SI Batteries
 

I've read some thread on dodgy temp gauges and MPG gauges. My temp gauges swings to red when starting and remains there then drops to blue after 5mins then centres. My MPG gauge only reachs 20MPG when hard accelerating and sit off the scale past 50MPG when driving normally.. Not what it used to do.
So threads say it could be the SI Batteries that need replacing, is this common and has anyone done this?

Its definitely instrument cluster related, any help?