Thnx both of you. Shinobi actually comes in 2 flavours; one with a suicide battery hidden inside a custom Z80 used for the sound system and one without a battery and with a standard Z80. I don't know why Sega made 2 kinds...maby one was made by another company on lincense?!
I have an original Shinobi of the 2nd kind (standard Z80), so it will never loose sound do to flat battery.