I indeed need to retain RAM, not EPROM (because it is retained already). I am not using external RAM, just internal.

My design is indeed not build with low power in mind. Besides the battery back-up, there is no use for it in my design. it will control a machine of about 2kW, so the controller power is not an issue.

I think I will go for the power fault comparator and add the battery to only the controller to power it when Vcc drops.

Thanks for all your input,

Marc