battery harness to the two-pin connector on the Controller board. The connector is keyed so that there is only one way to attach the harness to the connector. Reattach the battery access panel.
Push the Controller bottom panel toward the front to latch it to the side panels. Reinsert the six screws.
Insert the Controller back into its slot. The levers will catch. Push to lock them back in place.
Controller B will start charging its new battery as soon as its inserted. Reconnect the fiber and network exactly as it was before. I usually wait a minute before bringing the Controller back online. And that is done by going through the Storage Manager Client. You'll select the controller. Then select Advanced > Recovery > Place Controller > Online.
The last thing you need to do is reset the battery age back to zero. That's done by clicking on the "View" button and selecting "Batteries". Click "Reset" and answer "Yes" when prompted.
The array should now be in an optimal state.