Proceed CD Library

@tonyaudio 

https://en.wikipedia.org/wiki/List_of_battery_sizes#/media/File:Batteries_comparison_4,5_D_C_AA_AAA_...

Do you know the voltage?

4.85V 1/2C Cell Array Battery
Does this make sense to anyone?

@tonyaudio  

1/2C is the battery size (Dia.26x24mm L), 4 batteries in series = 4.8V (1.2Vx4). Without seeing the actual battery compartment, just a wide guess.
http://www.batteriesinaflash.com/special-application-111/assembly-cells/nicd/1-2c

@tonyaudio
Can you provide some picture of the battery compartment?
What you need is a 4.8V battery pack connect or clip to the battery compartment + and - terminal to make it work or see if the Communicator (the controller) still in working condition.
If you got 4 of those 1/2C rechargeable NiCad battery on hand, add some conductive spacer to make it to correct length.

My suggestion is cut a piece of 1" dia wood to 1/2"-5/8" length, drill and insert a 3/16" dia x 3/4" copper rod in the center.

Tony, have you try those 1/2C rechargeable NiCad battery?

the controller says "batteries need recharging"

Could be the resistance of the tin foil caused voltage drop.