Can a RAID card be used just to connect an optical drive

Since the controller has already evicted the drive, it is no longer part of the physical array.

This means you can safely swap it out for a new one.

You should run /c0 rescan after plugging in the new drive, followed by /c0 show; you should see the new drive mentioned as a Spare.

Then you can give the command to rebuild (this will happen automatically with the default configuration settings)

Clarification

This is a hack, not a proper fix which would be buying a new BBU.

I pulled this hack due to cost restraints (a new BBU costs more than 10x the cost of a used one,) resource restraints (an impending holiday already brought the economy of the country to a halt so no purchasing is possible at the time,) and the fact that I am the go-to guy for troubleshooting virtually anything, hardware or software, in my team. I was also the chief engineer in a few of the team's projects and have soldered prototype boards and even previously fixed a colleague's graphics card, so the team trust me with my soldering skills which is involved in this hack.

Also, since this is handling Li-po cells which may catch fire if mistreated, you have to be extremely careful when dealing them. I am a professional on this, being a trained engineer in both electronics engineering and computer engineering; and the hacker's kit Li-po edition is a custom purpose-built tool for troubleshooting Li-po and Li-ion batteries.

If you are not a multitalented engineer and troubleshooter, if you are not willing to take the risk, if you are not willing to stomach any loss if the hack fell apart, and/or if your team don't trust you in those (most of the time,) do not pull hacks like this.

Original answer

I have managed to rescue this BBU after hacking it. The seller tells me that the used BBU is sold as-is without any warranty, and this gave me the license to hack.

The battery controller chip by Linear Technology requires a minimum cell voltage of 3V to operate according to its datasheet, but my battery cell measures only 2.4V. No wonder why it won't work, the chip is in its low cell voltage protection mode.

I took the battery pack itself apart to find out what the remaining three out of five wires from the pack are connected to, but that proved not relevant. After I peeled off the paper top of the battery I am greeted with two solder blobs for the cell itself. A quick confirmation with a multimeter, the cell measures 2.4V, flat dead. Time to break out my hacker's kit, Li-po edition.

Since I already have two nice big solder blobs for terminals of the cell, I soldered wires to it attaching the cell directly to my TP4056-based Li-po charger module with integrated DW01A cell protection unit, bypassing the onboard protection unit and charging unit. DW01A have a cutoff voltage of 1.8V and TP4056 can charge cells with a starting voltage of 2V. After a nice night of 1A fast charging with TP4056, the cell is fully charged at 4.3V.

Sealing the battery pack back with a few strips of sticky tape, the BBU is properly detected and working again.

To answer my own questions:

1. The BBU is fixed.
2. There is no firmware incompatibility. The cause of the issue is just a flat battery.
3. The cell can be force charged with appropriate charger, bypassing the onboard protection unit.
4. The cell is a standard size and can be replaced if it is dead.
5. It is fixed, no need of writing it off.

Possible future experiments:

• Replace the Li-po cell with another, brand new cell (since I bought this one used and battery cells wear out eventually,) maybe physically larger thanks to the form factor, or some remotely mounted high capacity 18650 Li-ion cells.