Korg Poly800/EX800 Users

Have had my Poly 800 since the late '80s when my dad brought it home 
for me one day. It's been sitting in a cupboard for the last few 
years (well 10 or so!) and I finally dug it out today to give it a 
clean up. Oh how I wish I took those batteries out before I stored 
it! Have now taken it apart and cleaned up the battery compartment 
and its working well although it loses its patches when I switch it 
off. Is this the internal battery needing replacing?

Any help appreciated.

Greg