Does your radio work, but your tachometer doesn't? It is probably your SI board batteries!  
 
This is a set of two new replacement SI batteries for the BMW E30. They are pre-laser welded to the bus bars for easy installation into your SI board. 
 
Note: Not all SI boards use these batteries and opt for a different style, so make sure to check your SI board first before ordering. I have attached an example photo of what your SI board should look like to make use of these batteries. 
 
Voltage - Rated
3 V
 
Capacity
850mAh
 
Size / Dimension
0.57" Dia x 0.98" H (14.5mm x 25.0mm)
 
Termination Style
PC Pin (Horizontal Mount)
 
Discharge Rate
0.5mA
 
Weight
9 g