Regarding the button cell.
The "button cell" on the Control Surface (CS) processor is the security chip (displayed as 18 digits on the REPORT screen) which is embedded in your 36 digit password (a file STRAND.PAS) for enabling channels/applications.
In order to retain your console's channels and apps, it would need to be transferred along with it's associated password to the newer console.
Regarding the Nicad.
From memory, the current running show and any modifications made by the operator were stored in RAM on the CS Processor. This show and any changes made by the operator would be automatically transferred (backed up) to the hard drive on a regular (I don't know the frequency) basis during normal operation and saved as DAT files in the 220os directory.
If for any reason the console crashes or has a brown out, the Nicad (NiMH) would retain the RAM information on the CS processor so that on recovery, the latest information at time of the console going down would be retained (and subsequently automatically backed up to the hard drive).
I have removed the nicad on one of my consoles for peace of mind. The console is not used in any anger or subject to loads of show critical operator modifications so I'm not really worried about the data on the CS processor. Each time the console power cycles, it displays the message loading CS processor (as expected) as it cannot find any data on the CS processor and transfers the latest info it had from the hard drive to the CS processor.
This is something I'm prepared to live with especially on a 510 which has a multitude of screws to remove for internal inspecton.
For other 500 series consoles I've started to wire the Nicad "OFF" board so any leakage will not damage the CS processor.
So choices are either
1. Mount Nicad back in it's original position on the CS processor and regularly inspect.
2. Leave out the Nicad and live with the latest automatically backed up copy on the hard drive. Try it a few times to see how it goes, you can always add the Nicad if operating this way is not acceptable.
3. Mount the Nicad safely off board.