Battery for User Storage in Battery Backed RAM

I have an FBL2360 motor controller and I would like to utilize the user storage in battery backed RAM feature with the BEE command. I have tried the command and it does save data to RAM, but when I power off the controller it loses the memory.

I took the cover off the controller and saw the user battery, BT1, is not supplied with the unit. Does any one know the battery type which should be installed here?

Thanks!