Normally the are capacitors inside the cluster, see here http://bmwe32.masscom.net/johan/dash...dash_caps.html
sometimes also the area under the push bottom for the reset of the day trip is oxidized or the pin is stuck.
Older clusters like from E28 still had real batteries inside and they leaked and destroyed the board.
As far as I know only the E34 EURO version Low cluster = without check control (02/1988 - 08/1990 - 520i, 524td - maker: Motometer) had 2 batteries inside , all others have capacitors, see here, the first version is the one with the 2 batteries
http://www.netforce-player.de/pet-ra...AGEN/KI/KI.htm