Kawai Synthesizers

Hi,

I recently bought a Kawai K4 secondhand. However, when I power up the 
synth I get the display reading "check internal battery". I have also 
found that all the sounds are wiped off aswell.

I have opened the K4 but cannot find the internal battery.

Where can I find the internal battery? and is it easy to replace? 
Also, what kind of battery should I buy?

Please advise


Arthur