Using the diode method for switching from the higher voltage mains DC source to the batteries, you can detect when the hard connection has been disconnected because there will be a drop in power on your power source to the battery voltage.

I don't know what microcontroller you are using, but generally a uC can run on sleep mode just maintaining an xtal clock for a very long time. Some could do it for years on a couple AA's.

DS1307 is a 5V part only. Keep that in mind if you are running a 3.3V uC.

Thank you very much johnnies!

Originally Posted by Mr RB

You just need 2 schottky diodes, you place one directly between the +5v rail and micro power pin and another diode between a 3-cell 4.5v battery and the micro power pin.

It's an easy enough mod you can do it by cutting one PCB track and soldering the diodes on the bottom.

About Mr RB's plan, I should have another part in the system that will let the MCU know when it is driven by batteries, so the MCU could get into sleep mode, with only its clock running, right?

One more thing please.
If the power supply wire that connects to the Power pins of the MCU, connects to other devices, then its not good right?
I mean, i'll have to re-trace the wires, isnt it?

Thanks.
I'm using a CC2430 micro-controller.
Currently the MCU is powered only when the product is connected to the mains.
The time-ticking is being operated by a RTC timer.