unresponsive keyboard and mouse on restart

I have a late 2014 27" 5k iMac running Catalina 10.15.7. A week ago when restarting, both the keyboard and mouse were completely unresponsive so I could not enter my password. The only solution that worked was unplugging the iMac; the keyboard then worked on restart. I reset the NVRAM and SMC after unplugging each time, so the keyboard was hopefully active. However, both a Safe Boot and a normal restart still resulted in unresponsive keyboard and mouse. I then updated to 10.15.7, but no improvement.

I restarted from an external SSD which was a recent CCC backup with Catalina 10.15.6. This works perfectly with the keyboard and mouse! I used Disc Utility to do first aid on the iMac internal SSD with no faults found.

Is it possible that the NVRAM battery is failing after 6 years, even though volume/time zone etc setting still are OK? Is there any way to check this?